Sensilla on the third antennal segment of Drosophila melanogaster meigen (Diptera : Drosophilidae)

Int. J. InsectMorphoL &Embryol., Vol. 13, No. 1, pp. 51 to 63, 1984. Printed in Great Britain.

SENSILLA

0020- 7322/84$3.00+ .00 © 1984PergamonPressLtd.

ON THE THIRD ANTENNAL

SEGMENT

OF

DROSOPHILA MELANOGASTER MEIGEN (DIPTERA" DROSOPHILIDAE) S U N I T A V E N K A T E S H a n d R . NARESH SINGH Molecular Biology Unit, Tata Institute of Fundamental Research, Homi Bhabha Road, Bombay 400 005, India (Accepted 16 August 1983)

A b s t r a c t - - T h e distribution and morphology of the sensilla on the 3rd antennal segment of Drosophila melanogaster Meigen (Diptera : Drosophilidae) were studied with light and electron microscopy. Four types of hairs were identified. Three types o f hairs innervated by dendrites are sensiila basiconica, sensilla coeloconica and sensilla trichodea. They occur a m o n g s t a large number of the 4th type of uninnervated hairs or spinules. Sensilla basiconica and coeloconica can be easily identified by light microscopy on staining with 0.1010 silver nitrate in 70°10 ethanol. The tips o f sensilla basiconica and coeloconica appear dark brown. Most of the sensilla trichodea and spinules remain unstained. Sensilla hasiconica conform to the single-walled, muitiporous sensilla, having poretubules and branched dendrites. Sensilla coeloconica are double-walled and have longitudinal channels near the tip. No wall pores are found on sensilla trichodea. Dendrites do not branch in sensilla coeloconica and trichodea. A mechanosensory dendrite with characteristic tubular body is absent in these sensilla. Populations o f sensilla basiconica and sensilla trichodea occur in diametrically opposite, distinct regions on the 3rd antennal s e g m e n t - - t h e former in the dorsomedial and the latter in the ventrolateral regions, whereas sensilla coeioconica are distributed on most of the anterior and posterior surfaces, including the cavity wails of the sacculus. The axons are arranged in distinct groups in the antennal nerves at the stalk of the 3rd segment. This grouping becomes more pronounced in the nerve prior to its entry into the brain.

Index descriptors (in addition to those in title): Chemosensilla, fine structure, odor, olfaction, ultra-structure.

INTRODUCTION

THE SENSILLA for detection of odor in

Drosophila are

located mainly on the 3rd antennal

segment (Barrows, 1907; Ferris, 1950). The sensory neurons from these sensilla project into the ipsilateral antennal glomeruli and to a lesser extent to the contralateral antennal glomeruli (Stocker, 1979; Stocker et al., 1983). A light microscopic study of the different types of sensilla found on the body of the larva was clone by Hertweck (1931) and Power (1943; 1946) studied the antennal sensory areas in the brain of the adult. Mindek (1968) described the distribution of the 3 types of receptors--sensilla basiconica, sensilla coeloconica and sensilla trichodea on the 3rd antennal segment by light microscopy. The external morphology of Drosophila antenna by scanning electron microscopy has been illustrated by Hodgkin and Bryant (1978). Although considerable information is available on the fine structure of the antennal sensilla of larger flies, such as Sarcophaga and Phormia (for reviews see Snodgrass, 1944; Boeckh et al., 1965; Vinnikov, 1969; Slifer, 1970; Dethier, 1976; Altner, 1977; Zacharuk, 1980) little is known about the fine structure of the antennal sensilla of Drosophila. For this reason, we decided to study the sensilla on 51

52

SUNITA VENKATESHand R. NARESH SINGH

the 3rd a n t e n n a l segment o f D. melanogaster by light a n d electron m i c r o s c o p y . In a d d i t i o n , these studies were u n d e r t a k e n so that the structure could, in future, be c o r r e l a t e d with the p h y s i o l o g y o f the o l f a c t o r y system o f Drosophila. M o r e i m p o r t a n t l y , a d e s c r i p t i o n o f the a n a t o m y o f the w i l d - t y p e a n t e n n a is n e e d e d for worRers using o l f a c t o r y m u t a n t s o f Drosophila ( R o d r i g u e s , 1980) to investigate the n e u r o p h y s i o l o g y a n d n e u r o a n a t o m y o f the o l f a c t o r y system. F o r the s a m e reasons, the cross sections o f the a n t e n n a l nerves were e x a m i n e d a n d the n u m b e r o f a x o n s present d e t e r m i n e d . MATERIALS AND METHODS Four-to-5-day-old D. melanogaster Canton S male flies were used for fine structural studies. Flies of both sexes were used for light microscopy.

Light microscopy For preparing stained whole mounts, entire heads were kept in 0.1°70 AgNO3 in 70% ethanol for 2 days at room temperature. They were then washed, dehydrated in a graded series of ethanol and cleared overnight in xylene. Antennae were removed, washed in xylene and embedded in "Permount" between 2 coverslips. Photomicrographs were taken with a Zeiss RS Ill photomicroscope. The distribution of sensory hairs was mapped from a photographic series of silver-stained whole mounts of the antenna. The negatives were projected through a Carl Zeiss DL 5.2 microfilm reader and traced at final magnification x 3,400.

Electron microscopy Heads with intact antennae but one eye cut off were fixed overnight with Karnovsky's (1965) paraformaldehyde-glutaraldehyde fixative adjusted to pH 7.4 at room temperature. They were washed with 0.1 M sodium phosphate buffer (pH 7.4) and post-fixed with veronal acetate-buffered 1% osmium tetroxide (pH 7.4) for 3 hr at 4°C. They were washed in veronal acetate buffer and block-stained with 1% uranyl acetate plus 0.1% phosphotungstic acid, dissolved in 70% ethanol. The specimens were further dehydrated with a graded series of ethanol, followed by propylene oxide and embedded in Durcupan ACM (Fluka) in flat embedding forms. Polymerisation was at 60°C for about 36 hr. Ultrathin silvery sections were cut on a LKB Ultrotome III, collected on Formvar (polyvinyl formal) coated slotgrids and stained with a 5% aqueous solution of uranyl acetate at 60°C for 5 min, followed by lead citrate (Reynolds, 1963) for 5 min at room temperature. Sections were examined with a Jeol JEM 100 S electron microscope. Ultrathin sectioning of 14 specimens of the antenna was done for these studies. The number of electron micrographs taken was about 2,800. RESULTS Cross-sections o f the 3rd a n t e n n a l segment are r o u g h l y t r i a n g u l a r (Fig. l). W h e n the a n t e n n a is at rest, the largest a n d flattest surface faces a n t e r i o r l y with the o t h e r 2 surfaces fitting into a c o n c a v i t y in the head capsule. This flattest surface is d e s i g n a t e d as a n t e r i o r . T h e next largest surface, d e s i g n a t e d as p o s t e r i o r , b e a r s the a r i s t a on the p r o x i m a l region. A w a y f r o m the a r i s t a a n d n e a r the p r e f r o n s , the surface j o i n i n g the a n t e r i o r a n d p o s t e r i o r surfaces is the smallest o f the 3 a n d we designate it as the m e d i a l surface (Fig. 1). F o r reliable i d e n t i f i c a t i o n o f the sensilla with the light m i c r o s c o p e f r o m w h o l e m o u n t s o f the a n t e n n a e , we utilized 0 . 1 % AgNO3 s o l u t i o n in 70% e t h a n o l as a stain. W e f o u n d 4 m o r p h o l o g i c a l types o f hairs on the 3rd a n t e n n a l segment. T h e y c o n f o r m to the categories o f sensilla b a s i c o n i c a , sensilla c o e l o c o n i c a , sensilla t r i c h o d e a a n d spinules (Fig. 2 a - d ) . E t h a n o l i c A g N O , stains the distal p o r t i o n s o f sensilla b a s i c o n i c a (Fig. 2 a , b ) a n d sensilla c o e l o c o n i c a d a r k b r o w n (Fig. 2c,d). L o c a l i z a t i o n o f sensilla c o e l o c o n i c a , which are o f t e n h i d d e n by spinules, b e c o m e s very c o n v e n i e n t a n d reliable after their n a r r o w tips b e c o m e d a r k b r o w n with silver stain (Fig. 2a,c,d). T h e spinules a n d m o s t o f the sensilla t r i c h o d e a , which are like p o i n t e d straight t h o r n s , r e m a i n u n s t a i n e d (Fig. 2). A few sensilla t r i c h o d e a , which pick u p silver stain, have d a m a g e d tips. U s i n g this silver staining technique, we have r e - e x a m i n e d the d i s t r i b u t i o n o f each t y p e o f sensillum l o c a t e d on the surface o f the

Sensilla on the Third Antennal Segment of Drosophila melanogaster Meigen

53

/

PX

b FIG. 1. A schematic representation of the 3rd antennal segment showing resting and extended positions (thin profiles) with respect to frons. Upper diagrams are transverse sections at level of sacculus and pedicel (right) and slightly more distal (left) to show typical triangular cross-section, demonstrating nomenclature used in this account. Lower diagrams show medial and lateral view of 3rd antennal segment.

3rd antennal segment (Mindek, 1968). The results are shown in Fig. 3 a - e . The structures of the 4 morphological types of hairs were studied through series of electron micrographs. Sensilla basiconica, sensilla coeloconica and sensilla trichodea are sensory, because dendrites are present in their hair-shafts. The large number of spinules are uninnervated hairs. Amongst the 3 major types of dendrite-containing bristles, sensilla trichodea are refractory to silver stain. Only sensilla basiconica and coeloconica are argyrophilic, the former are shown by TEM to be multiporous and the latter multichannel sensilla. Transmission electron microscopy shows that in silver-stained sensilla basiconica, silver ions enter through the wall pores and get precipitated in the lumen (Fig. 5g) and in sensilla coeloconica; the silver ions enter through the channels and are precipitated in the lumen as well as permeating through the bristle cuticle (Fig. 5h). Sensiila basiconica A silver-stained sensillum basiconicum appears like a club or a baton with a dark brown distal half (Fig. 2a,b). Sensilla basiconica occur in a single patch over the entire medial surface and the dorso-medial regions of the anterior and posterior surfaces (Figs. 2a,b; 3a,b). Two conspicuous types of sensilla basiconica are distinguished by their hair sizes. Eleven to 17 sensilla basiconica, which are 2 - 3 . 5 p.m thick and 8 - 1 1 p.m long, are prominently arranged in a crescent near the distal border of the sacculus on the posterior surface (Fig. 2a, marked TB). Similarly, 7 - 11 sensilla basiconica are located on the proximal region of the anterior surface (Fig. 2b) and 1 4 - 2 2 are found on the medial surface (Fig. 3a,b). These are henceforth referred to as thick sensilla basiconica. The rest

54

SUNITA VENKATESHand R. NARESH SINGH

U

I~G. 2. a. Photomicrograph of an ethanolic silver nitrate-stained whole mount preparation of an antenna showing darkly stained sensilla basiconica on its posterior surface, x 450. b. Anterior view photomicrograph of whole mount antennal preparation with distal tips of sensilla basiconica darkly stained with ethanolic silver nitrate solution, x 450. c. Sensilla coeloconica inside sacculus on proximal region of posterior surface, x 450. d. Silver stained sensilla coeloconica on proximal region of posterior surface, x 450. of the sensilla basiconica, n u m b e r i n g a b o u t 180 a n d which are 1 - 1 . 5 lxm thick a n d 5 - 7 txm long, are referred to as slender sensilla basiconica (Fig. 2a,b, m a r k e d SB). Sensilla basiconica consist o f 2 - 4 sense cells, sheath cells, a n d the hair. The distal process of the sense cell, the dendrite, is partitioned as a n i n n e r a n d outer segment. A ciliary a p p a r a t u s is f o u n d at the t r a n s i t i o n p o i n t between these 2 segments (Fig. 4e, m a r k e d CL). T h e outer segment is divided at the hair base into several branches, which

Sensilla on the Third Antennai Segment of Drosophila melanogaster Meigen

/

N~

55

- ~/'\?

Q

C

e

"~'

f,

3

FIG. 3. Diagrams showing distribution of (a) sensiila basiconica on posterior surface, (b) sensilla basiconica on anterior surface, (c) sensilla coeioconica on posterior surface, (d) sensilla coeloconica on anterior surface, (e) sensilla trichodea on posterior surface, and (f) sensilla trichodea on anterior surface, x 400.

c o n t i n u e to the hair tip (Fig. 4(1). These small d i a m e t e r branchlets (Fig. 4b) o f t e n appear circular in cross section a n d their n u m b e r varies greatly between 7 a n d 180. Usually, 1 - 5 m i c r o t u b u l e s are visible in the cross section o f each b r a n c h l e t (Fig. 4c). Occasionally, 1 or 2 large diameter b r a n c h e s c o n t a i n i n g m a n y m i c r o t u b u l e s are seen in cross-sections o f the

56

SUNITA VENKATESHand R. NARESH SINGH

hair shaft (Fig. 4b). The wall of the hair is 0.2 txm thick and is penetrated by pores. About 1,200 pores are present on a thick sensillum basiconicum and about 300 pores on the slender sensillum basiconicum (an average of 11 TEM observations each from longitudinal grazing sections). Pore tubules are found inside pore cavities (Fig. 4a). Similar pore tubules were observed by Slifer and Sekhon (1964), Ernst (1969) and Schneider (1969) in the sensilla of other insects, The sensilla basiconica on the 3rd antennal segment of Drosophila are thus structurally similar to the thin-walled pegs occurring on the antennal flagellum of the flesh-fly Sarcophaga argyrostoma (Slifer and Sekhon, 1964). In the thick sensilla basiconica innervated by 4 sense cells, 2 cells are always less electron-dense. Of these 2, one has a large-diameter dendrite and the other a small one. These 2 cells are sometimes arranged diametrically opposite (Fig. 4g) and at times placed adjacent to each other (Fig. 4f). The other 2 electron-dense dendrites in these sensilla are almost of the same size (Fig. 4f,g). In thick sensilla basiconica innervated by 2 sense cells, one of the cells is sometimes less electron-dense, but this difference is not so marked as in those with 4 neurons. Of the 21 thick sensilla basiconica analysed from the medial surface, 11 were found to be innervated by 4 neurons, and 10 by 2 neurons. The distribution of neurons in these sensilla is given in Table I. Earlier, Falk et al. (1976) observed that in labellar taste bristles of Drosophila, one of the 4 sense cells was less electron-dense than others.

Sensiila coeioconica Unlike the flesh-fly, which has several sensory pits on the 3rd antennal segment (Slifer and Sekhon, 1964), D. melanogaster has only one sensory pit sacculus (Figs. 1, 2c). The sacculus is multichambered and its inside walls are lined with 35 - 40 sensilla coeloconica. The rest of the sensilla coeloconica, numbering about 100, are located in individual depressions and are interspersed with sensilla basiconica and trichodea on the anterior and posterior surfaces of the 3rd antennal segment (Fig. 3c,d). Proximal to the sacculus, on the posterior surface, is an isolated patch of 9 - 11 sensilla coeloconica (Fig. 2d). All these sensilla coeloconica are double-walled, longitudinally grooved hairs with channels near the distal tip (Fig. 5d). Proximally, the hairs are smooth-walled (Fig. 5e,f). In cross-section, these sensilla typically have 7 - I I peripheral lumina (Fig. 5 a - c ) surrounding a central lumen containing 1 - 3 dendrites (Fig. 5c,e). The dendrites do not extend up to the tip of the hair (Fig. 5a,b). Near the tip the hair lumen is filled with electron-dense material (Fig. 5b, marked DM). The dendrites probably communicate with the exterior through the channels that penetrate the wall. Unlike sensilla basiconica, the outer segment of the dendrite is unbranched in sensilla coeloconica. Sensilla coeloconica located in the sacculus are innervated by 2 or 3 sense cells, whereas those on the surface are innervated by 1, 2 or 3 neurons (Table 2). In Sarcophaga, the number of grooved hairs was small (Slifer and Sekhon, 1964). In Drosophila, however, such grooved hairs are quite abundant numbering about 140 in all (Fig. 3c,d).

Sensilla trichodea Sensilla trichodea are found on the ventrolateral regions of the anterior and posterior surfaces of the 3rd antennal segment (Figs. 2a,b; 3e, f). Unlike sensilla basiconica and coeloconica, no pores or channels are detectable in the 0.35 Ixm thick wall of these sensilla. Unlike sensilla basiconica, the outer segment of the dendrite is unbranched

Sensilla on the Third Antennal Segment of

Drosophila melanogaster Meigen

FIG. 4. Captions on p. 58.

57

58

SUNITA VENKATESHand R. NARESH SINGH TABLE l. NEURONALCOMPOSITIONOF DIFFERENTIALLYSTAINEDTHICK SENSILLA BASICONICAPRESENT ON THE MEDIAL SURFACE OF THE 3RD ANTENNAL SEGMENTOF Drosophila melanogaster Neurons in a sensillum

Distribution of neurons in a sensillum

No. sensilla analysed

4

2 dark and 2 light cells placed adjacently

6

4

2 dark and 2 light cells placed diametrically opposite

5

2

1 light and 1 dark cell

7

2

Both similarly stained

3

TABLE 2. NEURONAL COMPOSITION OF VARIOUS TYPES OF SENSILLA PRESENT ON THE THIRD ANTENNAL SEGMENT OF O. melanogaster, DETERMINEDBY TRANSMISSIONELECTRON MICROSCOPY Neurons per sensillum

Observed number of sensilla

Sensilla basiconica

2 4

42 36

Sensilla coeloconica from sacculus

2 3

56 24

Sensilla coeloconica from surface

1 2 3

10 24 25

Sensilla trichodea

1 2 3

21 45 20

Sensillum type

(Fig. 5i, marked D). Sensilla trichodea are innervated by 1, 2 or 3 dendrites (Table 2). The dendrites do not extend up to the bristle tip. The distribution of sensory cells in various individual sensilla basiconica, coeloconica and trichodea is givefi in Table 2.

FiG. 4. a. A longitudinal section of a sensillum basiconicum showing wall pores and pore tubules. x 35,000. b. and c. Cross-sections of a sensillum basiconicum with small diameter dendrite branchlets in lumen. Most of dendrites are between 0.1 and 0.5 lam diameter and at times accompany a larger diameter dendrite(*). Note microtubules in c. x 21,000 and 24,500 respectively. d. Division of a dendrite into small diameter processes near base of the peg of a thick sensillum basiconicum, x 12,300. e. Two dendrites D, and D~ shown side by side in same sinus. Ciliary region of dendrite D, is in same plane of section. Neurons (cell bodies) N, and N2 are assumed to give rise to dendrites D, and D2, respectively, x 6,300. f. and g. Transverse sections of thick sensilla basiconica from medial surface. Note less electron-dense dendrites arranged adjacently in f and diametrically opposite in g. × 8,750 and 11,700 respectively.

Sensilla on the Third Antennal Segment of

Drosophila melanogaster Meigen

e

g

FIG. 5. Captions on p. 60.

59

60

SUNITA VENKATESHand R. NARESH SINGH

Cross-sections of the antennal nerves T h e stalk o f the 3rd a n t e n n a l segment a c c o m m o d a t e s 2 nerves, the larger has a b o u t 970 axons a n d the smaller, a b o u t 290 axons. T h e axons in these nerves show consistent g r o u p i n g (Fig. 6a). Before e n t r y into the b r a i n , the 2 nerves f r o m the 3rd a n t e n n a l segment, a l o n g with a x o n s o r i g i n a t i n g f r o m the 1st a n d 2nd a n t e n n a l segments unite to f o r m a single a n t e n n a l nerve on either side. This a n t e n n a l nerve has a b o u t 1,800 axons a s s e m b l e d in several g r o u p s , in an o r d e r e d fashion. T h e r e are 3 g r o u p s o f only small d i a m e t e r a x o n s , one g r o u p o f very large axons, a n d one m i x e d g r o u p o f large a n d small d i a m e t e r a x o n s (Fig. 6b). T h e discrete g r o u p i n g o f a x o n s in i d e n t i f i a b l e g r o u p s a c c o r d i n g to size is m o r e p r o m i n e n t b e f o r e e n t r y into the b r a i n (cf. Figs. 6a a n d b). DISCUSSION Silver staining reveals t h a t sensilla b a s i c o n i c a a n d t r i c h o d e a occur on the a n t e n n a in 2 c o n s p i c u o u s patches. Sensilla b a s i c o n i c a are c o n c e n t r a t e d on the m e d i a l a n d d o r s o m e d i a l regions, whereas sensilla t r i c h o d e a are m a i n l y l o c a t e d on the v e n t r o l a t e r a l regions. Sensilla c o e l o c o n i c a a r e s p r e a d over the entire a n t e r i o r a n d p o s t e r i o r surfaces (Fig. 3 a - f). T h e o c c u r r e n c e o f such a p a t t e r n o f sensilla on the 3rd a n t e n n a l segment can be e x p l a i n e d in the simplest m a n n e r b y c o n s i d e r i n g the i n v o l v e m e n t o f 2 or 3 m o r p h o g e n s d u r i n g d e v e l o p m e n t . O n e m o r p h o g e n is r e s p o n s i b l e for the f o r m a t i o n o f sensilla b a s i c o n i c a a n d the o t h e r for sensilla t r i c h o d e a . T h e f o r m a t i o n o f sensilla c o e l o c o n i c a m a y be d e t e r m i n e d j o i n t l y b y b o t h m o r p h o g e n s . A l t e r n a t i v e l y , if 3 m o r p h o g e n s a r e involved, the f o r m a t i o n o f sensilla c o e l o c o n i c a m a y be c o n t r o l l e d b y the 3rd m o r p h o g e n (for l i t e r a t u r e see B a b l o y a n t z a n d H i e r n a u x , 1974). O u t o f a b o u t 150 sensilla t r i c h o d e a present on the 3rd a n t e n n a l segment, o n l y one o r 2 pick up silver stain, which are likely to have their tips d a m a g e d . A l t h o u g h sensilla t r i c h o d e a have d e n d r i t e s in their l u m e n , the presence o f a t e r m i n a l p o r e o r wall pores was not d e t e c t a b l e b y T E M . A b s e n c e o f wall p o r e s a n d being r e f r a c t o r y to silver stain indicate t h a t p o s s i b l y sensilla t r i c h o d e a are n o t c h e m o r e c e p t o r s . T h e y c o u l d be t h e r m o - o r hygroreceptors, or some other form of receptor. T h e q u e s t i o n comes to ones m i n d w h e t h e r thick sensilla b a s i c o n i c a have s o m e specific f u n c t i o n d i f f e r e n t f r o m the rest o f the slender sensilla b a s i c o n i c a . A similar q u e s t i o n applies to the specificity a n d f u n c t i o n a l role o f the sensilla c o e l o c o n i c a l o c a t e d in the sacculus a n d on the surface o f the 3rd a n t e n n a l segment. Since these sensilla d i f f e r in l o c a t i o n a n d c o m p o s i t i o n , it is likely t h a t they d i f f e r in f u n c t i o n o r specificity or b o t h . Single-walled sensilla with wall p o r e s a n d p o r e tubules in Periplaneta americana have sensory cells o f 4 types with specific sensitivity for s h o r t - c h a i n n - a l c o h o l s a n d terpenes. Based on their m a x i m u m r e s p o n s e to a certain chemical, they were called p e n t a n o l - , FIG. 5. Transverse sections of a sensillum coeloconicum from sacculus (a) at its distal tip ( x 56,700); b. proximal to that in a. Lumen is filled with electron-dense material DM, (x 58,300); c. Proximal to that in b. Dendrites are present in central lumen. Peripheral lumina are distinctly seen, x 27,500. d. Longitudinal section of a sensillum coeloconicum from sacculus. Note channels present near distal tip of hair, x 21,500. e. Section near base of hair. Note that hair is almost smooth-walled, x 21,000. f. Section at base of hair. Cuticle is smooth all around and 3 dendrites are seen in lumen of hair, x 36,000. g. Electron micrograph of cross-section of a silver-stained sensillum basiconicum. Note silver grains G present in lumen, x 29,300. h. Electron micrograph of longitudinal section of a silverstained sensillum coeloconicum from sacculus. Silver grains are present near channels as well as in matrix of cuticle, × 9,300. i. A longitudinal section through a sensillum trichodeum. Dendrites D extend undivided through lumen of hair, x 7,000.

Sensilla on the Third Antennal Segment of

Drosophila melanogaster Meigen

61

FIG. 6. a. Transverse section of larger branch of antennal nerve through stalk of the 3rd antennal segment. Note grouping of axons, x 9,300. b. Cross-section of antennal nerve before entry into brain. Note that grouping of axons is distinct. x 3,000.

hexanol-, octanol-, and alcohol-terpene sensilla (Altner et al., 1977). Porous olfactory sensilla of Bombyx mori, which have been investigated electrophysiologically, usually have 2 dendrites, each functioning as a specific receptor for Bombykol and Bombykal respectively (Steinbrecht, 1980). By analogy, multidendritic sensilla basiconica with wall pores and pore tubules located on the 3rd antennal segment of Drosophila are good candidates for olfactory sensilla; the differential staining properties and the variation in

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SUNITA VENKATESH and R. NARESH SINGH

size of the 4 dendrites in these sensilla suggests that they are p r o b a b l y m u l t i f u n c t i o n a l . M o r a n et al. (1971) suggested that pale staining of c y t o p l a s m reflects high water c o n t e n t . By a n a l o g y with hairs f o u n d o n locust a n t e n n a (Boeckh, 1967) double-walled sensilla coeloconica o f D r o s o p h i l a could be olfactory a n d thermosensitive receptors. The a n t e n n a l nerves show distinct g r o u p i n g of axons according to size (Fig. 6a,b). This g r o u p i n g o f a x o n s is consistent with earlier findings (Stocker, 1979). T w o m e c h a n i s m s m a y b e responsible for such a g r o u p i n g of axons within the nerve. It m a y b e either f u n c t i o n specific or location specific. A s s e m b l a g e of axons in b u n d l e s has been described earlier in other insect sensory nerves (Steinbrecht, 1969). A n i m p o r t a n t feature of sensilla o n the 3rd a n t e n n a l segment is that they do n o t have a n y characteristic m e c h a n o s e n s o r y dendrite with a t u b u l a r b o d y ( T h u r m , 1964, 1965). T o some extent, sensilla that are physiologically chemo-, hygro- or thermosensitive can be distinguished f r o m the structure a l o n e (Altner, 1977). However, for Drosophila, concrete evidence for f u n c t i o n can only come f r o m electrophysiological experiments where f u n c t i o n a l characteristics a n d responses o f sensilla are i n d i v i d u a l l y determined. In the absence of such recordings in published literature we draw analogies from other insects, where such studies have been made. This present a c c o u n t is likely to be a useful reference for future experiments with electrophysiology a n d genetics of olfaction in Drosophila. Acknowledgements--We are grateful to Professor O. Siddiqi for his support of these studies. We thank Drs. R. F. Stocker, U. W. Kenkare and Veronica Rodrigues for helpful suggestions. We express gratitude to the following colleagues at the European Molecular Biology Laboratory, Heidelberg: Drs. Hilary Anderson; Ursula Bassemir; Camilla and N. Strausfeld for help with the preparation of the manuscript and to Ms. Petra Riedinger, for the drawing.

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APPENDIX

Abbreviations used in figures. A = arista; AS = anterior surface; C = sensilla coeloconica; CH = channel; CL = ciliary region; D = dendrite; DM = dense material; G = silver grain; MS = medial surface; N = neuron; P = pore; PS = posterior surface; PT = pore tubule; PX = proximal; S = sacculus; SB = slender sensilla basiconica; SD = small diameter dendrite branchlets; SP = spinules; T = sensilla trichodea; TB = thick sensilla basiconica. A number, as a suffix, is used to distinguish between 2 similar structures, eg. D~ and D~ denote 2 different dendrites.