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The function of the tergal glands in the Queensland fruit fly, Dacus tyroni
7. Insect Physiol.,
1967,Vol.
13, pp. 1875 to 1883. Pwgamon Press Ltd. Printed in Great Britain
THE FUNCTION OF THE TERGAL GLANDS IN THE QUEENSLAND FRUIT FLY, DACUS TYRONl JEREMY
J. T. EVANS*
and PETER
J. STANBURY
School of Biological Sciences, University of Sydney, Sydney, New South Wales, Australia (Received
22 May
1967)
Abstxaclt-The paired glands on the fifth abdominal tergite of adult Dams tryok produce both an aqueous and a waxy secretion. In cleaning itself, the fly periodically wipes these secretions over the surface of its body. The function of the glands appears to be to reduce water loss, especially during a short period immediately following emergence. Other experimental results indicate that the glands do not secrete a pheromone with any effect on the rates at which the females mature or mate. INTRODUCTION sexes o:f the Queensland fruit fly, Dacus tryok (Froggatt) (family: Trypedidae), and of many species related to it, have a pair of glandular areas on the fifth abdominal te.rgite (EVANS, 1967a, b). These tergal glands appear to differ fundamentally in structure from any other known insect gland. The tergal glands of D. tryoni produce both an aqeuous and a waxy secretion, and droplets of the former accumulate under a surface film of the latter. In the process of cleaning itself, the fly wipes these secretions over the surface of its body (EVANS,1967,a). The function of the tergal glands is quite unknown but most of the numerous possibilities can be placed under one of the following six headings:
BOTH
1. sensory The tergal glands are chiefly differentiated from the rest of the tergal integument in that most of their bristles are not innervated but have enlarged trichogen and tormogen cells that are clearly modified for secretion (EVANS, 1967a). Thus, the function of the tergal glands is probably not sensory but secretory. 2. The secret&z of a Petone
active in ~ep7~d~~~i~~
In sexually mixed populations of D. tryoni, the females mature most rapidly under crowded conditions (PRITCHARD,personal communication, 1966). In locusts, the mature male produces a pheromone that can accelerate the maturation of younger males or of females (NORRIS, 1954). One possible function of the tergal glands in I). ~fryon~might therefore be to produce a pheromone that can accelerate the maturatia~n of one or both sexes. * Present address: Department Victoria 30.52, Australia.
of Zoology, 1875
University
of Melbourne,
Parkville,
1876
JEREMY
J. T. EVANSand PETERJ. STANBIJRY
In the field, D. tryoni mates with the approach of dusk. At this time, BartonBrowne (ANDREWARTHA and BIRCH, 1954) has observed adults of the species congregating together in an orchard. When sexually aroused, the male of the species vibrates its wings back and forth over the large bristles of the sex comb on its abdomen; simultaneously, it moves its hind legs rapidly over the posterior end of its abdomen and, thus, over the tergal glands (PRITCHARD,personal communication, 1966). These actions generate a readily audible buzz and probably also hastens the evaporation of the aqueous component of the tergal gland secretion. It is possible that this buzz or a pheromone from the tergal glands, or both, is responsible for bringing the flies together prior to mating. 3. The secretion of a repellant According to BATEMAN(personal communication, 1966), recently emerged adults of D. tryoni are frequently attacked and killed by ants. In the silkworm, Bombyx mori, and the rice stem borer, Chile simplex, KOIDSUMI(1957) has reported that the cuticle contains lipids which behave as fungicides. It is therefore possible that the tergal glands secrete a substance that is capable of discouraging attack by ants, fungi, or some other natural enemy. 4. The excretion of waste substances 5. The excretion of water 6. The secretion of wax
The trichogen cells associated with most of the bristles on the abdomen of D. tryoni appear to be connected with canals that run through the cuticle; each canal ends in a pore-plate. Several observations suggest that the trichogen cells secrete wax through these canals (EVANS,1967b). The tergal glands differ from the rest of the tergal integument in that both their trichogen cells and pore-plates are exceptionally large. In these respects and several others, the tergal glands appear to be specialized for the secretion of wax. Spread over the surface of the body, this wax might reduce the loss of water through the cuticle. The experiments described below were carried out to establish which of these functions, if any, is performed by the tergal glands. MATERIALS AND METHODS The experiments described in this paper were carried out with stocks of D. tryoni that were collected and maintained as previously reported (EVANS, 1967a). Unless otherwise stated, all of the experiments and weighings were performed, not under the conditions of rearing (25”C, 75 per cent r.h.), but at 20°C + O.S”C and 65 + 5 per cent r.h. In several of the experiments, flies were kept without food or water; these individuals normally survived for at least 24 hr. To determine the rates at which the tergal glands accumulated secretion under different conditions, flies were anaesthetized lightly with ether and then secured to plasticine by their legs and wings. When restrained in this fashion, the flies could not disperse the secretion of the glands over the surfaces of their bodies.
FUNCTION
OF TEFlGAL
GLANDS
IN
DACUS
1877
Each set of conditions investigated was applied separately to 20 females and 20 males. Any secretion of the glands at the beginning of an experiment was removed with a slip of jilter paper. Periodically thereafter, the amount of secretion that had accumulated on each gland was rated on an arbitrary scale, running from zero (no secretion) to four (a bulging droplet of secretion covering the whole gland). For each experimental group of flies, the mean rating per gland could then be plotted against time. In other experiments, the glands were prevented from secreting by painting nail varnish over most of the fifth abdominal tergum. This treatment evidently failed to damage the glands since subsequent removal of the varnish was always followed by a resumption of secretion; the same was true even of recently emerged flies. As controls, other groups of flies were painted on an abdominal tergum other than the fifth. Some of the experiments also included groups of flies that were not painted at all. RESULTS
Factors affecting the accumulation
of secretion
The average amounts of secretion accumulated by the tergal glands of normal flies during th’e course of a typical experiment are shown in Fig. 1. The glands in males and females generally accumulated secretion at similar rates, which is not surprising in view of the fact that the glands in the two sexes are similar both in appearance and structure (EVANS, 1967a). In individual flies, the two glands frequently secreted at different rates. In the absence of dietary protein, flies might be expected to show a lower turnover of protein and, consequently, to excrete nitrogen in smaller amounts. If the function. of the tergal glands were to excrete nitrogenous wastes, the glands of any flies deprived of protein might well accumulate secretion at exceptionally slow rates. To test this possibility, flies were supplied with sugar and water but not with the us#ualyeast hydrolysate for the first 4 weeks after their emergence. The tergal glands of these experimental flies accumulated secretion at normal rates. We concluded that the glands are probably not responsible for the excretion of nitrogenous wastes. If the function of the tergal glands were to eliminate excess water, supplying flies with either sugar or water alone should cause dramatic alterations in the rates at which their glands accumulate secretion. Groups of flies, approximately 2 weeks old, were therefore attached to plasticine and supplied with either sugar or water. But in both cases, the glands again accumulated secretion at normal rates. Other flies of t.he same age were placed in a large desiccator over silica gel (0 per cent r.h.). The glands of these flies still accumulated secretion, including the aqueous component, although they did so more slowly than usual. Eflect of tergal glands on survival
The impofiance of the tergal glands to the survival of the fly was investigated by comparing 1:he death rates of groups of flies painted on their glands with those
1878
JEWSMYJ. T., EVANSand PETW J. STANBURY
3*=*
0
10
20 TIME
IN
30 HOURS
40
!
50
FIG. 1. The mean amount of secretion accumulated per tergal gland in a typical experiment on groups of20 male and 20 female flies, respectively. These flies were attached to plasticine within a day of their emergence and then placed at 20°C and 65 per cent r.h. The amount of secretion was rated on sn arbitrary scale (O-4).
of groups painted elsewhere on their abdominal terga. Each group contained 50 females and 50 males and was placed in a smali cage which was regularly supplied with fresh sugar, yeast hydrolysate, and water. Dead flies were periodic~ly removed from each cage and counted. With flies less than approximately 6 hr old, both the experimental and control treatments appeared to cause the immediate death of a small number of individuals (Fig. 2). Thereaft er, the flies painted on their tergal glands died at a markedly faster rate than did those painted elsewhere. With flies more than about a week old, painting over the glands no longer had any noticeable effect on the subsequent death rate (Fig. 3).
EIfects of tergal glands on rates of maturation and mating To explore whether or not the tergal glands secrete a pheromone capable of accelerating either maturation or copulation, we studied the effects of painting over the tergal glands on the rates at which female flies mature and become inseminated.
FUNCTION
OF TERGAL
GLANDS
IN
* GROUP A - GLANDS
PA~NTEO
o GROUP B - PAINTED
ELSEWHERE
5
IN
DAYS
FIG. 2. Curves describing the survival at 25°C in two initially comprised 50 males and 50 females. Within in group A were painted on the fifth abdomina1 terga, whiIe those in group B were painted on the
a z z
20
15
10 TIME
1879
DACUS
groups of flies, each of which 3 hr of emergence, the flies which bear the tergat glands, first or second.
I.8
3 ul
l
GROUP
A -
GLANDS
PAINTED
-
PAINTED
ELSEWHERE
I.7 !i t.6 0
10
20 TIME
30
40
50
60
IN DAYS
FIG. 3. Curves describing the surviva1 at 2OT in two groups of flies, each of which initially comprised SO males and SO females. On the tenth day after emergence the flies in group A were painted on the fifth abdominal terga, which bear the tergal glands,, while those in group B were painted on the first or second.
1880
JEREMY
J. T. EVANSand PETWJ. STANBURY
Five days after their emergence, definite numbers of males and females (usually 60 and 120, respectively) were placed together in each of three identical cages. The flies in one cage were painted with nail varnish on the fifth abdominal tergite while those in another were painted on an abdominal tergite other than the fifth; those in the third cage were not painted at all. The cages were then placed well apart from each other, in a clean, well-ventilated room, containing no other flies. The room was maintained at 25°C and ambient humidity. The cages were regularly supplied with fresh sugar, yeast hydrolysate, and water. At daily intervals, 10 females were taken at random from each cage together with enough males to maintain the original sex ratio. In some experiments, the two sexes were separated from each other soon after their emergence and kept apart until the start of the experiment. By dissecting the females in each sample, we determined the proportion of these containing one or more fully grown eggs and, by microscopical examination of their spermathecae, we determined the proportion of the same females that had been inseminated. In four different experiments, the three experimental treatments described made no appreciable difference to the rates of either egg maturation or insemination. Thus, in the laboratory at least, the tergal glands would appear to participate in neither of these processes. During the course of these experiments, a small proportion of the flies involved lost their spot of varnish. It is, of course, possible that these individuals supplied enough of some pheromone to invalidate our conclusion. E#ect of tergal glands on water balance Another series of experiments tested the possibility that the tergal glands influence the rates at which flies lose water. In these experiments, groups of variously painted flies were weighed over 20-hr periods. Groups of flies were placed in 2 x 1 in. glass tubes and each of these was then closed with a piece of nylon gauze and a rubber band. The initial weight of each group of flies was obtained by weighing the tube, etc., before and after adding the flies. The tubes of flies were subsequently reweighed after 6 hr and again after a further 14 hr. Any tubes that contained dead flies were disregarded. We assumed that the changes in weight exhibited by the flies during these experiments were due chiefly to exchange of water. Three separate experiments set up with flies less than 1 hr old gave remarkable results (Table 1). When unpainted, these flies at first either lost very little water or, on occasion, gained some. When painted on an abdominal tergite other than the fifth, they lost water to a slight extent, but when painted on the fifth abdominal tergite they lost water at an appreciable rate. Analysis of variance demonstrated that the flies painted on the fifth abdominal tergum lost water at significantly greater rates (PC O-01) than did any of the other experimental groups. Between the sixth and twentieth hour of each experiment, however, all three groups of flies lost water at appreciable rates that were not significantly different from each other.
FUNCTION
OF TWOAL
TAB~~-~P~~TAGE~IG~~OSS~
GLANDS
&~OFGROUPS*
O-6 6-Z!O
OFYOUNG~FLIES
painted
Areas
Time period (hrf
None
First abdominal tergite
Fifth abdominal tergite
0.03 0.32
0.01 0.22
0.17 0.24
* Each group comprised 5 males and 5 females; conditions 20°C and 65 pe:r cent r.h. t Less than 1 hr old at start of experiment.
In two experiments experimental appreciable
1881
IN DACUS
groups rates.
during the experiment:
set up with flies, 8 and 18 hr old, respectively, all of the lost water .during both experimental periods at similar,
In four experiments on g-day-old flies (Table Z), the members of all three experimental groups also lost appreciable amounts of water during both experimental periods, and analysis of variance showed that the experimental treatments made no significant difference to the rate of loss. The same analysis also demonstrated that preliminary etherization significantly increased the rate of water loss during the first experimental period (P < 0.025) but not during the second. The ether possibly damaged some component of the cuticle that normally retards the passage of water to the outside. In addition, the analysis demonstrated that the males lost water more rapidly than did the females (3’ < O-01) but since the males are smaller thsrn the females, this result is not surprising. TABLE ~-THE
Loss&hr
P~C~AGS~IG~
OFGROUPS*
OF %-DAY-OLD
FLIES
Areas painted Time period (hr1 No ether O-6 6-20 Ether? O-6 6-20
None
First abdominal tergite
Fifth abdominal tergite
Male
Female
Male
Female
Male
Female
040 0.58
040 0.35
050 051
0.33 0.32
0.50 0.43
053 0.42
0.67 0.52
050 0‘41
0*50 0.49
o-50 042
0.48 O-56
0.38 0.35
* Each group comprised 10 flies; conditions during the experiment: cent r.h. t Brief etherization to facilitate setting up the experiment.
20°C and 65 per
1882
JEREMY J. T. EVANS ANDPETER J, STANBURY DISCUSSION
The results of our experiments suggest that the tergal glands do not secrete a pheromone that is capable of influencing either the maturation or the mating of the female fly. It is of course still possible that they secrete a pheromone that has some other effect. The results of other experiments suggest that the glands do not excrete nitrogenous waste. Furthermore, several independent results indicate that the glands are not responsible for disposing of excess water. Thus, painting over the tergal glands sometimes increased, but never decreased the rates at which flies lost water. Moreover, when flies were secured to plasticine and supplied with either sugar or water alone, their glands continued to accumulate secretion at normal rates. Even at 0 per cent r.h., the aqueous secretion still accumulated on the glands to some extent; it is clear that this secretion does not readily evaporate. During approximately the first 6 hr after emergence, adults of D, tryoni lost little or no water, whereas older flies lost water at appreciable rates. But when the tergal glands of the recently emerged flies were painted over, these flies also lost water in considerable amounts. Thus, for a brief period following emergence, the tergal glands would appear to be retarding water loss. Presumably, the waxy component of the glandular secretion, spread over the surface of the body by the cleaning movements of the fly, is primarily responsible for this effect. Yet, certain observations made by BEAMENT(1964) suggest that the aqueous component of the secretion may assist in this function. When he placed droplets of water on the cuticle of the cockroach, ~er~~~~~et~,these became covered with a film of cuticular grease. In time, the droplets disappeared, leaving the surface of the cuticle in an exceptionally hydrophobic condition. To explain this, he suggested that the water was orienting the bipolar molecules of the cuticular grease and then depositing these undisturbed onto the surface of the cuticle. Beament also maintains that an oriented monolayer of grease or wax is particularly resistant to the passage of water. Thus, the waxy and aqueous components of the tergal gland secretion in D. tryoni may co-operate to waterproof the cuticle. The trichogen cells in the tergal glands are considerably enlarged and are clearly modified to produce the gland’s waxy secretion (EVANS,1967a). But several observations suggest that the smaller trichogen cells under the rest of the abdominal cuticle also secrete wax onto the cuticular surface (EVANS, 1967b). In the hours immediately after emergence, therefore, the trichogen cells of the tergal glands may be alone in secreting wax and thus waterproofing the animal; later on, the trichogen cells over the abdomen as a whole may develop the same ability. This sequence of events could explain why painting over the tergal glands increased the loss of water only in recently emerged flies. It might also explain why painting over the glands tended to increase the death rate of recently emerged flies but had no effect on older ones. A~~owle~~~~s-We are grateful to Professor L. C. BIRCHand Dr. M. for giving us access to the facilities of their laboratory and for their many heipful
A. BATEMAN suggestions.
FUNCTIONOF TERGALGLANDSIN DACUS
1883
In addition, we profited especially from the comments of Dr. J. GEE, Dr. DARCY GILMOLIR, Dr. VALERIEB. MORRIS, and Dr. G. PRITCHARD.For proficient technical assistance, we thank Mrs. ANlrl~BARNACOAT, Mrs. VIVIAN HUNTER,and Miss LYN GRANT. REFERENCES ANDREWASTHA II. G. and BIRCH L. C. (1954) The Distribution and Abundance of Animals. Chicago University Press, Chicago. BEAMENTJ. W. L. (1964) The active and passive transport of water in insects. A&. Insect Physiol. 2, 6’7-129. EVANSJ. J. T. (1.967a) The integument of the Queensland fruit fly, Dacus tryoni (Frogg.)-I. The tergal glands. 2. Zellforsch. microsk. Anat. 81, 18-33. EVANSJ. J. T. (196713) The integument of the Queensland fruit fly, Dacus tryoni (Frogg.)-II. Development and ultrastructure of the integument and bristles. Z. Zellforsch. microsk. Anat. 81, 34-48. KOIDSUMIK. (1957) Antifungal action of cuticular lipids in insects r. Insect. Physiol. 1, 40-51. NORRISN. J. (1054) Sexual maturation in the desert locust. Antilocust BulZ. No. 18.
1967,Vol.
13, pp. 1875 to 1883. Pwgamon Press Ltd. Printed in Great Britain
THE FUNCTION OF THE TERGAL GLANDS IN THE QUEENSLAND FRUIT FLY, DACUS TYRONl JEREMY
J. T. EVANS*
and PETER
J. STANBURY
School of Biological Sciences, University of Sydney, Sydney, New South Wales, Australia (Received
22 May
1967)
Abstxaclt-The paired glands on the fifth abdominal tergite of adult Dams tryok produce both an aqueous and a waxy secretion. In cleaning itself, the fly periodically wipes these secretions over the surface of its body. The function of the glands appears to be to reduce water loss, especially during a short period immediately following emergence. Other experimental results indicate that the glands do not secrete a pheromone with any effect on the rates at which the females mature or mate. INTRODUCTION sexes o:f the Queensland fruit fly, Dacus tryok (Froggatt) (family: Trypedidae), and of many species related to it, have a pair of glandular areas on the fifth abdominal te.rgite (EVANS, 1967a, b). These tergal glands appear to differ fundamentally in structure from any other known insect gland. The tergal glands of D. tryoni produce both an aqeuous and a waxy secretion, and droplets of the former accumulate under a surface film of the latter. In the process of cleaning itself, the fly wipes these secretions over the surface of its body (EVANS,1967,a). The function of the tergal glands is quite unknown but most of the numerous possibilities can be placed under one of the following six headings:
BOTH
1. sensory The tergal glands are chiefly differentiated from the rest of the tergal integument in that most of their bristles are not innervated but have enlarged trichogen and tormogen cells that are clearly modified for secretion (EVANS, 1967a). Thus, the function of the tergal glands is probably not sensory but secretory. 2. The secret&z of a Petone
active in ~ep7~d~~~i~~
In sexually mixed populations of D. tryoni, the females mature most rapidly under crowded conditions (PRITCHARD,personal communication, 1966). In locusts, the mature male produces a pheromone that can accelerate the maturation of younger males or of females (NORRIS, 1954). One possible function of the tergal glands in I). ~fryon~might therefore be to produce a pheromone that can accelerate the maturatia~n of one or both sexes. * Present address: Department Victoria 30.52, Australia.
of Zoology, 1875
University
of Melbourne,
Parkville,
1876
JEREMY
J. T. EVANSand PETERJ. STANBIJRY
In the field, D. tryoni mates with the approach of dusk. At this time, BartonBrowne (ANDREWARTHA and BIRCH, 1954) has observed adults of the species congregating together in an orchard. When sexually aroused, the male of the species vibrates its wings back and forth over the large bristles of the sex comb on its abdomen; simultaneously, it moves its hind legs rapidly over the posterior end of its abdomen and, thus, over the tergal glands (PRITCHARD,personal communication, 1966). These actions generate a readily audible buzz and probably also hastens the evaporation of the aqueous component of the tergal gland secretion. It is possible that this buzz or a pheromone from the tergal glands, or both, is responsible for bringing the flies together prior to mating. 3. The secretion of a repellant According to BATEMAN(personal communication, 1966), recently emerged adults of D. tryoni are frequently attacked and killed by ants. In the silkworm, Bombyx mori, and the rice stem borer, Chile simplex, KOIDSUMI(1957) has reported that the cuticle contains lipids which behave as fungicides. It is therefore possible that the tergal glands secrete a substance that is capable of discouraging attack by ants, fungi, or some other natural enemy. 4. The excretion of waste substances 5. The excretion of water 6. The secretion of wax
The trichogen cells associated with most of the bristles on the abdomen of D. tryoni appear to be connected with canals that run through the cuticle; each canal ends in a pore-plate. Several observations suggest that the trichogen cells secrete wax through these canals (EVANS,1967b). The tergal glands differ from the rest of the tergal integument in that both their trichogen cells and pore-plates are exceptionally large. In these respects and several others, the tergal glands appear to be specialized for the secretion of wax. Spread over the surface of the body, this wax might reduce the loss of water through the cuticle. The experiments described below were carried out to establish which of these functions, if any, is performed by the tergal glands. MATERIALS AND METHODS The experiments described in this paper were carried out with stocks of D. tryoni that were collected and maintained as previously reported (EVANS, 1967a). Unless otherwise stated, all of the experiments and weighings were performed, not under the conditions of rearing (25”C, 75 per cent r.h.), but at 20°C + O.S”C and 65 + 5 per cent r.h. In several of the experiments, flies were kept without food or water; these individuals normally survived for at least 24 hr. To determine the rates at which the tergal glands accumulated secretion under different conditions, flies were anaesthetized lightly with ether and then secured to plasticine by their legs and wings. When restrained in this fashion, the flies could not disperse the secretion of the glands over the surfaces of their bodies.
FUNCTION
OF TEFlGAL
GLANDS
IN
DACUS
1877
Each set of conditions investigated was applied separately to 20 females and 20 males. Any secretion of the glands at the beginning of an experiment was removed with a slip of jilter paper. Periodically thereafter, the amount of secretion that had accumulated on each gland was rated on an arbitrary scale, running from zero (no secretion) to four (a bulging droplet of secretion covering the whole gland). For each experimental group of flies, the mean rating per gland could then be plotted against time. In other experiments, the glands were prevented from secreting by painting nail varnish over most of the fifth abdominal tergum. This treatment evidently failed to damage the glands since subsequent removal of the varnish was always followed by a resumption of secretion; the same was true even of recently emerged flies. As controls, other groups of flies were painted on an abdominal tergum other than the fifth. Some of the experiments also included groups of flies that were not painted at all. RESULTS
Factors affecting the accumulation
of secretion
The average amounts of secretion accumulated by the tergal glands of normal flies during th’e course of a typical experiment are shown in Fig. 1. The glands in males and females generally accumulated secretion at similar rates, which is not surprising in view of the fact that the glands in the two sexes are similar both in appearance and structure (EVANS, 1967a). In individual flies, the two glands frequently secreted at different rates. In the absence of dietary protein, flies might be expected to show a lower turnover of protein and, consequently, to excrete nitrogen in smaller amounts. If the function. of the tergal glands were to excrete nitrogenous wastes, the glands of any flies deprived of protein might well accumulate secretion at exceptionally slow rates. To test this possibility, flies were supplied with sugar and water but not with the us#ualyeast hydrolysate for the first 4 weeks after their emergence. The tergal glands of these experimental flies accumulated secretion at normal rates. We concluded that the glands are probably not responsible for the excretion of nitrogenous wastes. If the function of the tergal glands were to eliminate excess water, supplying flies with either sugar or water alone should cause dramatic alterations in the rates at which their glands accumulate secretion. Groups of flies, approximately 2 weeks old, were therefore attached to plasticine and supplied with either sugar or water. But in both cases, the glands again accumulated secretion at normal rates. Other flies of t.he same age were placed in a large desiccator over silica gel (0 per cent r.h.). The glands of these flies still accumulated secretion, including the aqueous component, although they did so more slowly than usual. Eflect of tergal glands on survival
The impofiance of the tergal glands to the survival of the fly was investigated by comparing 1:he death rates of groups of flies painted on their glands with those
1878
JEWSMYJ. T., EVANSand PETW J. STANBURY
3*=*
0
10
20 TIME
IN
30 HOURS
40
!
50
FIG. 1. The mean amount of secretion accumulated per tergal gland in a typical experiment on groups of20 male and 20 female flies, respectively. These flies were attached to plasticine within a day of their emergence and then placed at 20°C and 65 per cent r.h. The amount of secretion was rated on sn arbitrary scale (O-4).
of groups painted elsewhere on their abdominal terga. Each group contained 50 females and 50 males and was placed in a smali cage which was regularly supplied with fresh sugar, yeast hydrolysate, and water. Dead flies were periodic~ly removed from each cage and counted. With flies less than approximately 6 hr old, both the experimental and control treatments appeared to cause the immediate death of a small number of individuals (Fig. 2). Thereaft er, the flies painted on their tergal glands died at a markedly faster rate than did those painted elsewhere. With flies more than about a week old, painting over the glands no longer had any noticeable effect on the subsequent death rate (Fig. 3).
EIfects of tergal glands on rates of maturation and mating To explore whether or not the tergal glands secrete a pheromone capable of accelerating either maturation or copulation, we studied the effects of painting over the tergal glands on the rates at which female flies mature and become inseminated.
FUNCTION
OF TERGAL
GLANDS
IN
* GROUP A - GLANDS
PA~NTEO
o GROUP B - PAINTED
ELSEWHERE
5
IN
DAYS
FIG. 2. Curves describing the survival at 25°C in two initially comprised 50 males and 50 females. Within in group A were painted on the fifth abdomina1 terga, whiIe those in group B were painted on the
a z z
20
15
10 TIME
1879
DACUS
groups of flies, each of which 3 hr of emergence, the flies which bear the tergat glands, first or second.
I.8
3 ul
l
GROUP
A -
GLANDS
PAINTED
-
PAINTED
ELSEWHERE
I.7 !i t.6 0
10
20 TIME
30
40
50
60
IN DAYS
FIG. 3. Curves describing the surviva1 at 2OT in two groups of flies, each of which initially comprised SO males and SO females. On the tenth day after emergence the flies in group A were painted on the fifth abdominal terga, which bear the tergal glands,, while those in group B were painted on the first or second.
1880
JEREMY
J. T. EVANSand PETWJ. STANBURY
Five days after their emergence, definite numbers of males and females (usually 60 and 120, respectively) were placed together in each of three identical cages. The flies in one cage were painted with nail varnish on the fifth abdominal tergite while those in another were painted on an abdominal tergite other than the fifth; those in the third cage were not painted at all. The cages were then placed well apart from each other, in a clean, well-ventilated room, containing no other flies. The room was maintained at 25°C and ambient humidity. The cages were regularly supplied with fresh sugar, yeast hydrolysate, and water. At daily intervals, 10 females were taken at random from each cage together with enough males to maintain the original sex ratio. In some experiments, the two sexes were separated from each other soon after their emergence and kept apart until the start of the experiment. By dissecting the females in each sample, we determined the proportion of these containing one or more fully grown eggs and, by microscopical examination of their spermathecae, we determined the proportion of the same females that had been inseminated. In four different experiments, the three experimental treatments described made no appreciable difference to the rates of either egg maturation or insemination. Thus, in the laboratory at least, the tergal glands would appear to participate in neither of these processes. During the course of these experiments, a small proportion of the flies involved lost their spot of varnish. It is, of course, possible that these individuals supplied enough of some pheromone to invalidate our conclusion. E#ect of tergal glands on water balance Another series of experiments tested the possibility that the tergal glands influence the rates at which flies lose water. In these experiments, groups of variously painted flies were weighed over 20-hr periods. Groups of flies were placed in 2 x 1 in. glass tubes and each of these was then closed with a piece of nylon gauze and a rubber band. The initial weight of each group of flies was obtained by weighing the tube, etc., before and after adding the flies. The tubes of flies were subsequently reweighed after 6 hr and again after a further 14 hr. Any tubes that contained dead flies were disregarded. We assumed that the changes in weight exhibited by the flies during these experiments were due chiefly to exchange of water. Three separate experiments set up with flies less than 1 hr old gave remarkable results (Table 1). When unpainted, these flies at first either lost very little water or, on occasion, gained some. When painted on an abdominal tergite other than the fifth, they lost water to a slight extent, but when painted on the fifth abdominal tergite they lost water at an appreciable rate. Analysis of variance demonstrated that the flies painted on the fifth abdominal tergum lost water at significantly greater rates (PC O-01) than did any of the other experimental groups. Between the sixth and twentieth hour of each experiment, however, all three groups of flies lost water at appreciable rates that were not significantly different from each other.
FUNCTION
OF TWOAL
TAB~~-~P~~TAGE~IG~~OSS~
GLANDS
&~OFGROUPS*
O-6 6-Z!O
OFYOUNG~FLIES
painted
Areas
Time period (hrf
None
First abdominal tergite
Fifth abdominal tergite
0.03 0.32
0.01 0.22
0.17 0.24
* Each group comprised 5 males and 5 females; conditions 20°C and 65 pe:r cent r.h. t Less than 1 hr old at start of experiment.
In two experiments experimental appreciable
1881
IN DACUS
groups rates.
during the experiment:
set up with flies, 8 and 18 hr old, respectively, all of the lost water .during both experimental periods at similar,
In four experiments on g-day-old flies (Table Z), the members of all three experimental groups also lost appreciable amounts of water during both experimental periods, and analysis of variance showed that the experimental treatments made no significant difference to the rate of loss. The same analysis also demonstrated that preliminary etherization significantly increased the rate of water loss during the first experimental period (P < 0.025) but not during the second. The ether possibly damaged some component of the cuticle that normally retards the passage of water to the outside. In addition, the analysis demonstrated that the males lost water more rapidly than did the females (3’ < O-01) but since the males are smaller thsrn the females, this result is not surprising. TABLE ~-THE
Loss&hr
P~C~AGS~IG~
OFGROUPS*
OF %-DAY-OLD
FLIES
Areas painted Time period (hr1 No ether O-6 6-20 Ether? O-6 6-20
None
First abdominal tergite
Fifth abdominal tergite
Male
Female
Male
Female
Male
Female
040 0.58
040 0.35
050 051
0.33 0.32
0.50 0.43
053 0.42
0.67 0.52
050 0‘41
0*50 0.49
o-50 042
0.48 O-56
0.38 0.35
* Each group comprised 10 flies; conditions during the experiment: cent r.h. t Brief etherization to facilitate setting up the experiment.
20°C and 65 per
1882
JEREMY J. T. EVANS ANDPETER J, STANBURY DISCUSSION
The results of our experiments suggest that the tergal glands do not secrete a pheromone that is capable of influencing either the maturation or the mating of the female fly. It is of course still possible that they secrete a pheromone that has some other effect. The results of other experiments suggest that the glands do not excrete nitrogenous waste. Furthermore, several independent results indicate that the glands are not responsible for disposing of excess water. Thus, painting over the tergal glands sometimes increased, but never decreased the rates at which flies lost water. Moreover, when flies were secured to plasticine and supplied with either sugar or water alone, their glands continued to accumulate secretion at normal rates. Even at 0 per cent r.h., the aqueous secretion still accumulated on the glands to some extent; it is clear that this secretion does not readily evaporate. During approximately the first 6 hr after emergence, adults of D, tryoni lost little or no water, whereas older flies lost water at appreciable rates. But when the tergal glands of the recently emerged flies were painted over, these flies also lost water in considerable amounts. Thus, for a brief period following emergence, the tergal glands would appear to be retarding water loss. Presumably, the waxy component of the glandular secretion, spread over the surface of the body by the cleaning movements of the fly, is primarily responsible for this effect. Yet, certain observations made by BEAMENT(1964) suggest that the aqueous component of the secretion may assist in this function. When he placed droplets of water on the cuticle of the cockroach, ~er~~~~~et~,these became covered with a film of cuticular grease. In time, the droplets disappeared, leaving the surface of the cuticle in an exceptionally hydrophobic condition. To explain this, he suggested that the water was orienting the bipolar molecules of the cuticular grease and then depositing these undisturbed onto the surface of the cuticle. Beament also maintains that an oriented monolayer of grease or wax is particularly resistant to the passage of water. Thus, the waxy and aqueous components of the tergal gland secretion in D. tryoni may co-operate to waterproof the cuticle. The trichogen cells in the tergal glands are considerably enlarged and are clearly modified to produce the gland’s waxy secretion (EVANS,1967a). But several observations suggest that the smaller trichogen cells under the rest of the abdominal cuticle also secrete wax onto the cuticular surface (EVANS, 1967b). In the hours immediately after emergence, therefore, the trichogen cells of the tergal glands may be alone in secreting wax and thus waterproofing the animal; later on, the trichogen cells over the abdomen as a whole may develop the same ability. This sequence of events could explain why painting over the tergal glands increased the loss of water only in recently emerged flies. It might also explain why painting over the glands tended to increase the death rate of recently emerged flies but had no effect on older ones. A~~owle~~~~s-We are grateful to Professor L. C. BIRCHand Dr. M. for giving us access to the facilities of their laboratory and for their many heipful
A. BATEMAN suggestions.
FUNCTIONOF TERGALGLANDSIN DACUS
1883
In addition, we profited especially from the comments of Dr. J. GEE, Dr. DARCY GILMOLIR, Dr. VALERIEB. MORRIS, and Dr. G. PRITCHARD.For proficient technical assistance, we thank Mrs. ANlrl~BARNACOAT, Mrs. VIVIAN HUNTER,and Miss LYN GRANT. REFERENCES ANDREWASTHA II. G. and BIRCH L. C. (1954) The Distribution and Abundance of Animals. Chicago University Press, Chicago. BEAMENTJ. W. L. (1964) The active and passive transport of water in insects. A&. Insect Physiol. 2, 6’7-129. EVANSJ. J. T. (1.967a) The integument of the Queensland fruit fly, Dacus tryoni (Frogg.)-I. The tergal glands. 2. Zellforsch. microsk. Anat. 81, 18-33. EVANSJ. J. T. (196713) The integument of the Queensland fruit fly, Dacus tryoni (Frogg.)-II. Development and ultrastructure of the integument and bristles. Z. Zellforsch. microsk. Anat. 81, 34-48. KOIDSUMIK. (1957) Antifungal action of cuticular lipids in insects r. Insect. Physiol. 1, 40-51. NORRISN. J. (1054) Sexual maturation in the desert locust. Antilocust BulZ. No. 18.