Mechanism for range fractionation in chordotonal organs of Locusta migratoria (L) and Valanga sp. (Orthoptera : Acrididae)

lnt, J. Insect Morphol. & Embryol., Vol. 20, No. 1/2, pp. 25-39, 1991 Printed in Great Britain

0020-7322/91 $3.00 + .00 ~) 1991 Pergamon Press plc

MECHANISM FOR RANGE FRACTIONATION IN CHORDOTONAL ORGANS OF LOCUSTA MIGRATORIA AND VALANGA SP. ( O R T H O P T E R A • ACRIDIDAE)

(L)

LAURENCE H . FIELD Department of Zoology, University of Canterbury, Christchurch 1, New Zealand

(Accepted 25 September 1990) Abstract--The femoral chordotonal organ (FCO) of Locusta migratoria and Valanga sp. (Orthoptera : Acrididae) is a leg stretch receptor containing scolopophorous sensory neurones embedded in a ligament, which emerges distally from the body of the organ and connects to a distal apodeme. The ligament is pulled when the tibia is flexed. Thus the FCO bridges the femur-tibia joint. The ligament is divided into separate strands, each of which is composed of several or more, long attachment cells. These cells link individual scolopidia to the apodeme. An additional unloading strand connects the body of the FCO to a static point on the femoral integument. Because the strands are inserted along the apodeme in a sequential array, and because the unloading strand holds all the tension on the FCO when the ligament is relaxed (tibia extended), a mechanism for gradual, sequential uptake of tension by the ligament strands exists when the ligament is pulled (tibia flexed). This leads to a range fractionation of stretch-sensitive neurone responses during tibial flexion, and of relaxation-sensitive neurone responses during tibial extension. Observations on the ultrastructural distribution of desmosomes suggest that groups of attachment cells may be functionally connected and may collectively transmit force to specific groups of neurones.

Index descriptors (in addition to those in title): Locust, femoral chordotonal organ, range fractionation, stretch receptor, scolopidium, attachment cell.

INTRODUCTION IN A NUMBERof vertebrate and invertebrate sensory receptors, the individual neurones respond to only a fraction of the full physiological stimulus range. Generally, the overall output at any one stimulus level is a composite of the differing responses of several neurones (Cohen, 1964). This "range fractionation principle" (Cohen, 1964) has been shown in sensory systems, such as temperature receptors in the tongue (Dodt and Zotterman, 1952), arthropod limb proprioceptors (Pringle, 1956; Cohen, 1963; Young, 1970; Zill, 1985), auditory receptors (Kalmring et al., 1978), mechanoreceptor sensilla (Shomozawa and Kanou, 1984), and spatial equilibrium receptors in vertebrates (Lowenstein and Roberts, 1949) and invertebrates (Cohen, 1960). Little is known about the mechanisms underlying range fractionation. Shimozawa and Kanou (1984) showed that for cercal filiform hair sensilla in the cockroach, the different sensory thresholds to air-current stimulation depend directly upon sensillum length. For multiple-neurone proprioceptors, particularly chordotonal organs (which bear ciliated scolopidia at the tips of the neurone dendrites), several attempts have been made to 25

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L.H. FIELD

relate ultrastructure and/or physiology to differential sensitivity of the afferent cells. For example, a tibial-tarsal chordotonal organ in the cockroach leg is stretched by a branched ligament. Young (1970) postulated that, for this organ, neurone specificity was due in part to different lengths and elasticity of the branches. The crista acoustica of insect tympanal (auditory) organs contains sensory neurons, which are arranged in a row, and ordered with respect to size. The individual neurones have narrow bandwidth sensitivities to sound, and are tonotopically arranged along the row (Oldfield, 1985; Oldfield, et al., 1986). Although such an elegantly arranged system would suggest a clear mechanical or histological basis for the range fractionation of the neurones, Oldfield et al. (1986) were unable to determine the underlying mechanism. They showed that neither the length of individual sensilla (neurone plus scolopidium) nor the frequency response of the tympanal m e m b r a n e were responsible for the tuning of the neurones. Moulins (1976) reviewed the structure of many arthropod chordotonal receptors, and concluded that differences in ultrastructural attachment of the ciliated scolopidia to cells of the ligamentous suspensory tissue are responsible for differences in extension- or relaxation-sensitivity of single neurones. Implicit in his conclusion is the possibility that gradation in such ultrastructural connections would lead to range fractionation. So far there has been no demonstration of the basis of range fractionation in a multiple-neurone proprioceptor. The femoral chordotonal organ of the locust is the largest receptor in the leg, and one of the most thoroughly studied insect proprioceptors (Usherwood et al., 1968; Burns, 1974; Field and Rind, 1977, 1980; Field and Pfliiger, 1989; Matheson, 1990; Matheson and Field, 1990). It is of the strand, or connective type (Moulins, 1976), in which the scolopidia are e m b e d d e d in an elastic strand that spans the joint. Physiological responses of the hind leg (metathoracic) chordotonal organ ( m t F C O ) are known to comprise range fractionated discharges of individual units (Burns, 1974; Zill, 1985), although a thorough study of this is needed in light of newly discovered neurones in the m t F C O (Matheson and Field, 1990). In addition, it is clear that the responses of this chordotonal organ very accurately and reliably code for joint position, since the locust can learn to repeatedly set its leg to new positions to within a few degrees (Zill and Forman, 1983; F o r m a n and Zill, 1984). This p a p e r gives new descriptions of the anatomy and ultrastructure of the m t F C O ligament. The ligament is mechanically specialized to provide differential activation of the sensory neurones, and could therefore serve as a basis for range fractionation in this organ.

MATERIALS AND METHODS Adult locusts (Locusta migratoria and Valanga sp.) of both sexes were obtained from laboratory cultures. Observations of normal ligament movement were made through a small window dissected on the anterior surface of the distal femur of locusts restrained dorsal side down on plasticine. The tibia was moved manually with a micromanipulator, while viewing through a binocular microscope. Joint angle was monitored against a small protractor next to the leg. For light microscopy, dissections (n = 18) were fixed in situ with 5% formaldehyde, dehydrated, dissected free of the leg, cleared and mounted on glass slides using Permount (Fisher Scientific Co., NJ). A Leitz Orthoplan microscope with Nomarsky optics was used for viewing and photography. Specimens for electron microscopicexamination were dissected out, and fixed, in 2.5% glutaraldehyde in 0.1 M cacodylate buffer (pH 7.4) for 8 h at 4°C, before washing overnight in buffer (4°C). Postfixation in 1% osmium tetroxide (buffered as above) for 2-4 h at 4°C was followed by another overnight wash in buffer (4°C). They were then dehydrated through an ethanol series. For TEM examination, the tissue was embedded in

Chordotonalorganrangefractionation

27

Spurr's low viscosityresin and sectionedon an LKB 8800 ultramicrotomeusing glass knives. Sectionswere stained in 3% uranylacetate followedby lead citrate, and viewedwith a Jeol 1200EXTEM at an accelerating voltage of 80 kV. For SEM examination,the tissue was critical-point dried, sputter-coated with gold, and viewed with a CambridgeStereoscan250 SEM.

RESULTS

Morphology The mtFCO is located in the distal femur and is innervated by nerve 5B1. It is anchored proximally by connective tissue attachments to the dorsal femoral cuticle, and distally by a narrow cuticular apodeme, originating at the base of the extensor tibiae muscle apodeme (Usherwood et al., 1968) (Fig. la). The main strand of the chordotonal organ is a ligament of elastic tissue joining the body of the organ to the apodeme (Fig. lb). Movement of the tibia displaces the apodeme and the ligament transmits the resultant force to approximately 92 sensory neurones (Matheson and Field, 1990) embedded in the main body of the mtFCO. Br/iunig (1985) showed that a single additional sensory neurone with a central cell body innervates, and is activated by extension of, the thin flexor strand (Field and Burrows, 1982), which connects the mtFCO to the flexor muscle apodeme (Fig. 1). The main ligament is composed of many fine strands that may be visualized by laterally displacing the apodeme (Fig. lc). A detailed examination of these strands using light microscopy showed that distally in the ligament they separate into 2 bundles (Fig. 2a). One (e, end) attaches to the end of the apodeme, while the other (s, side) becomes progressively subdivided and attaches to the side of the apodeme (Fig. 2a, c). The inset of Fig. 2a shows that the fine branches sometimes become further subdivided before merging onto the hypodermis surrounding the apodeme. An approximately linear sequence of attachment of fine strands occurs along the proximal 900 Ixm of the apodeme (left side, Fig. 2a). A terminal segment about 110 ixm long (arrow, Fig. 2b) is articulated onto the end of the apodeme. The "e" bundle becomes constricted before attaching to the articulated segment, and dark-field microscopy revealed an apparent extension (arrow in inset of Fig. 2b) of the terminal segment, which may lend additional stiffening to the bundle. Either or both of these structures may represent the tubular cone which occurs at the proximal tip of the apodeme in the bush cricket (Theophilidis, 1986). Figure 2c shows a sharper view of the more-or-less sequential branching of fine strands from the "s" bundle onto the apodeme. It also demonstrates that at a femur-tibia angle of about 100° some of the fine strands (distal) of the "s" bundle are taut, while others (proximal) are relaxed.

Fine structure and histology of the ligament The TEM study showed that the finest strands are single attachment cells, each of which connects one scolopidium, in the body of the FCO, to the apodeme. Proximally, each attachment cell surrounds the electron-dense cap of a scolopidium (Young, 1970), whose tip may be seen in cross section in the center of Fig. 3b. At this level, the attachment cells contain many microtubu|es in a concentric or whorled pattern (Fig. 3be), similar to that observed by Young (1970) in the cockroach tibia-tarsal chordotonal organ. Arrow 1 in Fig. 4a indicates the approximate level at which the sections were taken. In this region, the ligament is still a single thick structure. The attachment cells are

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L.H. FIELD

a

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flexor apodeme tibia

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Fro. 1. Location and morphology of metathoracic femoral chordotonal organ (mtFCO) in hind leg. Left side is proximal. (a) Chordotonal organ is anchored dorsally in distal femur (inset) and is attached to tibia by FCO apodeme and flexor strand. (b) Light micrograph of an mtFCO in vivo, showing chordotonal organ ligament (arrow). (c) Lateral displacement of apodeme (arrowhead) reveals branched nature of ligament. Scale = 0.4 ram.

Chordotonal organ range fractionation

Fl6.2. Details of ligament branches and apodeme structure. Right side is proximal in a and b. Left side is proximal in e. (a) "e" Bundle attaches to proximal tip of apodeme, while "s" bundle breaks up into many branched strands, which attach to side of apodeme. Inset shows fine strands merging with hypodermal layer covering apodeme. (b) Proximally, apodeme is articulated at tip. Terminal segment (arrowhead) appears to give off an extension into "e" bundle of strands, indicated by arrowhead in dark-field photomicrograph of inset. (c) A composite photomicrograph showing sequential fashion in which fine strands branch from "s" bundle and insert distally along apodeme. Scale bars: a and c, 250 ~.m; b (and inset of a, b), 100 ~m.

29

30

L.H. FIELD

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FIG. 3. Ligament ultrastructure. (a) Transverse section of ligament (taken at arrow 2, Fig. 4a) showing dense extracellular fibres (arrow) surrounding each attachment cell (ac), and lightly-staining outer perineurial sheath (pn). (b) Section through single scolopidial cap (c), which is surrounded by an attachment cell (ac) containing concentrically arranged microtubules (magnified in d). (c) Section through proximal end of ligament (arrow 1 in Fig. 4a) showing 8 attachment cells (ac) surrounded by

Chordotonal organ range fractionation

31

surrounded by cells (Fig. 3c) that have presumably secreted an extracellular bilayer of dark and light, electron dense, connective tissue that makes up the perineurium of the chordotonal organ. The ligament thus conforms to the "cellular strand" class of chordotonal organs set out by Moulins (1976). Distally (but before branching), the dark inner electron-dense perineurial layer breaks up into longitudinal fibers (still extracellular) that partially surround each attachment cell (Fig. 3a, f and arrow 2 in Fig. 4a), and only the light perineurial band surrounds the ligament (Fig. 3a) (this may be seen as an outer sheath around the ligament in Fig. 5a). The microtubules have now become very densely packed in the attachment cells and are no longer arranged in circular patterns (Fig. 3f). A wholemount of the mtFCO (Fig. 4a) shows that the fine strands (attachment cells) within the ligament generally run parallel to each other between the body of the organ and the apodeme. As the strands approach the organ from the apodeme, they spread out over the neurones without noticeable crossover (Fig. 4b, c). This has important implications in attempting to determine which neurones are activated by which strands. When prepared as in Fig. 4a, the ligament appears to be a flat structure, and it is possible to see that the fine strands of the "s" bundle appear to arise from the small distal neurones, and some more proximal large neurones on the ventral (lower in this view) side of the organ. Fine strands from the "e" bundle appear to arise from the larger neurones of the dorsal side in Figs 4a, b and d. A further aspect of organization apparent in these preparations is seen in the sequence of Figs 4b--d, as the plane of focus is moved from the top to the bottom of the organ. The strands spread over one side of the chordotonal organ body (Fig. 4b), while the other side is occupied primarily by the neurone cell bodies (Fig. 4d).

Mechanisms appropriate for range fractionation Careful dissection of the ligament of the mtFCO, and an associated SEM study, (T. Matheson, pers. commun.) revealed a fine suspensory strand, which connects a point on the ligament, just proximal to the branched region, to a more distal point on the cuticle of the femur (Fig. 5). The strand arises from the perineurium sheath of the FCO (Fig. 5a, and inset) and extends distally (for about half the length of the apodeme) to the cuticle, where it splays out and fuses with the hypodermis (Fig. 5a, b). By virtue of remaining taut when the apodeme moves proximally (as in Fig. 5b), this strand takes the strain off the chordotonal ligament strands and allows them to become fully relaxed at extreme leg extension. For this reason the strand has been termed the unloading strand. When the leg flexes (apodeme moves distally as in Fig. 5c), the strain is gradually transferred from the unloading strand back to the chordotonal ligament strands, until all the fine strands are stretched at full flexion. Range fractionation in this system occurs as the apodeme moves, for example distally, and progressively takes up strain on the fine strands in a sequential fashion, owing to

scolopidial cells (sc), which probably secrete outer bilayered perineurium (pn). (e) Whorled arrangement of microtubules from lower left attachment cell in c. (f) Attachment cell in distal portion of ligament, showing densely-packed microtubules (enlarged in inset) and surrounded by densely staining extracellular fibres and perineurium (pn). Scale bars: a, 4 ~.m; b, 1 jxm; c, 2 p.m; d, 50 nm; e and f, 200 nm; f inset, 50 nm.

32

L.H. FIELD

FIG. 4. Wholemount of mtFCO showing arrangement of strands in ligament using Nomasky illumination. Left side is proximal. (a) Strands of the "e" and "s" bundles lie parallel within ligament. "s" bundle appears to supply tiny distal neurones (ds) as well as ventral (lower, in Figs) proximal neurones (pr). (b-d) Successive photographs of ligament fibres as plane of focus is moved through mtFCO. Note how ligament strands fan out over one side of organ (h), while neurone somata are concentrated on other side (d). Arrows indicate levels of TEM sections for (1): Figs 3b-e, and (2): Figs 3a, f. Scale bars: a, 200 ixm (inset, 85 vLm); b--d, 120 i~m.

Chordotonal organ range fractionation

33

b

i:~:~.~:i:~.::~:.:/:!:!:.:i:.:i:.:!;i~:.~:.~:~.:.:!:.~:i:i:i::.:.:.::.:::i;i.;i:.:.:!~:i.i.i:~:i:!:i:i~ ~:i~!i~i:~:~:!:~;~;i:~:~i:~:~ ~ ~ :~:~:~:~:~:~:.......:~:~?~:::::::~:~:..~:`:~:'.~::.`::....::::::::::::~:~::::.~.::..~..:.`...~~`~

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45" FIG. 5. Morphology and operation of unloading strand. Proximal is on left side. (a) SEM showing proximal attachment of unloading strand (US) to ligament (I) and distal attachment to hypodermis. Proximal end is fused to ligament perineurium (pn) in inset. In this position, "e" bundle of ligament is buckled, while "s" bundle is still taut. (b) At a tibial setting of 130°, unloading strand takes up tension as ligament strands relax. (c) At 45°, apodeme moves distally (fight), pulling ligament strands and removing tension from unloading strand, which, however, is elastic and remains taut. Scale: a, 100 ~m; inset, 25 IJ,m.

34

L . H . FIELD

their sequential insection onto the apodeme (Figs lc, 2). Strand recruitment is shown in a series of photomicrographs from a live preparation, as the tibia was flexed from 120° to 70 ° (Fig. 6). At 120° (top), much of the "e" bundle is buckled, while the "s" bundle is taut. Progressive distal movement of the apodeme (towards the lower left in figure) recruits more and more of the buckled strands as force is transferred to them. Stretchsensitive neurones should be sequentially recruited as each strand takes up tension. This is confirmed by whole nerve recordings from the FCO during tibial flexion (Fig. 7). As the leg angle becomes more flexed, more and larger, neurones are recruited. In a similar fashion, relaxation-sensitive neurones should be sequentially activated as the apodeme moves proximally during tibial extension. The flexor stand forms an additional distal anchoring point of the FCO. However, instead of attaching to the tibia, it is attached to the apodeme of the flexor tibiae muscle (Fig. la). Extension of the tibia stretches this strand, which in turn induces discharge of some FCO neurones as the tibia moves towards angles greater than 80 ° (Field and Burrows, 1982; Zill, 1985). It was therefore of interest to examine the flexor strand to determine whether it also had specialized connections to the neurones. Careful inspection of wholemounts (e.g. Fig. 4) showed that this structure attaches to the outer neural lamella covering the body of FCO, rather than invading the FCO with attachment-cell-like strands connected to neurones. The material from the flexor strand spreads over the sides of the FCO, as well as forming several branched endings along the ventral margin of the main body. Although it may well distort the FCO when the tibia is extended, there does not appear to be any mechanism for mechanical range fractionation.

DISCUSSION The finding that the main ligament of the locust mtFCO has a well organized structural connection to the neurones of this sense organ has important implications for understanding how other strand mechanoreceptors may activate sensory neurones. Previously is has been assumed that strands or ligaments are band-like structures that transmit force simultaneously to all of the component neurones of a sense organ, and that any range fractionation in the system must be due to differences in firing thresholds of individual neurones. It is now clear that a detailed examination of the ligaments of other strand receptors, and particularly other chordotonal organs, could reveal a highly organised pattern of mechanical connections, which account for the graded activation of neurones within the stimulus range of each organ. The extent to which the individual attachment cells of other strand chordotonal organs are collectively ensheathed vs loosely grouped as free strands, as in the present study, may vary considerably. With the exception of the tibial-tarsal chordotonal organ (cockroach), most appear to be bundled together by the neural lamella. The tibial-tarsai organ represents an intermediate situation in which the ligament is subdivided into 2 additional side branches. Each of the 3 branches contains bundles of attachment cells (Young, 1970). If collectively ensheathed, tension on single strands may influence the tension on parallel neighbouring strands. In fact, nearly all published ultrastructural studies of insect strand chordotonal organs have described at least some desmosomes between adjacent attachment cells (e.g. Young, 1970; Fuller and Ernst, 1973; Slifer and Sekhon,

Chordotonal organ range fractionation

35

120 °

110"

90*

80*

70*

FIG. 6. Successive uptake of tension by ligament strands (mostly "e" bundle) as apodeme is moved distally (toward left) during tibial flexion between 120° and 70 °. Preparation is in vivo. e = "e" bundle; s = "s" bundle. Scale: 80 p.m.

36

L . H . FreED

90*

so*

FIG. 7. Recordings from m t F C O nerve showing firing of tonic neurones at tibial tarsal angles between 130 ° and 10°. Each new setting recruits more small units, as well as new larger units, resulting in range fractionation. Position-monitor trace (dark line) changes in each record. Scale: 0.4 s.

1975; Moulins, 1976; Matheson, pers. commun.; Field and Pfliiger, pers. observ.). The distribution and density of desmosomes within a chordotonal organ would determine the extent of lateral transfer of force among the attachment cells. Although desmosome distribution has not been addressed in any study, it is possible to glean some conclusions from available information. First, it is clear that there are many desmosomes in the proximal neurone somata regions of chordotonal organs. These connect together the scolopale cells as well as those attachment cells that penetrate to the more proximal neurones. Second, in the distal regions of the chordotonal organ ligaments, attachment cells are sometimes grouped in two's or three's by extensive desmosome connections. Third, at the point of anchorage of the ligament to the integument, attachment cells have many hemi-desmosomes (although this does not bear upon the present argument). These

Chordotonalorganrangefractionation

37

conclusions are drawn from transverse sections. Apparently no published information gives details of the longitudinal distribution of desmosomes along groups of attachment cells. These observations suggest that groups of attachment cells are functionally connected, and collectively transmit force to specific groups of neurones. It remains to be determined whether the strands that branch just before attaching to the apodeme of the mtFCO (Fig. 2a) comprise such groups. The role of the flexor strand (Fig. 1) in activating the FCO neurones in an organized way remains unclear. Zill (1985) suggested that the flexor strand activates tonic neurones sensitive to extension of the tibia between 80° and 160° (in his "ventral group" of receptor neurones), while the main ligament activates the "dorsal group" of tonic neurones, which respond to flexion of the tibia in the range 0°-80 °. In this view (Zill, 1985), the dorsal group would be stretch-sensitive (pulled by the main ligament) and the ventral group would be both stretch and bend sensitive (pulled and bent by the flexor strand). However, recent evidence indicates that the situation may be more complicated. In a comprehensive intracellular study of identified cells, Matheson (1990) showed that tibialextension-sensitive neurones are not grouped in a ventral cluster, but instead occur in medial or medio-dorsal positions, and that some respond maximally at angles <80 °, while others respond maximally at >80 ° . Further, he showed that the more ventral neurones include some sensitive to tibial flexion in addition to those sensitive to tibial extension. Another exception to Zill's scheme is the discovery of dorsal neurones that respond to tibial extension movements. Therefore it appears that the flexor strand does not have a simple functional attachment to ventral neurones. Several structural observations also suggest that the flexor strand has an indirect effect on the FCO neurones. For example, the flexor strand appears to attach only to the ventral surface of the body of the organ, rather than invading it internally to make direct connections to scolopidia or neurones. Further, all the neurones of the FCO appear to have attachment cells, and hence direct, connections to the main ligament; presumably all neurones are activated by this ligament. Finally, the site of attachment of the flexor strand to the FCO is ventral and proximal to most of the scolopidia in the organ, thus making it unlikely that the flexor strand exerts direct pull on the scolopidia. Therefore, the likely role of the flexor strand may only provide lateral tension on the body of the FCO during tibial extension, modifying any existing tension from the main ligament as it becomes unloaded. This must somehow lead to activation of units during tibial extension, since discharges from at least some neurones can be recorded at this time. However, the actual contribution of neurones activated by the flexor strand to resistance reflexes must be minimal, since Field and Burrows (1982) found that separate stretching of the flexor strand (while holding the main apodeme static) resulted in little or no excitatory synaptic input to flexor motor neurones. Implicit in the above discussion is the assumption that scolopidia are activated by stretch or longitudinal pull by the apodeme. Some confusion occurs over the possible existence and ultrastructural basis of relaxation-sensitivity in insect FCO scolopidia. Moulin's (1976) review of the evidence from insects and crustaceans concluded that relaxation-sensitivity is always associated with ultrastructural specializations. The scolopidia of relaxation-sensitive neurones in other chordotonal organs are inserted into an extracellular collagenous matrix of a connective tissue strand, which contains branched "strand cells". Ultrastructural studies of insect FCO's (e.g. Howse, 1968; Young, 1970; Fuller and Ernst, 1973; Slifer and Sekhon, 1975; Moran et al., 1977; pers.

38

L.H. FIELD

observ.) show that their ligament is not of the extracellular connective tissue type; rather it is of the type described in the present study: purely cellular tissue composed of attachment cells linking the scolopidia to the integument or an apodeme. In other chordotonal organs of both insects and crustacea, this cellular strand type of ultrastructure is associated with stretch-sensitive scolopidia, leading to the conclusion (on ultrastructural grounds) that insect FCO scolopidia should be stretch-sensitive only. A further refinement, based upon high-voltage T E M studies of Moran et al. (1977), is that a scolopidium can be activated by slight lateral displacement of the distal cap of the scolopidium, which would result if the scolopidium were inserted at a slight angle to the direction of pull of the ligament (as found for some scolopidia in the mesothoracic FCO and the mtFCO (Moran et al., 1977; Zill, 1985)). Physiological evidence, however, strongly implicates the presence of relaxation-sensitive scolopidia in insect FCO and the coxal CO (Hustert, 1982). Velocity/position sensitive units which respond to tibial extension have been shown for the stick insect (Hofmann et al., 1985) and locust chordotonal organs (Matheson, 1990). These are known to be activated by relaxation of the main ligament during tibia extension, because (a) in the stick insect that is the only ligament in the FCO, and (b) in the locust, Matheson (1990) immobilized the flexor strand (thus eliminating its effect) and stimulated only the main ligament. Therefore, it is not clear how relaxation-sensitivity is related to ultrastructure in the insect FCO. The presence of relaxation-sensitive units could complicate further the organization of range fractionation in the FCO. For example, are some strand groups connected only to relaxation units, while others are dedicated only to stretch units, or are the strands individually mixed among the 2 unit types? Preliminary physiological evidence suggests that the isolated bundles of strands (as seen in Fig. 2) attach to both types of units (pers. observ.). This study has dealt only with an organized structural mechanism for systematically distributing ligament forces to the scolopidia in the FCO. However, it should be realized that additional range fractionation could also be imposed by variation in electrical excitation threshold of the neurones. Such a system is thought to underlie range fractionation in the complex tibial organ (for sound and vibration detection) of insects (Kalmring et al., 1978) and the filiform sensory hairs of the cricket cercus (Shimozawa and Kanou, 1984). Acknowledgements--I thank Tom Matheson for permission to use his SEM photograph for Fig. 5a and for

preparation of FCO material for the TEM micrographs in Fig. 3. I am also grateful to him for helpful discussions about chordotonal organ function, and for kindly reading the manuscript. Thanks go to Jan McKenzie for assistance with the transmission electron microscopy.

REFERENCES BASSLER,U. 1983. Neural Basis of Elementary Behavior in Stick Insects. Studies of Brain Function. Vol. 10, Springer, Berlin. BR.~UNIG,P. 1985.Strand receptors associated with the femoral chordotonal organs of locust legs. J. Exp. Biol. 116: 331-41. BURNS,M. D. 1974. Structure and physiologyof the locust femoral chordotonal organ. J. Insect Physiol. 20: 1319-39. COHEN, M. J. 1960. The response patterns of single receptors in the crustacean statocyst. Proc. Roy. Soc. (London). B152: 30-49. COHEN,M. J. 1964. The peripheral organizationof sensory systems, pp. 273-292. In R. F. REISS,(ed.) Neural Theory and Modeling. Stanford University Press, Stanford.

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