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Some structures of insects as seen with the scanning electron microscope
Micron, 1969, 1:84-108
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with VI plates
Some structures of insects as seen with the scanning electron microscope H. E. H I N T O N , F.R.S. Department o1" Zoology, University of Bristol, U.K.
Manuscript received 24 September, 1969
With the advent of the scanning electron microscope it has becomepossible to examine the geometry of many of the structures of insects, particularly hard suface structures, at a resolution never achieved before. In this paper micrographs are presented of a number of relatively little known insect structures. They clearly illustrate the advantages of the scanning microscope over any other available instrument. The biological significance of some of the structures illustrated is discussed in detail, especially those to do with physical gills, the anti-reflection coating of the eyes of some insects, diffraction gratings in beetles, and stridulatory or sound producing organs of some moths and butterflies. Il est devenu possible d'examiner la gdometrie de beaucoup des structures d'insectes, particulikrement les structures de surface dure, a une rdsolution jamais atteinte avant, grace a l' avknement du microscope dlectronique it balayage. Dans cette revue on prdsente des micrographiques d'un certain nombre de structures relativement peu connues d'insectes. Ils montrent nettement les avantages du microscope it balayage sur tout autre instrument disponible. On discute en ddtail de l' importance biologique de certaines des structures exposdes, spdcialement de celles qui se rapportent aux bronchies physiques, au rev~tement anti-rdflectif sur les yeux de certains insectes, aux rdseaux des coldopt~res, et aux organes stridulatoires ou producteurs de sons de certains ldpidoptkres et de certains papillons. Es ist jetzt mOglich die Geometrie vieler Insekten Strukturen mit abtastender ~Jbermikroskopie zu untersuchen, insbesondere die harten Oberfliichen Strukturen, mit einer derartigen AuflOsung hie bevor erreichbar. In diesen Artikel werden Mikrographien einiger verhiiltnissm~sig wenig bekannten Insekten wiedergegeben. Diese zeigen ganz klar die Vorteile des abtastenden Mikroskops verglichen mit irgendeinen anderen Apparat. Die biologische Bedeutung einiger dieser Strukturen wird ausfuhrlich besprochen, besonders solche die die physischen Kiemen, die nichtreflektierenden Schicht der Augen einiger Insekten, die Diffraktionsgitter yon Kiifern und die Zirpoder Lauterzeugenden Organe einiger Motten und Schmetterlinge betreffen.
RESPIRATORY STRUCTURES In recent years, work with the scanning electron microscope has made it possible to study with much greater precision than before the geometry of certain respiratory structures in insects, particularly surface hairs and meshworks that hold the films or bubbles of air used for respiration by many aquatic insects. Among aquatic insects there are two quite different kinds of physical gills that are used to extract dissolved oxygen from the ambient water. One of these may be called a compressible or shrinking physical gill and the other an incompressible physical gill. The latter is now generally known as a plastron.
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Compressible or shrinking physical gill Many aquatic insects carry a bubble of air held between hydrofuge hairs on some part of their surface. A number of writers had long ago realised that insects with bubbles remained beneath the surface for periods longer than could be accounted for by the load of oxygen taken below. They therefore suggested thai the bubbles of air were in some way acting as gills that enabled the insects to extract additional supplies of oxygen from the ambient water. It was left to Ege (1915) to provide tile tirst satisfactory experimental and theoretical analysis of the way in which the bubbh" of air (or film) was being used as a gill. As the insect withdraws oxygen from the bubble, the partial pressure of ox}gen in the bubble falls and the partial pressure of nitrogen rises. The titll in the partial pressure of oxygen causes this gas to diffuse into the bubble and the rise in the partial pressure of nitrogen causes this gas to diffuse out of the bubble. However, because oxygen is much more soluble in water than nitrogen, the water-air interface of the bubble is more permeable for oxygen than for nitrogen. Because of this ditti-rence in the solubilities of the two gases there will be a greater tendency for equilibrium to be restored by oxygen diffusing into the bubble than by nitrogen diffusing out of it. However, some nitrogen is continually leaking out of the bubble so that the surface area ot't he bubble eventually becomes too small for the insect to satisfy its oxygen requirements by diffusion of oxygen into the bubble from the ambient water. The insect must then go to the surface and obtain another bubble. Ege has shown that under certain conditions the backswimmer (.\blonecta) extracts from the ambient water as much as 13 times the amount of oxygen that the bubble originally contained. Other writers have shown that at low temperatures when insects are not very active and not much oxygen is used, the bubbles of some may last for several days. But these bubbles shrink inexorably as the nitrogen leaks out and sooner or later the insect must surface to refill the gas space. In the compressible type of gill the most important gas is nitrogen. This has been very clearly demonstrated by Popham (1954), who kept water boatmcn (Corixidae) in well oxygenated water but with access only to pure nitrogen in the gas phase above the water. Under these conditions the bugs were able to respire normally even though the bubbles they obtained at the surface were not air but pure nitrogen. The duration of the compressible gill is determined by the rate at which nitrogen is lost. The rate of loss of any gas depends upon the product of solubility and diffusion coefficient (aD). The substitution of inert gases for nitrogen in the bubble will therefore shorten or lengthen the life of the bubble according to the ratio (~.I)) in,.,~ ~'a~/(~D)x:. In experiments in which a number of inert gases were substituted for nitrogen in both the bubbles and the ambien{ water, Rahn and Paganelli (1968) found that only neon rivals nitrogen. However, if nitrogen is replaced by the synthetic gas sulfur hexafluoride, the duration of the bubble is increased by a factor of 4 because of the very low solubility and high molecular weight of this gas. Some, if not much, of the carbon dioxide excreted by the insect will pass into the compressible gill through the spiracles or openings of the breathing tubes. Although carbon dioxide diffuses more slowly than nitrogen or oxygen, it is very much more soluble than either of these gases, and (eD)co 2 is about 20 times greater than (0cD)o2. Thus, the effect of carbon dioxide on the volume and pressure of the gas in the bubble
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is small and probably does not materially affect the lifetime of the bubble under water. The efficiency of the compressible gill is affected by diffusion boundary layers. The boundary layers will be thin when the insect is swimming and there is a rapid movement of water past the surface of the bubble. When the insect is stationary, the diffusion boundary layer will become thicker and the gill will not be as efficient. Many insects reduce the boundary layers by driving water past the bubble. For instance, the backswimmers use their paddle-like hind legs to do this, and it has been shown that the rate of oxygen uptake from the ambient water decreases when tethered bugs are prevented from using their paddles.
The plastron or incompressible physical gill A plastron is a gas film of constant volume and an extensive water-air interface. Such films are held in position by hydrofuge hairs or hydrofuge meshworks of various kinds (Plate I, Figure 2; Plate II, Figures 3-5), and they resist wetting under the hydrostatic pressures to which they are normally subjected in nature. In well-aerated water a plastron enables an animal to remain immersed indefinitely, when it obtains the oxygen it requires from the ambient water. In the plastron, unlike the compressible gill, nitrogen plays no essential role other than its contribution to the back pressure of the system. Removal of all nitrogen from the plastron space without substituting another gas would effectively increase the pressure on the water-air interface of the plastron by 0.79 atm, which would be enough to wet the plastrons of many kinds of insects. Nearly all aquatic insects with a plastron are found in waters in which the oxygen pressure is maintained at a fairly high level, such as fast streams, the littoral zones of large lakes, and intertidal areas. Their restriction to these kinds of habitats is no accident because, as has been noted before (Hinton, 1959), the plastron becomes an efficient means of extracting oxygen from the insect should the oxygen pressure of the environment fall below that of their tissues. A few adult beetles appear to be the only plastron-bearing insects that live in standing water, such as marshes and ponds, in which there may be from time to time a sudden and severe fall in oxygen pressure, especially at night. What the beetles do when this happens is not known, but they can always climb out of the water when the oxygen pressure falls enough to cause the plastron to work in reverse. In this connection it may be noted that most of the plastronbearing beetles in marshes and ponds are the only plastron-bearing beetles that are good swimmers: most beetles with a plastron cannot swim and are confined to fast streams and the littoral of large lakes. The well-oxygenated waters in which the plastron breathers are found are characterised by frequent and often large fluctuations in water level, e.g. intertidal areas and small mountain streams. These environments are thus alternately dry and flooded; and the great selective advantage of the plastron can only be fully understood in relation to this particular feature of the environment. In water, the plastron provides the insect with a relatively enormous area for the extraction of dissolved oxygen without necessarily involving any reduction in the impermeability of its cuticle. In air, the plastron provides a direct route for the entry of atmospheric oxygen that does not involve water-loss over a large area because the connection between the plastron and the internal tissues may be very restricted. Thus the capacity of the insect to avoid
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an undue loss of water when its habitat is dry is not necessarily impaired bv the possession of a plastron. Once it becomes possible to distinguish the special t~atures of" aquatic cnvironments that confer a great selective advantage upon the plastron method of respiration, it also becomes possible to distinguish the same features in other environments that at first sight may appear to be very different indeed. We have seen that the essential features of the aquatic environments in which plastron respiration is so advantageous are: (1) that it is alternately dry and flooded; and (2) that when flooded the oxygen pressure in the water is maintained at a high level. Both of these essential features of aquatic environments in which plastrons are evolved are reproduced in the environment of" many terrestrial insects: the difference is merely that for the terrestrial insects the flooded periods are less frequent and do not last as long. Whenever it rains heavily, a very large number of terrestrial insects are submerged beneath a layer of water. The immobile stages of these insects--eggs and p u p a e - - a r e normally glued or otherwise fastened to the substrate and therefore necessarily remain submerged until it has stopped raining and the water has evaporated or flowed away. Thus in most climates many of the terrestrial insects are alternately dry and flooded. To be submerged in water for several hours or even days, a period that may even exceed the duration of the egg stage, is no rare and isolated event but is a normal hazard of their environment. It therefore seems likely that many terrestrial insects will be adapted to respiration under water in a manner no less complex than many aquatic insects. Among terrestrial insects, plastron respiration was first reported for the eggs of some flies (Hinton, 1959; 1960). It is now known that instances of plastron respiration among terrestrial insects are vastly more numerous than among aquatic ones. Reviews have been published of plastron respiration in adult insects (Thorpe, 1950; Crisp, 1964), pupal insects (Hinton, 1968), and insect eggs (Hinton, 1969a).
Evolution of the plastron Plastron respiration has been independently evolved a great many times, e.g. at least 4 times in bugs of the family Naucoridae, many more than 8 times in adult beetles (Hinton, 1969c), at least 15 times among insect pupae (Hinton, 1968), and undoubtedly hundreds of times among aquatic and terrestrial insect eggs. Plastrons evolve in a number of different ways. Among adult bugs and beetles it is probable that they usually evolve by increasing the density of hydrofuge hairs concerned in the retention of the compressible type of bubble until the bubble can be held against a pressure difference. An increase in density of the hairs is often accompanied by a change in size and shape, and possibly also contact angle, that further assist in the retention of the bubble or film against a pressure difference. In the account given in the previous section, the compressible gill was treated as if there was an exact equalisation of the pressure in the gas space and ambient water. In practice, however, the hydrofuge hairs or other structures that hold the bubble beneath the water deform its surface, that is, they hold it under a slight pressure difference that will slightly prolong the life of the compressible gill. However, there is rarely any question about being able to distinguish the compressible gill from the plastron: the sorts of insects using the two kinds of physical gill are rather different as are also their habitats. Swimming insects with compressible gills are not common in torrential
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streams--the characteristic habitat of non-swimming plastron-breathers--for the simple reason that their ground speeds are so much less than those of fast streams. The spiracles or openings of the breathing tubes (tracheae) of insects are hydrofuge and resist the entry of water under more or less considerable hydrostatic pressure. When insects are submerged in water, a water/air interface is established across the spiracles, which thus inevitably function as plastrons. Their total area in relation to the weight of the animal usually does not exceed about 1500 ~m2/mg, and both terrestrial and aquatic insects readily drown if kept submerged. The chrysalis of an Australian fly (Eutanyderus) has a plastron-bearing spiracular gill. In this fly the relation of the plastron water-air interface to weight is about 15000 ptmZ/mg, that is, an order of magnitude above that of insects with normally formed spiracles. Most terrestrial and aquatic insects drown fairly quickly if their spiracles are not exposed to air at relatively frequent intervals. In other words, oxygen uptake through normally formed spiracles adapted for atmospheric respiration is insignificant in relation to needs, and such insects cannot be called plastron-breathers. Thus there is, paradoxically, plastron respiration in insects without a plastron, that is without structures especially evolved for plastron respiration (Hinton, 1966). Apart from the chrysalis of the fly Eutanyderus nearly all plastron-breathers have a ratio of 105 to 2 × 106 ~mZ/mg. However, there can hardly be any doubt that plastron respiration becomes significant before even the ratio found in Eutanyderus is achieved. The stages in the evolution of the plastron of insect eggs are particularly easy to comprehend. The majority of terrestrial insect eggs have meshworks in the shell that hold a layer of air. A number of aquatic eggs have similar respiratory systems. The species that have air-containing meshworks, usually in the inner half of the shell, also have aeropyles or holes that extend to the surface of the shell and effect the continuity of the air in the shell and the ambient atmosphere. The aeropyles are hydrofuge, and it requires considerable excess pressure to drive water through them. Thus, when the egg is covered with water, a water-air interface is inevitably established across the opening of each aeropyle. When this happens there is some plastron respiration, but in most terrestrial eggs the water-air interface across all of the aeropyles is not sufficiently great for plastron respiration to satisfy a significant part of oxygen requirements. In very many terrestrial eggs the total area of the aeropyle openings per milligram of wet tissue is several orders of magnitude too little for there to be effective plastron breathing. However, no great change of structure has to occur for a significant part of the oxygen demands of the egg to be satisfied by plastron respiration when it is covered by water. It is only necessary to increase the number of aeropyles, or enlarge their openings, or both. In the egg shown in Plate I, Figure 1, assuming it to be spherical and to have a diameter of 1 mm, 0.25o/o of the surface would have to be a water-air interface for there to be 15,000 p.m2 per milligram, whereas in fact in this egg only about 0.15% of the surface is aeropyle openings. The mosquito egg shown in Plate I. Figure 2 is a prolate spheroid (b/a=0.2) 0.15 mm wide. Only 0.03% of its surthce has to be water-air interface to satisfy the ratio 15,000 ~m2/mg, whereas in fact more than 30% is water-air interface.
Respiratory eff~ieney of plastrons If a plastron is to be an efficient respiratory structure it must (a) resist wetting at the
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hydrostatic pressures to which it is normally subjected in nature; (b resist loss of waterproofing from surface-active materials; (c) have a satisfactory rati(~ between the rate of oxygen consumption and the area of the plastron;and (dl the drop in oxygen pressure along the plastron must be small enough so that the whole of tile plastron is effectively used. Using these criteria, the efficiency of the plastrons of many insects in the adult, pupal, and egg stages has been compared (Thorpe, 1950; Crisp, 1964: Hinton, 1968, 1969a). Here only some of the most interesting observations that have been made in relation to the resistance to wetting by excess pressures are discussed. Most terrestrial insect eggs occur in places where they can be struck bv drops of rain. Assuming a raindrop to be spherical, the pressure it exerts on striking a plastron is equivalent to a head of water about 1,000 times its diameter. A very large raindrop of a diameter of 4 mm will thus exert an excess pressure equivalent to about 31 cm Hg. But this pressure is exerted only for about a millisecond, and it has been shown that the least resistant plastrons will withstand such pressures for about 30 minutes (Hinton, 1960). The hydrostatic pressure required to force water through the plastron network and wet the plastron varies greatly according to the kind of insect, and in a few instances an excess pressure of as much as 3 atm may be required, e.g. aduhs of the bug Aphdocheirus. Broadly speaking, there is a relation between the environment of the insect and the excess pressure required to wet the plastron, but in all instances there appears to be a wide safety margin. Wetting by excess pressures always occurs before there is a mechanical breakdown of the plastron meshwork. The resistance of the plastron to excess pressures varies directly as the surface tension of the water. For instance, when the surface tension is lowered to about 25 dyne/cm, which corresponds to a reduction of the contact angle to 55-6ff', the plastrons of most aquatic insects are wetted at virtually no excess pressure. However, most plastronbreathers are only found in clean water where the oxygen pressure is consistently high. The surface tension of the streams in which they live is usually about 70 dyne/era or a little more, and in a few of the upland streams it may be as high as 73.5 dyne/cm. A few years ago it was found that the plastrons of many kinds of terrestrial eggs were more resistant to excess pressures in clean water (72.8 dyne/cm) than the plastrons of many aquatic insects (Hinton, 1960). This was puzzling because the plastrons of aquatic insects, even those living in shallow streams, are ordinarily exposed to much greater hydrostatic pressures than terrestrial eggs, particularly when the streams are flooded. Experiments with many kinds of eggs that are laid in decomposing organic materials eventually provided an explanation of the paradox. Eggs laid in decomposing organic matler are often exposed to concentrations of surface active substances that rarely if ever occur in streams. It was found that the surface tension of the temporary pools of rain water on cow pats is reduced to about 50 dyne/cm. Under comparable conditions the surface tension of water on the surface of decomposing flesh is reduced to about 40 dyne/cm (Hinton, 1960). The reason for the smaller reduction in surface tension of water standing on cow pats is probably that the dung consists largely of lignin and undigested cellulose, and these decompose very slowly. Thus the concentrations of organic acids and other surface-active substances in cow dung do not reach the levels to bc tbund in liquifying flesh or liquifying vegetable materials from which most of the fats and pro'ceins have not been removed.
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Changes in the geometry or the nature of the surface of the plagtron network that increase its resistance to wetting by surface-active substances, also increase its resistance to wetting in clean water by excess pressures because, as already noted, wetting always occurs before there is a mechanical breakdown of the plastron. It therefore necessarily follows that selection for greater resistance to wetting by surface-active substances inevitably results in an increase in the resistance to wetting by excess pressures. In many environments the very flooding necessary to produce high pressures will also dilute the surface-active substances and so increase surface tension. However, eggs of flies are often laid in decomposing materials in cracks and crevices. When such niches are flooded there may be no persistent turbulence, and the surface tension of the water immediately around the eggs will be lowered by surface-active substances diffusing out of the decomposing materials in which the eggs are laid. The molecules of most surface-active materials are large, and their rates of diffusion therefore small. Thus conditions exist for a thick diffusion boundary layer of surface-active substances to be formed around the eggs provided the water is not moving. Eggs of flies can be placed in two groups according to the resistance of their plastrons to excess pressures: (a) those normally found in dung, especially cow dung, and (b) those normally found in decaying vegetable and animal materials. In the first group the resistance of the plastron falls off rapidly on exposure to excess pressures of over 30 cm Hg for 30 minutes. The second group includes the eggs of such flies as Drosophila and the common blowflies (Calliphora, Lucilia), and their plastrons resist excess pressures of 60 to 100 cm Hg for 30 minutes or more. The eggs of the fly Sepsis (Sepsidae) were found in cow dung and so should have belonged to the first group. However, the resistance of their plastrons to excess pressures resembled that of the eggs of the second group. This difficulty was resolved when it was later found that the Sepsidae are unusual in that, although they commonly lay in cow dung, in nature they will also lay eggs on dead animals. CORNEAL NIPPLES In order to reduce reflection from the surface and thereby increase transmission, lenses are often coated with a thin layer of material of the appropriate refractive index. In industry this process is often spoken of as 'blooming' the lens. Reflection in the middle part of the spectrum is prevented, and there is therefore often an indistinct purplish sheen produced by a mixture of blue and red light. This blooming effect is precisely what has been evolved on the cornea of the compound eyes of the adults of many moths and butterflies as well as other insects such as some caddis flies, mosquitoes, and Neuroptera. On the eyes of insects the anti-reflection coating consists of conical tubercles about 200 nm high and about 200 nm from centre to centre. The significance of these tubercles was first noted by Bernhard and Miller (1962), who called them corneal nipples. Scanning electron micrographs of the corneal nipples of three common American butterflies are shown in Plate II, Figures 7-9. In figure 7 the surface of the cornea has been scratched with a pin in order to show more clearly the height of the nipples. The fact that the nipples do serve as an antireflection coating is easily demonstrated by comparing the reflectance of the eyes of insects with and without nipples, for instance, the reflectance from the cornea of a moth with that from the cornea of a beetle.
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For a full account of what is known of the functions of the corneal nipples, the reader is referred to the summaries of Bernhard et al. (1965) and Miller et al. (1968). These writers liken the anti-reflection coating of corneal nipples to an impedence transformer. The nipples make a gradual transition between the refractive indices of the air and the cornea. Besides reducing the amount of radiant energy reflected from the air-cornea interface, they suggest that the corneal nipples have the following functions : (1) In insects that are inactive and vulnerable during the day, the corneal nipples will serve to reduce specular reflection from the eyes that might make them conspicuous to predators. (2) It is possible that at night the corneal nipples might be useful in increasing sensitivity near the absolute threshold of vision. (3) In dark-adapted eyes with crystalline tracts there is a strong tapetal reflection. Reflection of part of the tapetal light from the front corneal surface back into the rhabdoms as the remainder passes out of the eye could create ghost images and interfere with perception. Suppression of this reflection would increase night vision. DIFFRACTION COLOURS Nearly all of the iridescent colours of insects and other animals are due to interference by multiple thin films separated by a material of a slightly different refractive index. Mason (1927) first showed that the iridescence of scarab beetles of the genus Serica was due entirely to diffraction gratings on their elytra or wing cases, and until this year it had always been supposed that these were the only insects in which iridescence was due entirely to diffraction. However, recent work has shown that diffraction colours are widely distributed among beetles and also occur in a few other insects (Hinton, 1969b,c,1970; Hinton and Gibbs, 1969a,b; Hinton, Gibbs and Silberglied, 1969). A summary of the work done by myself during the last two years and that done in collaboration with D. F. Gibbs of the Department of Physics is now in press (Hinton, 1969d), and the reader is referred to this paper for a detailed summary of diffraction in insects. It is now known that diffraction gratings have been independently evolved in no less than 14 different groups of beetles. They are particularly common among ground beetles (Carabidae), rove beetles (Staphylinidae), scarab beetles (Scarabaeidae), and flower beetles (Phalacridae). But the number of times such structures have been independently evolved is in fact much more than 14 because it can be shown that in several of these 14 groups gratings have been evolved on more than on occasion (Hinton, 1970). In addition, the stridulatory files of wasps of the family Mutillidae also function as diffraction gratings. In all of these beetles and wasps there is evidence to suggest that the diffraction colours are of some significance, that is, they have a selective advantage: the diffraction colours function as warning colours that confer some immunity against casual predators. In all but the wasps it would appear that the gratings have no other function but to diffract light. In the mutillid wasps, a structure that was almost certainly first evolved as part of a stridulatory file became modified in such a way that not only did it continue to be used to produce sound but also diffraction colours, which enhanced the warning colours caused by pigments on other parts of their bodies. Among insects there are a number of structures that have grooves or ridges of
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sufficient regularity and the d g h t periodicity to diffract light, but t h e colours they can produce are nevertheless 'accidental' in the sense that they have no significance in the life of the insects. For instance, the most common and widely distributed sound producing organ is the stridulatory organ. This usually consists of a 'file' of straight and parallel ridges, as shown in Plate IV, Figures 20 and 21, and a 'scraper' that may be one or more ridges, spines, or knobs on another part of the body. The scraper is rubbed across the file to produce sound. In the great majority of instances the file is concealed by the part of the body that bears the scraper. In a number of beetles the ridges of the file are sufficiently close together to produce reasonably bright spectra, at least at glancing incidence. However, as these files are concealed from view the fact that they can produce colour is of no biological significance. In most butterflies and moths the ribs of the scales diffract light, but because the individual scales are too irregular in profile, the spectra produced by different parts of the same scale, or by adjacent scales, overlap and summate to white light. I have previously (Hinton, 1969b) drawn attention to the fact that insects with a grating are not only provided with a warning colour but, as some of the light is specularly reflected, with a reflectance and colour that makes it more difficult for a predator to estimate distance and therefore the size of the insect. A diffraction grating thus produces an unusual combination: a warning colour that simultaneously repels and deceives about distance, together with a reflectance (zero order) which also deceives about distance. It becomes difficult to estimate the distance away of an object, and therefore its size, when the appearance of the object, i.e. its reflectance, or both its reflectance and colour, change rapidly with the angle at which it is viewed. This difficulty arises irrespective of whether the predator is estimating distance by stereoscopic vision or by evoking parallax. When an insect has a diffraction grating there will be a great change in reflectance and colour with the angle at which it is viewed, particularly when it is illuminated by a small distant source of light like the sun. It is for this reason that the more perfect the grating the more the predator is likely to be confused providing that the angular separation between diffraction maxima is fairly small compared with the range of angles over which the predator views the insect when estimating distance by evoking parallax. We might therefore have expected the diffraction gratings to be more perfect than they are. For instance, the diffraction gratings of rove beetles (Plate III, Figures 10-15) and scarab beetles (Plate IV, Figures 16,19) are all far from perfect. And we know that this imperfection is not due to the fact that insects cannot evolve perfectly parallel lines because they repeatedly produce lines that are almgst perfectly parallel for the stridulatory files of their sound producing organs (Figures 20-21). It has been suggested (Hinton, 1969b) that a possible reason for the consistent imperfection of the diffraction gratings of beetles is that a perfect grating might produce too sharp a peak reflectance when reinforcement occurs. A very bright reflectance could well act as a beacon and so outweigh the benefits of confusing distance and shape. It would, of course, only act as a beacon to the kind of casual predator that was not repelled by the colour. The lines that produce the spectra are always normal to the major axis of the insect except when such lines occur on the appendages. The grating spacing varied from 0.85 to 3.3~tm among different beetles. There was sometimes considerable variation
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on different parts of the body of the same individual, e.g. in one species of rove beet,e the spacing was 1.4p.m on the wing cases and as m u c h as 3.3p.m on parts of the legs. In most groups of beetles the gratings are simple grooves or ridges, but in one g r o u n d beetle (Iridagonum quadripunctum) and in all scarab beetles (e.~o. PlateI V, Figures 16-19) there is a row of short spines or knobs of one kind or a n o t h e r on each ridge. "File iridescent surfaces of these insects are perhaps more properly described as two-dimensional arrays of structures that scatter light rather than as a simple system of grooves or ridges. T h e small spines on one line are not well aligned with those of adjacent lines. T h e effect of this irregularity is to degrade the resolution of the corresponding spectra, and in all beetles seen the resolution of spectra in a direction across the major axis of the insect was negligible. T h e fine spines on the grating lines on the dorsal surtacc ot iridescent scarab beetles are always pointed backwards. T h e y are responsible for the fact that these beetles have a strong sheen when viewed from the front but not when viewed from the back. This difference m a y be likened to that seen when the r u b b e d pile of a carpet is viewed from a direction that traps the light as c o m p a r e d with a direction that reflects the light. I f there is a selective a d v a n t a g e in having a sheen when seen from the front we might expect a similar a d v a n t a g e in having a sheen when seen from the back. However, it is possible that it is better to have a sheen when viewed only from the fi'ont t h a n none at all. T h e fine spines m a y have something to do with water-repellancy, but we have not investigated this. Biologists often feel that they do not fully understand a structure or some other attribute of an organism until they have an idea of the selective pressures concerned in its evolution. In this they are quite right, but the questions have to be related to natural situations. For instance, it would seem to be a waste of time to try to find out if there was a selective advantage in crabs and lobsters turning red when they are boiled. STRIDULATORY ORGANS Insects produce sounds in m a n y different ways but they nevertheless have only a limited range o f organs especially evolved for sound production, and the only ones with what might be described as a wind instrument are the death's head h a w k m o t h s (Acherontia) and some other Sphingidae. T h e most widely distributed s o u n d - p r o d u c i n g organ is the stridulatory organ briefly described in the previous section. T h e r e are hardly two parts of the body that rub or can be m a d e to rub against each other that are not in some insect modified to form a stridulatory organ. T h e advent of the scanning electron microscope has m a d e it possible to examine the geometry of the stridulatory organs with a little more precision than betbre, although these organs are usually large enough to be seen clearly with the light microscope. T h e sounds p r o d u c e d serve a variety of functions in the life of insects. T h e y are often c o n c e r n e d with sexual or social behaviour. T h e suggestion that certain moths echo-locate (Hinton, 1955) has been confirmed by the experiments o f K a y (1969). In a very large n u m b e r , if not the majority, of instances it would a p p e a r that the sounds are defensive and serve to repel casual predators and parasites. This would certainly seem to be true for the sounds p r o d u c e d in the pupal stage. T h e pupal stage has in a large measure disrupted relations with the external physical and biological e n v i r o n m e n t : the variations in its structure and colour, or of that of its cocoon if it
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has one, can in nearly every instance be related to its means of defence, its respiratory requirements, its method of avoiding excessive loss of moisture, or the method of escape employed by the adult. It is thus taking no great risk to say that any sound-producing organs evolved in the pupal stage are concerned in defence even when there is no direct proof that they scare away some enemies. In a study of the pupae of more than 700 different kinds of moths and butterflies (Hinton, 1948), stridulatory organs were found in a large number. Among the more interesting of these were organs in which the file was made by the caterpillar and the scraper was on the pupa so that the two essential parts of the functional organ were contributed by two quite different stages of the insect. When the caterpillar of some moths finishes building its cocoon, it then builds a stridulatory file on the inner wall of one end of the cocoon. The file is composed of very hard silken ridges that are straight and parallel. They are in such a position in the cocoon that when the caterpillar has metamorphosed into a pupa, the latter can rub the scraper at the end of its abdomen across the file previously made by the caterpillar. After a cocoon is made, its position may be altered because of some natural event--the stem to which it is attached may be blown to the ground. Sometimes the caterpillar reverses its orientation in a cocoon after an accident of the kind described and then pupates in a direction opposite to the normal one. It was therefore very interesting to find that the caterpillars of some moths build two stridulatory files, one at each end of the cocoon. Thus the pupa can still make sounds if the caterpillar reverses its position immediately before it changes into a pupa, after which the position of the insect in the cocoon cannot be reversed. When a stridulatory organ is functioning, the normal situation is for the file to be stationary and the scraper to be moved across the file. This is so even when both scraper and file are similar in structure. However, in some moths and butterflies the stridulatory organs between the abdominal segments produce sound in a manner very different to that previously described. The existence of stridulatory organs on the abdomen of the pupae of many species of Troides (Plate IV Figure 20) was recorded long ago (Hinton, 1948), but I have only recently realised that the manner in which they produce sound is very different from that previously recorded from stridulatory organs. When living pupae of Troides are suitably stimulated, they produce a very distinct sound by moving the abdominal segments so that the opposed surfaces of the stridulatory organ, which at rest are interlocked, are separated. Here the two parts of the stridulatory organ consist of straight and parallel ridges. The ridges of one half of the organ fit in the grooves between the ridges of the other half. The sound is produced by collisions as the ridges are pulled out of the grooves. The process is analagous to unzipping a zip fastener. As expected, the sound produced in this way is very impure as compared with that of the usual kind of stridulatory organ.
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REFERENCES BERNHARD, C. G. and MmLER, W. H., 1962. A corneal nipple pattern in insect compound eyes. Acta physiol, scand., 56: 385-386. BERNHARD, C. G., MXLLER, W. H. and MOLLER, A. R., 1965. The insect corneal nipple array. A biological broad-band impedence transformer that acts as an antireflection coating. Acta physiol, scand., 63 (Suppl. 43) : 1-79. CRISP, D. J., 1964. Plastron respiration. Recent Prog. Surface Sci., 2: 377-425. EGE, R., 1915. On the respiratory function of the air stores carried by some aquatic insects. Z. allg. Physiol., 17: 81-124. HINTON, H. E., 1948. Sound production in lepidopterous pupae. Entomologist, 81 : 254-269. HINTON, H. E., 1955. Sound producing organs in the Lepidoptera. Proc. R. ent. Soc. Lond. (C), 20: 5-6. HINTON, H. E., 1959. Plastron respiration in the eggs of Drosophila and other flies. Nature, Lond., 184: 280-281. HINTON, H. E., 1960. The structure and function of the respiratory horns of the eggs of some flies. Phil. Trans. R. Soc., (B) 243: 45-73. HXNTON, H. E., 1966. Respiratory adaptations of the pupae of beetles of the family Psephenidae. Phil. Trans. R. Soc., (B) 251: 211-245. HINTON, H. E., 1968. Spiracular gills. Adv. Insect Physiol., 5: 65-162. HINTON, H. E., 1969a. Respiratory systems of insect egg shells. A. Rev. Ent., 14: 343-368. HINTON, H. E., 1969b. Diffraction gratings in burying beetles (Nicrophorus). Entomologist, 102: 185-189. HINTON, H. E., 1969c. Plastron respiration in adult beetles of the suborder Myxophaga. J. Zool., Lond., 159: 131-137. HINTON, H. E., 1970. Some little known surface structures. Symp. R. ent. Soc. Lond., 5. In press. HIrqWON, H. E. and GraBs, D. F., 1969a. Diffraction gratings in Phalacrid beetles. Nature, Lond., 221: 953-954. HINTON, H. E. and GraBS, D. E 1969b. An electron microscope study of the diffraction gratings of some Carabid beetles. J. Insect Physiol., 15: 959-962. HINTON, H. E., GraBs, D. F. and SILBERGHEr), R., 1969. Stridulatory files as diffraction gratings in mutillid wasps. J. Insect Physiol., 15: 959-962. KAY, R. E., 1969. Acoustic signalling and its possible relationship to assembling and navigation in the moth, Heliothis zea. J. Insect Physiol., 15: 989-1001. MASON, C. W., 1927. Structural colors in insects. III. J. phys. Chem., Ithaca., 31: 1856-1872. MmLER, W. H., BERNHAgD, C. G. and ALLErq,J. L., 1968. The optics of the insect compound eyes. Science, Wash., 162: 760-767. POPHAM, E.J., 1954. A new and simple method of demonstrating the physical gill of aquatic insects. Proc. R. ent. Soc. Lond., (A) 29: 51-54. RAHN, R. and PAOANELH, C. V., 1968. Gas exchange in gas gills of diving insects. Resp. Physiol., 5: 145-164. THORPE, W. H., 1950. Plastron respiration in aquatic insects. Biol. Rev., 25: 344-390.
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PLATE I
Figure 1. Anterior end of the egg of the geometrid moth, Semiothisa s~naria, showing the micropyle through which the sperm enters and a n u m b e r of aeropyles opening on the apices of round tubercles.
Figure 2. Plastron network of side of egg of the mosquito Culexfat~eans.
Micron, 1969, 1:84-108 with VI plates
INSECT STRUCTURES SEEN WITH SEM
HINTON
98 Plate I
99
HINTON
PLATE II
Figure 3. Egg of the fly Fannia coracina. Figure 4. Plastron network of inner base of the wing of the egg ofFannia atripes. Figure 5. Plastron meshwork of the dorsal edge of the wing of the egg of Fannia polychaeta. Figure 6. Network of inner surface of the eggshell ofFannia canicularls. Figure 7. Corneal nipples of the butterfly Caligo braziliensis. Figure 8. Corneal nipples of the butterfly Ageroniaferonia. F(gure 9. Corneal nipples of the butterfly Danaus plexippus.
Micron, 1969, 1:84-108 with VI plates
I N S E C T S T R U C T U R E S SEEN W I T H S E M
100 Plate II
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PLATE III D I F F R A C T I O N GRATINGS OF ROVE BEETLES, S T A P H Y L I N I D A E
Figure 10. Figure 11. Figure 12. Figure 13.
Lordithon lunulatus, middle femur. Femur of the same species.
Tachyporus hypnorum, fourth abdominal sternite. Second abdominal sternite of the same species. T h e grating spacing appears less than it is because of foreshortening.
Figure 14. Quedius cinctus, front femur. Figure 15. Euryporus picipes, fifth abdominal tengite.
Micron, 1969, ! :84-108 with VI plates
INSECT STRUCTURES SEEN WITH SEM
HINTON
102 Plate III
103
HINT()N
P L A T E IV D I F F R A C T I O N G R A T I N G OF T H E E L Y T R A OF T H E SCARAB B E E T L E
Phyllotocus ruficollis
Figure 18. Diffraction grating of the side of the pronotum of Phyllotocus moestus. Figure 19. Diffraction grating of the elytra of the scarab beetle Sericesthis ,~eminata. Figure 20. Stridulatory file ~f the fifth abdominal segment of the butterfly Troides acercus. The-sound producing organ is between the fourth and fifth abdominal segments.
Figure 21. Stridulatory file of the cerambycid beetle, Cerambyx cerdo.
Micron, 1969, 1:84-108 with VI plates
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104 Plate IV
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PLATE V
Figure 22. Segments of 'dust hairs' of the upperside of the forewing of the male hesperid butterfly, Hylephila phylaeus. On each forewing there is a patch of about 111,000 segmented setae that function in distributing the scent of the male.
Figure 23. Bases of two scales from the upperside of the forewing of the male cabbage butterfly, Pieris brassicae. Figure 24. Placoid sensillae or olfactory pores on the apical segment of the antenna of the worker honeybee.
Figure 25. A segment near the base of the antenna of the common silverfish (Lepisma) showing different kinds of sense organs.
Figure 26. Pollen-collecting hairs on the head of a worker honeybee. Figure 27. Attachment organs of the front tarsus of a male water beetle, Acilius sulcatus. With these modified setae the male holds the female when mating.
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107
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P L A T E VI CARPET
BEETLES OF THE FAMILY DERMESTIDAE
Figure 28. Anthrenusfuscus, scales of metasternal epimeron of adult. Figure 29. Spear-headed setae of the larvae of Anthrenus vorax. Figure 30. Spear-headed setae of the larvae of Anthrenus verbasci. Figure 31. Spear-headed setae of the larvae of Trogoderma glabrum. Figures 32 and 33. Spear-headed setae of the larvae of Anthrenocerus australia. The spear-headed setae of these larvae may have been evolved as a defence against spiders.
Micron, 1969, 1:84-108
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Plate VI
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84
with VI plates
Some structures of insects as seen with the scanning electron microscope H. E. H I N T O N , F.R.S. Department o1" Zoology, University of Bristol, U.K.
Manuscript received 24 September, 1969
With the advent of the scanning electron microscope it has becomepossible to examine the geometry of many of the structures of insects, particularly hard suface structures, at a resolution never achieved before. In this paper micrographs are presented of a number of relatively little known insect structures. They clearly illustrate the advantages of the scanning microscope over any other available instrument. The biological significance of some of the structures illustrated is discussed in detail, especially those to do with physical gills, the anti-reflection coating of the eyes of some insects, diffraction gratings in beetles, and stridulatory or sound producing organs of some moths and butterflies. Il est devenu possible d'examiner la gdometrie de beaucoup des structures d'insectes, particulikrement les structures de surface dure, a une rdsolution jamais atteinte avant, grace a l' avknement du microscope dlectronique it balayage. Dans cette revue on prdsente des micrographiques d'un certain nombre de structures relativement peu connues d'insectes. Ils montrent nettement les avantages du microscope it balayage sur tout autre instrument disponible. On discute en ddtail de l' importance biologique de certaines des structures exposdes, spdcialement de celles qui se rapportent aux bronchies physiques, au rev~tement anti-rdflectif sur les yeux de certains insectes, aux rdseaux des coldopt~res, et aux organes stridulatoires ou producteurs de sons de certains ldpidoptkres et de certains papillons. Es ist jetzt mOglich die Geometrie vieler Insekten Strukturen mit abtastender ~Jbermikroskopie zu untersuchen, insbesondere die harten Oberfliichen Strukturen, mit einer derartigen AuflOsung hie bevor erreichbar. In diesen Artikel werden Mikrographien einiger verhiiltnissm~sig wenig bekannten Insekten wiedergegeben. Diese zeigen ganz klar die Vorteile des abtastenden Mikroskops verglichen mit irgendeinen anderen Apparat. Die biologische Bedeutung einiger dieser Strukturen wird ausfuhrlich besprochen, besonders solche die die physischen Kiemen, die nichtreflektierenden Schicht der Augen einiger Insekten, die Diffraktionsgitter yon Kiifern und die Zirpoder Lauterzeugenden Organe einiger Motten und Schmetterlinge betreffen.
RESPIRATORY STRUCTURES In recent years, work with the scanning electron microscope has made it possible to study with much greater precision than before the geometry of certain respiratory structures in insects, particularly surface hairs and meshworks that hold the films or bubbles of air used for respiration by many aquatic insects. Among aquatic insects there are two quite different kinds of physical gills that are used to extract dissolved oxygen from the ambient water. One of these may be called a compressible or shrinking physical gill and the other an incompressible physical gill. The latter is now generally known as a plastron.
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Compressible or shrinking physical gill Many aquatic insects carry a bubble of air held between hydrofuge hairs on some part of their surface. A number of writers had long ago realised that insects with bubbles remained beneath the surface for periods longer than could be accounted for by the load of oxygen taken below. They therefore suggested thai the bubbles of air were in some way acting as gills that enabled the insects to extract additional supplies of oxygen from the ambient water. It was left to Ege (1915) to provide tile tirst satisfactory experimental and theoretical analysis of the way in which the bubbh" of air (or film) was being used as a gill. As the insect withdraws oxygen from the bubble, the partial pressure of ox}gen in the bubble falls and the partial pressure of nitrogen rises. The titll in the partial pressure of oxygen causes this gas to diffuse into the bubble and the rise in the partial pressure of nitrogen causes this gas to diffuse out of the bubble. However, because oxygen is much more soluble in water than nitrogen, the water-air interface of the bubble is more permeable for oxygen than for nitrogen. Because of this ditti-rence in the solubilities of the two gases there will be a greater tendency for equilibrium to be restored by oxygen diffusing into the bubble than by nitrogen diffusing out of it. However, some nitrogen is continually leaking out of the bubble so that the surface area ot't he bubble eventually becomes too small for the insect to satisfy its oxygen requirements by diffusion of oxygen into the bubble from the ambient water. The insect must then go to the surface and obtain another bubble. Ege has shown that under certain conditions the backswimmer (.\blonecta) extracts from the ambient water as much as 13 times the amount of oxygen that the bubble originally contained. Other writers have shown that at low temperatures when insects are not very active and not much oxygen is used, the bubbles of some may last for several days. But these bubbles shrink inexorably as the nitrogen leaks out and sooner or later the insect must surface to refill the gas space. In the compressible type of gill the most important gas is nitrogen. This has been very clearly demonstrated by Popham (1954), who kept water boatmcn (Corixidae) in well oxygenated water but with access only to pure nitrogen in the gas phase above the water. Under these conditions the bugs were able to respire normally even though the bubbles they obtained at the surface were not air but pure nitrogen. The duration of the compressible gill is determined by the rate at which nitrogen is lost. The rate of loss of any gas depends upon the product of solubility and diffusion coefficient (aD). The substitution of inert gases for nitrogen in the bubble will therefore shorten or lengthen the life of the bubble according to the ratio (~.I)) in,.,~ ~'a~/(~D)x:. In experiments in which a number of inert gases were substituted for nitrogen in both the bubbles and the ambien{ water, Rahn and Paganelli (1968) found that only neon rivals nitrogen. However, if nitrogen is replaced by the synthetic gas sulfur hexafluoride, the duration of the bubble is increased by a factor of 4 because of the very low solubility and high molecular weight of this gas. Some, if not much, of the carbon dioxide excreted by the insect will pass into the compressible gill through the spiracles or openings of the breathing tubes. Although carbon dioxide diffuses more slowly than nitrogen or oxygen, it is very much more soluble than either of these gases, and (eD)co 2 is about 20 times greater than (0cD)o2. Thus, the effect of carbon dioxide on the volume and pressure of the gas in the bubble
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is small and probably does not materially affect the lifetime of the bubble under water. The efficiency of the compressible gill is affected by diffusion boundary layers. The boundary layers will be thin when the insect is swimming and there is a rapid movement of water past the surface of the bubble. When the insect is stationary, the diffusion boundary layer will become thicker and the gill will not be as efficient. Many insects reduce the boundary layers by driving water past the bubble. For instance, the backswimmers use their paddle-like hind legs to do this, and it has been shown that the rate of oxygen uptake from the ambient water decreases when tethered bugs are prevented from using their paddles.
The plastron or incompressible physical gill A plastron is a gas film of constant volume and an extensive water-air interface. Such films are held in position by hydrofuge hairs or hydrofuge meshworks of various kinds (Plate I, Figure 2; Plate II, Figures 3-5), and they resist wetting under the hydrostatic pressures to which they are normally subjected in nature. In well-aerated water a plastron enables an animal to remain immersed indefinitely, when it obtains the oxygen it requires from the ambient water. In the plastron, unlike the compressible gill, nitrogen plays no essential role other than its contribution to the back pressure of the system. Removal of all nitrogen from the plastron space without substituting another gas would effectively increase the pressure on the water-air interface of the plastron by 0.79 atm, which would be enough to wet the plastrons of many kinds of insects. Nearly all aquatic insects with a plastron are found in waters in which the oxygen pressure is maintained at a fairly high level, such as fast streams, the littoral zones of large lakes, and intertidal areas. Their restriction to these kinds of habitats is no accident because, as has been noted before (Hinton, 1959), the plastron becomes an efficient means of extracting oxygen from the insect should the oxygen pressure of the environment fall below that of their tissues. A few adult beetles appear to be the only plastron-bearing insects that live in standing water, such as marshes and ponds, in which there may be from time to time a sudden and severe fall in oxygen pressure, especially at night. What the beetles do when this happens is not known, but they can always climb out of the water when the oxygen pressure falls enough to cause the plastron to work in reverse. In this connection it may be noted that most of the plastronbearing beetles in marshes and ponds are the only plastron-bearing beetles that are good swimmers: most beetles with a plastron cannot swim and are confined to fast streams and the littoral of large lakes. The well-oxygenated waters in which the plastron breathers are found are characterised by frequent and often large fluctuations in water level, e.g. intertidal areas and small mountain streams. These environments are thus alternately dry and flooded; and the great selective advantage of the plastron can only be fully understood in relation to this particular feature of the environment. In water, the plastron provides the insect with a relatively enormous area for the extraction of dissolved oxygen without necessarily involving any reduction in the impermeability of its cuticle. In air, the plastron provides a direct route for the entry of atmospheric oxygen that does not involve water-loss over a large area because the connection between the plastron and the internal tissues may be very restricted. Thus the capacity of the insect to avoid
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HINTON
an undue loss of water when its habitat is dry is not necessarily impaired bv the possession of a plastron. Once it becomes possible to distinguish the special t~atures of" aquatic cnvironments that confer a great selective advantage upon the plastron method of respiration, it also becomes possible to distinguish the same features in other environments that at first sight may appear to be very different indeed. We have seen that the essential features of the aquatic environments in which plastron respiration is so advantageous are: (1) that it is alternately dry and flooded; and (2) that when flooded the oxygen pressure in the water is maintained at a high level. Both of these essential features of aquatic environments in which plastrons are evolved are reproduced in the environment of" many terrestrial insects: the difference is merely that for the terrestrial insects the flooded periods are less frequent and do not last as long. Whenever it rains heavily, a very large number of terrestrial insects are submerged beneath a layer of water. The immobile stages of these insects--eggs and p u p a e - - a r e normally glued or otherwise fastened to the substrate and therefore necessarily remain submerged until it has stopped raining and the water has evaporated or flowed away. Thus in most climates many of the terrestrial insects are alternately dry and flooded. To be submerged in water for several hours or even days, a period that may even exceed the duration of the egg stage, is no rare and isolated event but is a normal hazard of their environment. It therefore seems likely that many terrestrial insects will be adapted to respiration under water in a manner no less complex than many aquatic insects. Among terrestrial insects, plastron respiration was first reported for the eggs of some flies (Hinton, 1959; 1960). It is now known that instances of plastron respiration among terrestrial insects are vastly more numerous than among aquatic ones. Reviews have been published of plastron respiration in adult insects (Thorpe, 1950; Crisp, 1964), pupal insects (Hinton, 1968), and insect eggs (Hinton, 1969a).
Evolution of the plastron Plastron respiration has been independently evolved a great many times, e.g. at least 4 times in bugs of the family Naucoridae, many more than 8 times in adult beetles (Hinton, 1969c), at least 15 times among insect pupae (Hinton, 1968), and undoubtedly hundreds of times among aquatic and terrestrial insect eggs. Plastrons evolve in a number of different ways. Among adult bugs and beetles it is probable that they usually evolve by increasing the density of hydrofuge hairs concerned in the retention of the compressible type of bubble until the bubble can be held against a pressure difference. An increase in density of the hairs is often accompanied by a change in size and shape, and possibly also contact angle, that further assist in the retention of the bubble or film against a pressure difference. In the account given in the previous section, the compressible gill was treated as if there was an exact equalisation of the pressure in the gas space and ambient water. In practice, however, the hydrofuge hairs or other structures that hold the bubble beneath the water deform its surface, that is, they hold it under a slight pressure difference that will slightly prolong the life of the compressible gill. However, there is rarely any question about being able to distinguish the compressible gill from the plastron: the sorts of insects using the two kinds of physical gill are rather different as are also their habitats. Swimming insects with compressible gills are not common in torrential
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streams--the characteristic habitat of non-swimming plastron-breathers--for the simple reason that their ground speeds are so much less than those of fast streams. The spiracles or openings of the breathing tubes (tracheae) of insects are hydrofuge and resist the entry of water under more or less considerable hydrostatic pressure. When insects are submerged in water, a water/air interface is established across the spiracles, which thus inevitably function as plastrons. Their total area in relation to the weight of the animal usually does not exceed about 1500 ~m2/mg, and both terrestrial and aquatic insects readily drown if kept submerged. The chrysalis of an Australian fly (Eutanyderus) has a plastron-bearing spiracular gill. In this fly the relation of the plastron water-air interface to weight is about 15000 ptmZ/mg, that is, an order of magnitude above that of insects with normally formed spiracles. Most terrestrial and aquatic insects drown fairly quickly if their spiracles are not exposed to air at relatively frequent intervals. In other words, oxygen uptake through normally formed spiracles adapted for atmospheric respiration is insignificant in relation to needs, and such insects cannot be called plastron-breathers. Thus there is, paradoxically, plastron respiration in insects without a plastron, that is without structures especially evolved for plastron respiration (Hinton, 1966). Apart from the chrysalis of the fly Eutanyderus nearly all plastron-breathers have a ratio of 105 to 2 × 106 ~mZ/mg. However, there can hardly be any doubt that plastron respiration becomes significant before even the ratio found in Eutanyderus is achieved. The stages in the evolution of the plastron of insect eggs are particularly easy to comprehend. The majority of terrestrial insect eggs have meshworks in the shell that hold a layer of air. A number of aquatic eggs have similar respiratory systems. The species that have air-containing meshworks, usually in the inner half of the shell, also have aeropyles or holes that extend to the surface of the shell and effect the continuity of the air in the shell and the ambient atmosphere. The aeropyles are hydrofuge, and it requires considerable excess pressure to drive water through them. Thus, when the egg is covered with water, a water-air interface is inevitably established across the opening of each aeropyle. When this happens there is some plastron respiration, but in most terrestrial eggs the water-air interface across all of the aeropyles is not sufficiently great for plastron respiration to satisfy a significant part of oxygen requirements. In very many terrestrial eggs the total area of the aeropyle openings per milligram of wet tissue is several orders of magnitude too little for there to be effective plastron breathing. However, no great change of structure has to occur for a significant part of the oxygen demands of the egg to be satisfied by plastron respiration when it is covered by water. It is only necessary to increase the number of aeropyles, or enlarge their openings, or both. In the egg shown in Plate I, Figure 1, assuming it to be spherical and to have a diameter of 1 mm, 0.25o/o of the surface would have to be a water-air interface for there to be 15,000 p.m2 per milligram, whereas in fact in this egg only about 0.15% of the surface is aeropyle openings. The mosquito egg shown in Plate I. Figure 2 is a prolate spheroid (b/a=0.2) 0.15 mm wide. Only 0.03% of its surthce has to be water-air interface to satisfy the ratio 15,000 ~m2/mg, whereas in fact more than 30% is water-air interface.
Respiratory eff~ieney of plastrons If a plastron is to be an efficient respiratory structure it must (a) resist wetting at the
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]IINT(~N
hydrostatic pressures to which it is normally subjected in nature; (b resist loss of waterproofing from surface-active materials; (c) have a satisfactory rati(~ between the rate of oxygen consumption and the area of the plastron;and (dl the drop in oxygen pressure along the plastron must be small enough so that the whole of tile plastron is effectively used. Using these criteria, the efficiency of the plastrons of many insects in the adult, pupal, and egg stages has been compared (Thorpe, 1950; Crisp, 1964: Hinton, 1968, 1969a). Here only some of the most interesting observations that have been made in relation to the resistance to wetting by excess pressures are discussed. Most terrestrial insect eggs occur in places where they can be struck bv drops of rain. Assuming a raindrop to be spherical, the pressure it exerts on striking a plastron is equivalent to a head of water about 1,000 times its diameter. A very large raindrop of a diameter of 4 mm will thus exert an excess pressure equivalent to about 31 cm Hg. But this pressure is exerted only for about a millisecond, and it has been shown that the least resistant plastrons will withstand such pressures for about 30 minutes (Hinton, 1960). The hydrostatic pressure required to force water through the plastron network and wet the plastron varies greatly according to the kind of insect, and in a few instances an excess pressure of as much as 3 atm may be required, e.g. aduhs of the bug Aphdocheirus. Broadly speaking, there is a relation between the environment of the insect and the excess pressure required to wet the plastron, but in all instances there appears to be a wide safety margin. Wetting by excess pressures always occurs before there is a mechanical breakdown of the plastron meshwork. The resistance of the plastron to excess pressures varies directly as the surface tension of the water. For instance, when the surface tension is lowered to about 25 dyne/cm, which corresponds to a reduction of the contact angle to 55-6ff', the plastrons of most aquatic insects are wetted at virtually no excess pressure. However, most plastronbreathers are only found in clean water where the oxygen pressure is consistently high. The surface tension of the streams in which they live is usually about 70 dyne/era or a little more, and in a few of the upland streams it may be as high as 73.5 dyne/cm. A few years ago it was found that the plastrons of many kinds of terrestrial eggs were more resistant to excess pressures in clean water (72.8 dyne/cm) than the plastrons of many aquatic insects (Hinton, 1960). This was puzzling because the plastrons of aquatic insects, even those living in shallow streams, are ordinarily exposed to much greater hydrostatic pressures than terrestrial eggs, particularly when the streams are flooded. Experiments with many kinds of eggs that are laid in decomposing organic materials eventually provided an explanation of the paradox. Eggs laid in decomposing organic matler are often exposed to concentrations of surface active substances that rarely if ever occur in streams. It was found that the surface tension of the temporary pools of rain water on cow pats is reduced to about 50 dyne/cm. Under comparable conditions the surface tension of water on the surface of decomposing flesh is reduced to about 40 dyne/cm (Hinton, 1960). The reason for the smaller reduction in surface tension of water standing on cow pats is probably that the dung consists largely of lignin and undigested cellulose, and these decompose very slowly. Thus the concentrations of organic acids and other surface-active substances in cow dung do not reach the levels to bc tbund in liquifying flesh or liquifying vegetable materials from which most of the fats and pro'ceins have not been removed.
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Changes in the geometry or the nature of the surface of the plagtron network that increase its resistance to wetting by surface-active substances, also increase its resistance to wetting in clean water by excess pressures because, as already noted, wetting always occurs before there is a mechanical breakdown of the plastron. It therefore necessarily follows that selection for greater resistance to wetting by surface-active substances inevitably results in an increase in the resistance to wetting by excess pressures. In many environments the very flooding necessary to produce high pressures will also dilute the surface-active substances and so increase surface tension. However, eggs of flies are often laid in decomposing materials in cracks and crevices. When such niches are flooded there may be no persistent turbulence, and the surface tension of the water immediately around the eggs will be lowered by surface-active substances diffusing out of the decomposing materials in which the eggs are laid. The molecules of most surface-active materials are large, and their rates of diffusion therefore small. Thus conditions exist for a thick diffusion boundary layer of surface-active substances to be formed around the eggs provided the water is not moving. Eggs of flies can be placed in two groups according to the resistance of their plastrons to excess pressures: (a) those normally found in dung, especially cow dung, and (b) those normally found in decaying vegetable and animal materials. In the first group the resistance of the plastron falls off rapidly on exposure to excess pressures of over 30 cm Hg for 30 minutes. The second group includes the eggs of such flies as Drosophila and the common blowflies (Calliphora, Lucilia), and their plastrons resist excess pressures of 60 to 100 cm Hg for 30 minutes or more. The eggs of the fly Sepsis (Sepsidae) were found in cow dung and so should have belonged to the first group. However, the resistance of their plastrons to excess pressures resembled that of the eggs of the second group. This difficulty was resolved when it was later found that the Sepsidae are unusual in that, although they commonly lay in cow dung, in nature they will also lay eggs on dead animals. CORNEAL NIPPLES In order to reduce reflection from the surface and thereby increase transmission, lenses are often coated with a thin layer of material of the appropriate refractive index. In industry this process is often spoken of as 'blooming' the lens. Reflection in the middle part of the spectrum is prevented, and there is therefore often an indistinct purplish sheen produced by a mixture of blue and red light. This blooming effect is precisely what has been evolved on the cornea of the compound eyes of the adults of many moths and butterflies as well as other insects such as some caddis flies, mosquitoes, and Neuroptera. On the eyes of insects the anti-reflection coating consists of conical tubercles about 200 nm high and about 200 nm from centre to centre. The significance of these tubercles was first noted by Bernhard and Miller (1962), who called them corneal nipples. Scanning electron micrographs of the corneal nipples of three common American butterflies are shown in Plate II, Figures 7-9. In figure 7 the surface of the cornea has been scratched with a pin in order to show more clearly the height of the nipples. The fact that the nipples do serve as an antireflection coating is easily demonstrated by comparing the reflectance of the eyes of insects with and without nipples, for instance, the reflectance from the cornea of a moth with that from the cornea of a beetle.
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For a full account of what is known of the functions of the corneal nipples, the reader is referred to the summaries of Bernhard et al. (1965) and Miller et al. (1968). These writers liken the anti-reflection coating of corneal nipples to an impedence transformer. The nipples make a gradual transition between the refractive indices of the air and the cornea. Besides reducing the amount of radiant energy reflected from the air-cornea interface, they suggest that the corneal nipples have the following functions : (1) In insects that are inactive and vulnerable during the day, the corneal nipples will serve to reduce specular reflection from the eyes that might make them conspicuous to predators. (2) It is possible that at night the corneal nipples might be useful in increasing sensitivity near the absolute threshold of vision. (3) In dark-adapted eyes with crystalline tracts there is a strong tapetal reflection. Reflection of part of the tapetal light from the front corneal surface back into the rhabdoms as the remainder passes out of the eye could create ghost images and interfere with perception. Suppression of this reflection would increase night vision. DIFFRACTION COLOURS Nearly all of the iridescent colours of insects and other animals are due to interference by multiple thin films separated by a material of a slightly different refractive index. Mason (1927) first showed that the iridescence of scarab beetles of the genus Serica was due entirely to diffraction gratings on their elytra or wing cases, and until this year it had always been supposed that these were the only insects in which iridescence was due entirely to diffraction. However, recent work has shown that diffraction colours are widely distributed among beetles and also occur in a few other insects (Hinton, 1969b,c,1970; Hinton and Gibbs, 1969a,b; Hinton, Gibbs and Silberglied, 1969). A summary of the work done by myself during the last two years and that done in collaboration with D. F. Gibbs of the Department of Physics is now in press (Hinton, 1969d), and the reader is referred to this paper for a detailed summary of diffraction in insects. It is now known that diffraction gratings have been independently evolved in no less than 14 different groups of beetles. They are particularly common among ground beetles (Carabidae), rove beetles (Staphylinidae), scarab beetles (Scarabaeidae), and flower beetles (Phalacridae). But the number of times such structures have been independently evolved is in fact much more than 14 because it can be shown that in several of these 14 groups gratings have been evolved on more than on occasion (Hinton, 1970). In addition, the stridulatory files of wasps of the family Mutillidae also function as diffraction gratings. In all of these beetles and wasps there is evidence to suggest that the diffraction colours are of some significance, that is, they have a selective advantage: the diffraction colours function as warning colours that confer some immunity against casual predators. In all but the wasps it would appear that the gratings have no other function but to diffract light. In the mutillid wasps, a structure that was almost certainly first evolved as part of a stridulatory file became modified in such a way that not only did it continue to be used to produce sound but also diffraction colours, which enhanced the warning colours caused by pigments on other parts of their bodies. Among insects there are a number of structures that have grooves or ridges of
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sufficient regularity and the d g h t periodicity to diffract light, but t h e colours they can produce are nevertheless 'accidental' in the sense that they have no significance in the life of the insects. For instance, the most common and widely distributed sound producing organ is the stridulatory organ. This usually consists of a 'file' of straight and parallel ridges, as shown in Plate IV, Figures 20 and 21, and a 'scraper' that may be one or more ridges, spines, or knobs on another part of the body. The scraper is rubbed across the file to produce sound. In the great majority of instances the file is concealed by the part of the body that bears the scraper. In a number of beetles the ridges of the file are sufficiently close together to produce reasonably bright spectra, at least at glancing incidence. However, as these files are concealed from view the fact that they can produce colour is of no biological significance. In most butterflies and moths the ribs of the scales diffract light, but because the individual scales are too irregular in profile, the spectra produced by different parts of the same scale, or by adjacent scales, overlap and summate to white light. I have previously (Hinton, 1969b) drawn attention to the fact that insects with a grating are not only provided with a warning colour but, as some of the light is specularly reflected, with a reflectance and colour that makes it more difficult for a predator to estimate distance and therefore the size of the insect. A diffraction grating thus produces an unusual combination: a warning colour that simultaneously repels and deceives about distance, together with a reflectance (zero order) which also deceives about distance. It becomes difficult to estimate the distance away of an object, and therefore its size, when the appearance of the object, i.e. its reflectance, or both its reflectance and colour, change rapidly with the angle at which it is viewed. This difficulty arises irrespective of whether the predator is estimating distance by stereoscopic vision or by evoking parallax. When an insect has a diffraction grating there will be a great change in reflectance and colour with the angle at which it is viewed, particularly when it is illuminated by a small distant source of light like the sun. It is for this reason that the more perfect the grating the more the predator is likely to be confused providing that the angular separation between diffraction maxima is fairly small compared with the range of angles over which the predator views the insect when estimating distance by evoking parallax. We might therefore have expected the diffraction gratings to be more perfect than they are. For instance, the diffraction gratings of rove beetles (Plate III, Figures 10-15) and scarab beetles (Plate IV, Figures 16,19) are all far from perfect. And we know that this imperfection is not due to the fact that insects cannot evolve perfectly parallel lines because they repeatedly produce lines that are almgst perfectly parallel for the stridulatory files of their sound producing organs (Figures 20-21). It has been suggested (Hinton, 1969b) that a possible reason for the consistent imperfection of the diffraction gratings of beetles is that a perfect grating might produce too sharp a peak reflectance when reinforcement occurs. A very bright reflectance could well act as a beacon and so outweigh the benefits of confusing distance and shape. It would, of course, only act as a beacon to the kind of casual predator that was not repelled by the colour. The lines that produce the spectra are always normal to the major axis of the insect except when such lines occur on the appendages. The grating spacing varied from 0.85 to 3.3~tm among different beetles. There was sometimes considerable variation
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on different parts of the body of the same individual, e.g. in one species of rove beet,e the spacing was 1.4p.m on the wing cases and as m u c h as 3.3p.m on parts of the legs. In most groups of beetles the gratings are simple grooves or ridges, but in one g r o u n d beetle (Iridagonum quadripunctum) and in all scarab beetles (e.~o. PlateI V, Figures 16-19) there is a row of short spines or knobs of one kind or a n o t h e r on each ridge. "File iridescent surfaces of these insects are perhaps more properly described as two-dimensional arrays of structures that scatter light rather than as a simple system of grooves or ridges. T h e small spines on one line are not well aligned with those of adjacent lines. T h e effect of this irregularity is to degrade the resolution of the corresponding spectra, and in all beetles seen the resolution of spectra in a direction across the major axis of the insect was negligible. T h e fine spines on the grating lines on the dorsal surtacc ot iridescent scarab beetles are always pointed backwards. T h e y are responsible for the fact that these beetles have a strong sheen when viewed from the front but not when viewed from the back. This difference m a y be likened to that seen when the r u b b e d pile of a carpet is viewed from a direction that traps the light as c o m p a r e d with a direction that reflects the light. I f there is a selective a d v a n t a g e in having a sheen when seen from the front we might expect a similar a d v a n t a g e in having a sheen when seen from the back. However, it is possible that it is better to have a sheen when viewed only from the fi'ont t h a n none at all. T h e fine spines m a y have something to do with water-repellancy, but we have not investigated this. Biologists often feel that they do not fully understand a structure or some other attribute of an organism until they have an idea of the selective pressures concerned in its evolution. In this they are quite right, but the questions have to be related to natural situations. For instance, it would seem to be a waste of time to try to find out if there was a selective advantage in crabs and lobsters turning red when they are boiled. STRIDULATORY ORGANS Insects produce sounds in m a n y different ways but they nevertheless have only a limited range o f organs especially evolved for sound production, and the only ones with what might be described as a wind instrument are the death's head h a w k m o t h s (Acherontia) and some other Sphingidae. T h e most widely distributed s o u n d - p r o d u c i n g organ is the stridulatory organ briefly described in the previous section. T h e r e are hardly two parts of the body that rub or can be m a d e to rub against each other that are not in some insect modified to form a stridulatory organ. T h e advent of the scanning electron microscope has m a d e it possible to examine the geometry of the stridulatory organs with a little more precision than betbre, although these organs are usually large enough to be seen clearly with the light microscope. T h e sounds p r o d u c e d serve a variety of functions in the life of insects. T h e y are often c o n c e r n e d with sexual or social behaviour. T h e suggestion that certain moths echo-locate (Hinton, 1955) has been confirmed by the experiments o f K a y (1969). In a very large n u m b e r , if not the majority, of instances it would a p p e a r that the sounds are defensive and serve to repel casual predators and parasites. This would certainly seem to be true for the sounds p r o d u c e d in the pupal stage. T h e pupal stage has in a large measure disrupted relations with the external physical and biological e n v i r o n m e n t : the variations in its structure and colour, or of that of its cocoon if it
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has one, can in nearly every instance be related to its means of defence, its respiratory requirements, its method of avoiding excessive loss of moisture, or the method of escape employed by the adult. It is thus taking no great risk to say that any sound-producing organs evolved in the pupal stage are concerned in defence even when there is no direct proof that they scare away some enemies. In a study of the pupae of more than 700 different kinds of moths and butterflies (Hinton, 1948), stridulatory organs were found in a large number. Among the more interesting of these were organs in which the file was made by the caterpillar and the scraper was on the pupa so that the two essential parts of the functional organ were contributed by two quite different stages of the insect. When the caterpillar of some moths finishes building its cocoon, it then builds a stridulatory file on the inner wall of one end of the cocoon. The file is composed of very hard silken ridges that are straight and parallel. They are in such a position in the cocoon that when the caterpillar has metamorphosed into a pupa, the latter can rub the scraper at the end of its abdomen across the file previously made by the caterpillar. After a cocoon is made, its position may be altered because of some natural event--the stem to which it is attached may be blown to the ground. Sometimes the caterpillar reverses its orientation in a cocoon after an accident of the kind described and then pupates in a direction opposite to the normal one. It was therefore very interesting to find that the caterpillars of some moths build two stridulatory files, one at each end of the cocoon. Thus the pupa can still make sounds if the caterpillar reverses its position immediately before it changes into a pupa, after which the position of the insect in the cocoon cannot be reversed. When a stridulatory organ is functioning, the normal situation is for the file to be stationary and the scraper to be moved across the file. This is so even when both scraper and file are similar in structure. However, in some moths and butterflies the stridulatory organs between the abdominal segments produce sound in a manner very different to that previously described. The existence of stridulatory organs on the abdomen of the pupae of many species of Troides (Plate IV Figure 20) was recorded long ago (Hinton, 1948), but I have only recently realised that the manner in which they produce sound is very different from that previously recorded from stridulatory organs. When living pupae of Troides are suitably stimulated, they produce a very distinct sound by moving the abdominal segments so that the opposed surfaces of the stridulatory organ, which at rest are interlocked, are separated. Here the two parts of the stridulatory organ consist of straight and parallel ridges. The ridges of one half of the organ fit in the grooves between the ridges of the other half. The sound is produced by collisions as the ridges are pulled out of the grooves. The process is analagous to unzipping a zip fastener. As expected, the sound produced in this way is very impure as compared with that of the usual kind of stridulatory organ.
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REFERENCES BERNHARD, C. G. and MmLER, W. H., 1962. A corneal nipple pattern in insect compound eyes. Acta physiol, scand., 56: 385-386. BERNHARD, C. G., MXLLER, W. H. and MOLLER, A. R., 1965. The insect corneal nipple array. A biological broad-band impedence transformer that acts as an antireflection coating. Acta physiol, scand., 63 (Suppl. 43) : 1-79. CRISP, D. J., 1964. Plastron respiration. Recent Prog. Surface Sci., 2: 377-425. EGE, R., 1915. On the respiratory function of the air stores carried by some aquatic insects. Z. allg. Physiol., 17: 81-124. HINTON, H. E., 1948. Sound production in lepidopterous pupae. Entomologist, 81 : 254-269. HINTON, H. E., 1955. Sound producing organs in the Lepidoptera. Proc. R. ent. Soc. Lond. (C), 20: 5-6. HINTON, H. E., 1959. Plastron respiration in the eggs of Drosophila and other flies. Nature, Lond., 184: 280-281. HINTON, H. E., 1960. The structure and function of the respiratory horns of the eggs of some flies. Phil. Trans. R. Soc., (B) 243: 45-73. HXNTON, H. E., 1966. Respiratory adaptations of the pupae of beetles of the family Psephenidae. Phil. Trans. R. Soc., (B) 251: 211-245. HINTON, H. E., 1968. Spiracular gills. Adv. Insect Physiol., 5: 65-162. HINTON, H. E., 1969a. Respiratory systems of insect egg shells. A. Rev. Ent., 14: 343-368. HINTON, H. E., 1969b. Diffraction gratings in burying beetles (Nicrophorus). Entomologist, 102: 185-189. HINTON, H. E., 1969c. Plastron respiration in adult beetles of the suborder Myxophaga. J. Zool., Lond., 159: 131-137. HINTON, H. E., 1970. Some little known surface structures. Symp. R. ent. Soc. Lond., 5. In press. HIrqWON, H. E. and GraBs, D. F., 1969a. Diffraction gratings in Phalacrid beetles. Nature, Lond., 221: 953-954. HINTON, H. E. and GraBS, D. E 1969b. An electron microscope study of the diffraction gratings of some Carabid beetles. J. Insect Physiol., 15: 959-962. HINTON, H. E., GraBs, D. F. and SILBERGHEr), R., 1969. Stridulatory files as diffraction gratings in mutillid wasps. J. Insect Physiol., 15: 959-962. KAY, R. E., 1969. Acoustic signalling and its possible relationship to assembling and navigation in the moth, Heliothis zea. J. Insect Physiol., 15: 989-1001. MASON, C. W., 1927. Structural colors in insects. III. J. phys. Chem., Ithaca., 31: 1856-1872. MmLER, W. H., BERNHAgD, C. G. and ALLErq,J. L., 1968. The optics of the insect compound eyes. Science, Wash., 162: 760-767. POPHAM, E.J., 1954. A new and simple method of demonstrating the physical gill of aquatic insects. Proc. R. ent. Soc. Lond., (A) 29: 51-54. RAHN, R. and PAOANELH, C. V., 1968. Gas exchange in gas gills of diving insects. Resp. Physiol., 5: 145-164. THORPE, W. H., 1950. Plastron respiration in aquatic insects. Biol. Rev., 25: 344-390.
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PLATE I
Figure 1. Anterior end of the egg of the geometrid moth, Semiothisa s~naria, showing the micropyle through which the sperm enters and a n u m b e r of aeropyles opening on the apices of round tubercles.
Figure 2. Plastron network of side of egg of the mosquito Culexfat~eans.
Micron, 1969, 1:84-108 with VI plates
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HINTON
98 Plate I
99
HINTON
PLATE II
Figure 3. Egg of the fly Fannia coracina. Figure 4. Plastron network of inner base of the wing of the egg ofFannia atripes. Figure 5. Plastron meshwork of the dorsal edge of the wing of the egg of Fannia polychaeta. Figure 6. Network of inner surface of the eggshell ofFannia canicularls. Figure 7. Corneal nipples of the butterfly Caligo braziliensis. Figure 8. Corneal nipples of the butterfly Ageroniaferonia. F(gure 9. Corneal nipples of the butterfly Danaus plexippus.
Micron, 1969, 1:84-108 with VI plates
I N S E C T S T R U C T U R E S SEEN W I T H S E M
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PLATE III D I F F R A C T I O N GRATINGS OF ROVE BEETLES, S T A P H Y L I N I D A E
Figure 10. Figure 11. Figure 12. Figure 13.
Lordithon lunulatus, middle femur. Femur of the same species.
Tachyporus hypnorum, fourth abdominal sternite. Second abdominal sternite of the same species. T h e grating spacing appears less than it is because of foreshortening.
Figure 14. Quedius cinctus, front femur. Figure 15. Euryporus picipes, fifth abdominal tengite.
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HINTON
102 Plate III
103
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P L A T E IV D I F F R A C T I O N G R A T I N G OF T H E E L Y T R A OF T H E SCARAB B E E T L E
Phyllotocus ruficollis
Figure 18. Diffraction grating of the side of the pronotum of Phyllotocus moestus. Figure 19. Diffraction grating of the elytra of the scarab beetle Sericesthis ,~eminata. Figure 20. Stridulatory file ~f the fifth abdominal segment of the butterfly Troides acercus. The-sound producing organ is between the fourth and fifth abdominal segments.
Figure 21. Stridulatory file of the cerambycid beetle, Cerambyx cerdo.
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104 Plate IV
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HINTON
PLATE V
Figure 22. Segments of 'dust hairs' of the upperside of the forewing of the male hesperid butterfly, Hylephila phylaeus. On each forewing there is a patch of about 111,000 segmented setae that function in distributing the scent of the male.
Figure 23. Bases of two scales from the upperside of the forewing of the male cabbage butterfly, Pieris brassicae. Figure 24. Placoid sensillae or olfactory pores on the apical segment of the antenna of the worker honeybee.
Figure 25. A segment near the base of the antenna of the common silverfish (Lepisma) showing different kinds of sense organs.
Figure 26. Pollen-collecting hairs on the head of a worker honeybee. Figure 27. Attachment organs of the front tarsus of a male water beetle, Acilius sulcatus. With these modified setae the male holds the female when mating.
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BEETLES OF THE FAMILY DERMESTIDAE
Figure 28. Anthrenusfuscus, scales of metasternal epimeron of adult. Figure 29. Spear-headed setae of the larvae of Anthrenus vorax. Figure 30. Spear-headed setae of the larvae of Anthrenus verbasci. Figure 31. Spear-headed setae of the larvae of Trogoderma glabrum. Figures 32 and 33. Spear-headed setae of the larvae of Anthrenocerus australia. The spear-headed setae of these larvae may have been evolved as a defence against spiders.
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Plate VI
HINTON