Morphology and fine structure of mouthpart sensilla in the dark-sided cutworm Euxoa messoria (Harris) (Lepidoptera : Noctuidae)

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M O R P H O L O G Y A N D FINE S T R U C T U R E OF M O U T H P A R T SENSILLA IN THE D A R K - S I D E D C U T W O R M EUXOA MESSORIA (HARRIS) ( L E P I D O P T E R A • N O C T U I D A E ) B. D. DEVITT and J. J. B. SMITH Department of Zoology, University of Toronto, Toronto, Ontario, Canada M5S 1A1 (Accepted 30 March 1982) Abstract--The present study describes the fine structure of sensilla on the maxillary palps, galeae, labial palps, and labrum of the dark-sided cutworm, Euxoa messoria (Harris) (Lepidoptera : Noctuidae). Ten types of sensilla were identified and a total of 144 bipolar sense cells were observed. Each palp bears a group of 8 terminal uniporous pegs, 1 large and 2 small subterminal multiporous plate sensilla and a digitiform sensillum, Also associated with the walls are 3 scolopidial sensilla and a campaniform-like sensillum. Each galea bears 2 large segmented uniporous pegs, a campaniform sensillum, 2 aporous papillae, and a small, spike-shaped, aporous peg. Each labial palp bears 2 cone-shaped sensilla. Externally, the labrum bears 12 aporous hairs, and internally, 6 aporous hairs and I pair of campaniform sensilla. The ultrastructural evidence suggests that the uniporous pcgs and multiporous plate sensilla are chemosensory and that the hairs, aporous papillae, aporous cones, digitiform sensillum, scolopidial sensilla, campaniform, and campaniformlike sensilla are mechanosensory. From the structural evidence, no role could be assigned to the aporous spike-shaped pegs on the galeae. The multiporous plate sensilla resemble olfactory sensilla, and appear to be a new type of sensillum. Two-thirds of the observed neurons innervate the ctlemosensilla. The maxillary palps have approximately 4 times as many contact chemosensory neurons as the galeae, suggesting that they have an important role in gustation. Index deseriplors (in addition to those in title): Galea, maxillary palp, labrum, labial palp, contact chemosensilla,'olfactory sensilla, campaniform sefisilla. 1NTRODUCTION THERE are n u m e r o u s reports on the type a n d n u m b e r o f sensilla f o u n d on the m o u t h p a r t s o f larval L e p i d o p t e r a . C h e m o s e n s i l l a have been l o cat ed on the galeae, the maxillary palps, an d the inner s u r f a c e o f the l a b r u m (Dethier, 1937; l s h i k a w a , 1965; S c h o o n h o v e n and Dethier, 1966; S c h o o n h o v e n , 1972, Ma, 1972, 1976; W i e c z o r e k , 1976; De Boer et al, 1977; Al b ert , 1980). E l e c t r o p h y s i o l o g i c a l evidence has indicated that g u st at i o n is m e d i a t e d t h r o u g h the medial a n d lateral u n i p o r o u s pegs on the galeae ( c o m m o n l y referred to as the medial and lateral sensilla styloconica) and, when present, t h r o u g h the e p i p h a r y n g e a l sensilla. T h e sensilla at the tips o f the m a x i l l a r y palps, which are typically a g r o u p o f 8 pegs (often referred to as sensilla basiconica), h a v e been sh o w n to be sensitive to o l f a c t o r y stimuli ( S c h o o n h o v e n a n d Dethier, 1966). In a d d i t i o n to the a b o v e chemosensilla, n u m e r o u s a p o r o u s pegs, c a m p a n i f o r m sensilla, and hairs h a v e also been o b s erv ed . I n f o r m a t i o n a b o u t the u l t r a s t r u c t u r e o f the m o u t h p a r t sensilla is limited to a d es cr i p t i o n o f the e p i p h a r y n g e a l sensilla a n d the u n i p o r o u s pegs on the galeae in Pieris brassicae (Ma, 1972). T h e present study c o n c e r n s the u l t r a s t r u c t u r e o f the sensilla on the m o u t h p a r t s o f larval L e p i d o p t e r a an d is i n t e n d e d to aid in u n d e r s t a n d i n g the sensory basis o f feeding b e h a v i o u r o f the d a r k - s i d e d c u t w o r m , E u x o a messoria. 255

256

B . D . DEVITT and J. J. B. SMI'[H MATERIALS

AND

METHODS

All sensilla were prepared from E u x o a messoria larvae 2 4 - 4 8 hr after they had moulted into the ultimate instar. Eggs were obtained from the Research Institute, Agriculture Canada, London, Ontario, Larvae were reared on an artificial diet (Devitt et al., 1980). For scanning electron microscopy (SEM), heads were fixed for 2 hr in cold glutaraldehyde (2.5%) in Millonig's buffer. Specimens were rinsed in several changes of distilled water and the surface was cleaned by placing the heads either in a hot (60°C) solution of 10% KOH for approximately 60 sec or in cold (4°C) 85°70 lactic acid for 20 rain. After rinsing, specimens were dehydrated through a graded acetone series, critical-point dried, and mounted on stubs with conductive paint. They were subsequently coated with gold and observed in a Cambridge Stereoscan Electron Microscope. For transmission electron microscopy (TEM) specimens were prepared following the method of Mclver and Siemicki (1978). Sections, cut on a diamond knife using an MT-2 Ultramicrotome, were double-stained with saturated uranyl acetate in methanol for 20 rain followed by lead citrate for 5 - 10 rain. For light microscopy, thick sections (I - 2 lam) were cut from Spurr-embedded specimens and stained with a solution of 1% toluidine or methylene blue in 1% borax. Whole m o u n t s were prepared by intravital staining for 60 rain with reduced 1% methylene blue (after Govind and Dandy, 1970). Following overnight fixation with cold (4°C) 10% a m m o n i u m molybdate, the heads were rinsed for 2 hr with distilled water, then the maxillae, labrum, and labium were removed and dehydrated through a graded ethanol series. The terminology of Zacharuk (1980) is used.

RESULTS

1. Maxillary palps The second segment of each palp bears a group of 8 terminal uniporous pegs, 3 subterminal multiporous plate sensilla and a digitiform sensillum. A campaniform-like sensillum and a group of 3 scolopidial sensilla are also found associated with the walls of this segment (Fig. I, Table I). Uniporous peg sensilla. Externally, these sensilla appear as small blunt-tipped structures ( 3 - 4 I.tm long). The cuticular covering is approximately 0.6 lam thick and differs from the cuticle of the paip by the addition of canaliculi and an inner electrondense ring of cuticle (Fig. 2). The inner ring extends for a short distance below the base of the peg and ends distally approximately 0.5 lam from the tip of the peg. A spongy electron-lucid layer of cuticle occurs at the base of the pegs and continues for a short distance into their walls (Fig. 2). This layer has an abundant supply of fine canals that appear to be continuous with the canaliculi of the pegs. The tips of the pegs are unusual in that they appear to consist of a ridge of cuticle surrounding a short papilla (150 nm long,) which bears a single terminal pore (130 nm in diameter) (Fig. 3). Each peg is innervated by a bundle of 4 (pegs 1 - 7) or 5 (peg 8) bipolar neurons. Electrophysiological evidence (unpublished) has indicated that a neuron sensitive to mechanical stimulation is present in pegs 1 - 3, 6, and 8. In addition, one dendrite from each bundle of neurons innervating these pegs was observed to terminate at or near the base of the peg (Fig. 4). The cytoplasm at the tips of these dendrites became granular with patches of electron-dense material appearing (Fig. 4). In considering the termination point of these dendrites and the electrophysiological observations, it appears that the electrondense patches may represent a simple form of tubular body. All the other dendrites were observed to enter the lumen of the pegs and extend, unbranched, to within 1 ~tm of the pore. The outer dendritic segments are encased by a thin dendritic sheath. From serial thin sections it was observed that the dendritic sheaths partially, and occasionally completely, separate the dendrites from one another. Near the base of the pegs, the dendritic sheath is closely apposed to the inner cuticular ring and the simply structured tubular body (Fig. 4). Three sheath cells, with their associated ciliary and sensillar sinuses, were observed with

Morphology of Mouthpart Sensilla of Euxoa messoria

25 7

[q(;. 1. Scanning electron micrograph of maxillary palp and galea. Tip of 2nd segment bears 8 small pegs (arrows showing pegs 3, 5 and 8) and the dorsal surface bears 1 large (Imps) and 2 small (smps) multiporous plate sensilla, and a digitiform sensillum (dfs). Galea bears 2 large segmented uniporous pegs (medial-m, and lateral-l), a campaniform sensillum (c), 2 small aporous papillae (ap), and a small aporous spike-shaped peg (pg). × 400. Insert shows, at higher magnification, digitiform sensillum and 3 multiporous plate sensilla, x 1,800. each b u n d l e o f dendrites. A n u n u s u a l f e a t u r e (which will be dealt with in m o r e detail in a future p a p e r ) was the t e r m i n a t i o n o f the cells well below the base o f the pegs, leaving the distal section o f sensillar sinus o p e n to the s u r r o u n d i n g h e m o l y m p h . Multiporous plate sensilla. In s c a n n i n g electron m i c r o g r a p h s these sensilla a p p e a r as shallow d e p r e s s i o n s o f slightly r o u g h e n e d cuticle (Fig. 1). T r a n s m i s s i o n electron m i c r o g r a p h s revealed that the sensilla are encased within the wall o f the p a l p a n d consist o f an i n n e r v a t e d c h a m b e r (referred to here as sensillar c h a m b e r ) , which is covered by a specialized c u t i c u l a r cap. T w o f o r m s o f these sensilla were o b s e r v e d a n d will be referred to as small ( S M P S ) a n d large ( L M P S ) m u l t i p o r o u s plate sensilla. T w o S M P S ( a p p r o x i m a t e l y 3 - 4 Ixm irl d i a m e t e r ) were o b s e r v e d (Fig. 1) near the base o f the d i g i t i f o r m sensillum on each palp. T h e sensillar c h a m b e r is a p p r o x i m a t e l y 1.5 ~tm d e e p (Fig. 5), a n d has a single basal o p e n i n g where the d e n d r i t e s enter. T h e cuticular cap

B . D . DEVITT and J. J. B. SMITH

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TABLE l , INNER'CATION AND SUOGESTED FUNCTION OF MOUTHPART SENSILLA OF LARVAL

Location

Type

Maxillary palp

Uniporous pegs

8

33

Small multiporous plates Large multiporous plates Digitiform Scolopidal Campaniform-like

2 1 1 3 1

4 3 1 3 1 Total 45

Uniporous pegs

2

10

Campaniform Aporous papilla Aporous peg

1 2 1

1 2 2 Total 15

Aporous cones

2

2

Galea

Labial palp Labrum

Number

Neurons

Campaniform 2 2 Hairs 18 18 Total (2 x maxillary palps, galea, labial palps + labrum) = 144 Total number of chemosensory neurons = 86

Euxoa messoria

Suggested function 28 - - Chemosensory 5 - - Mechanosensory Chemosensory Cbemosensory Mechanosensory ?? Mechanosensory Mechanosensory 8 - - Chemosensory 2 - - Mechanosensory Mechanosensory Mechanosensory ??? Mechanosensory ?? Mechanosensory Mechanosensory

consists of an outer porous layer (0.3 p.m thick) and an inner lamellar layer (0.5 I~m thick) (Figs. 5, 6). The outer layer is penetrated by numerous minute pores (5 - 6 nm wide) and pore tubules, which insert at the base of each pore. The tubules (approximately 15 nm wide) have electron-dense walls, and appear to be filled with a finely granular substance (Fig. 6). Basally the tubules divide and form an irregular mass between the 2 layers of the cap. The inner lamellar layer appears to consist of layers of electron-lucid material interspersed with channels containing the sensillum liquor. Some tubules from the outer cuticular layer continue into the lamellar layer but were not observed to extend beyond it. Thin sections obtained through the ciliary regions of the SMPS revealed that each sensillum is innervated by 2 neurons (I and II). Proximal portions of the outer segments of dendrite I are characterized by an electron-dense cytoplasm (Fig. 7). As this neuron enters the sensillar chamber it divides, forming several large branches. Distal to the ciliary region, dendrite I1 forms numerous small branches. Within the terminal portion of the dendritic channel, the small dendritic branches narrow, forming fine branches, and the liquor of the channel becomes more electron-dense. From above the ciliary region, the dendrites become completely encased by a dendritic sheath. Distally, this sheath becomes thinner (approximately 35 nm) and less electron-dense than the thicker (approximately 50 nm) proximal portions. The dendritic sheath becomes embedded in the lamellar layer of the cap (Fig. 5) and is closely associated with the electron-lucid layers. The lateral edge of the sheath was traced to within 20 nm of the outer cuticular layer. Three sheath cells and their associated ciliary and sensillar sinuses were observed with each SMPS. The sensillar sinus is filled with an intricate complex of interdigitating lamellae from the intermediate and outer sheath cells (Fig. 7). The outer cell encloses a large vacuole; it was not determined if this was continuous with the sensillar sinus. One LMPS (7 lain in diameter) was observed approximately 20 p.m below the distal surface of each palp (Fig. 1). The cuticular structure of the cap is similar to that observed in the SMPS (Figs. 8, 9). The outer tubular layer (0.5 lain thick) is slightly thicker and the

Morphology of Mouthpart Sensilla of Euxoa me~soria

Ft(,. 2. Transverse section distal to base of peg 3 of palp showing dendritic sheath (ds) surrounding 3 dendrites, canaliculi (ca), inner ring of cuticle (ic) and electron lucid layer of cuticle (s). × 15,360. FI6. 3. Transverse section through pore (p) of peg 1 of palp. Note dendritic sheath (ds) and association of canaliculi with epicuticular pores and sensillar channel (arrows). × 30,190. FJo. 4. Cross-section near base of peg 8 on palp showing dendritic sheath (ds) surrounding 5 dendrites (1 - 5) and inner ring of cuticle (ic). Note area of apposition between inner ring of cuticle and dendritic sheath (arrows) and patches of electron-dense material of tubular body of dendrite 1. × 27,763.

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Fl~;s. 5 - 7 on page 261.

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pores are larger (approximately 13 nm) than in the SMPS. The pore tubules, (15 nm wide), which extend from the base of the surface pores, are unbranched and appear to terminate in small pockets filled with a granular electron-dense substance (Fig. 8). The lamellar layer (approximately 0.9 ~tm thick) has a slightly more structured appearance than the corresponding layer of the smaller plates. Thin sections obtained through the ciliary region revealed that the LMPS are innervated by 3 neurons. At the base of the opening to the sensillar chamber, the dendrites begin to divide and eventually fill the chamber with small dendritic branches. In the LMPS, the sensillar chamber appears to be analogous to the dendritic chambers of other multiporous sensilla (Zacharuk, 1980). The dendritic branches extend to the edge of the lamellar layer, but were not observed to enter into it. The branches showed evidence of beading and of vesiculation. Interspersed a m o n g the dendritic branches are numerous bilaminate and trilaminate vesicles. In the LMPS, the dendritic sheath appears to be bilaminate, containing loops and folds and numerous perforations (Fig. 10). The distally terminating point of the dendritic sheath was not determined; however, it was not observed to enter into the sensillar (dendritic) chamber. The sheath is structurally similar to the felt-work at the tip of the dendritic sheath of hair sensilla on the terminalia of Aedes aegypti (Rossignol and Mclver, 1977). However, in the LMPS the felt-work structure was observed over the entire length of the dendritic sheath and not just in the terminal portions. Three sheath cells with their associated sinuses are associated with the dendrites innervating the LMPS. The intermediate cell encloses a large vacuole, which appears to be continuous with the sensillar sinus. The sheath cells terminate at the base of the dendritic chamber. Digitiform sensillum. In scanning electron micrographs this sensillum appears as a protuberance (25 I.tm long and 10 ~tm wide at the base) of the palp wall (Fig. 1). The inner wall of the sensillum inserts deeply into the palp, terminating within the hemocoel above the junction with the 2nd segment. The sensillum is innervated by a single nerve cell with a lamellated dendrite (Fig. 11), which was traced to within 5 lam of the tip. At the base of the cuticular protuberance the dendrite branches, and begins to form the lamellae. Distal to the ciliary region the dendrite is encased by a dendritic sheath. Two sheath ceils, which enclose the associated extracellular spaces, were observed. The outer sheath cell encloses a large vacuole, which appears to be continuous with the sensillar sinus. Distally, this cell terminates near the base of the sensillar channel. Campaniform-like sensillum. There are no external cuticular modifications associated with this sensillum. Approximately ¼ of the way down the dorsal wall a densely staining

FIG. 5. Cross-section through a small multiporous plate sensillum embedded within wall of maxillary palp (pc) showing outer layer o f cuticular cap (oc) which is perforated by pores and pore tubules (pt), inner lamellated layer of cap (1), sensillar chamber (sc) microvilli from sheath cells (m), and dendritic sheath (ds), which surrounds numerous large (Idb), small (sdb) and fine (fdb) dendritic branches. x 46,800. Fit;. 6. Higher magnification of cuticular cap of a small multiporous plate sensillum showing pores (p), branching pore tubules (pt), and lamellae (1) and channels (c) of inner lamellar layer, x 65,920. Fit;. 7. Cross-section through proximal portion of outer dendritic segments of dendrites (I and 11), which innervate one of the small multiporous plate sensilla, showing dendritic sheath (ds] surrounded by complex interdigitating lamellae of sheath cells, x 18,286.

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FIG. 8. Cross-section through proximal portion of large multiporous plate sensillum showing dendritic chamber (dc) encased by palp cuticle (pc) and outer (pc) and inner (1) layers of cuticular cap, dendritic branches (db) and distal projections of a sheath cell (m). × 12,096. Inset shows, at higher magnification, pores (p) and pore tubules (pt) of outer cuticular layer of cap. × 82,400. FIG. 9. Cross-section through distal portion of large multiporous plate sensillum showing outer (pc) and inner (1) layers of cap, dendritic branches (db) dendritic chamber (dc) and palp wall (pc). Note pore tubules (pt) extending into inner layer of cap and dendritic chamber, x 31,850. FtG. 10. Cross-section through 3 dendrites innervating large multiporous plate sensillum showing dendritic sheath (ds), sensillar sinus (ss), and microvilli (m) from a sheath cell. × 24,300.

Morphology of Mouthpart Sensilla of Euxoa messoria

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tubular body was observed to insert into an electron-lucid lamellated layer of cuticle (Fig. 12). The lamellae of this layer are arranged in a concave manner, which contrasts with the convex arrangement of the lamellae in the cuticle of the palp. The number of sheath cells associated with this sensillum was not determined. Scolopidial sensilla. Three scolopidia, each composed of a single bipolar sense cell and at least 2 sheath cells, were observed. These sensilla insert on the dorsal wall of the second segment, forming a roughly triangular grouping about 10 jam from the distal surface.

2. Galeae The distal surface of the galea is supplied with 6 sensilla: 2 small aporous papillae ( 3 - 5 ~m long), a small spike-shaped aporous peg (11 ~tm long), a large campaniform sensillum (11 ~tm in diameter), and the medial and lateral uniporous peg sensilla (Fig. 1, Table 1). Uniporous peg sensilla. The medial and lateral pegs consist of a small uniporous peg, 9 ~m long and 6 lam wide, on top of a bulbous base, 35 tam long and 40 lam wide. The structure of the cuticle in the peg differs from that in the base in that it is thinner and canaliculi are present. Present around the base of the pegs and in the proximal segments of their walls is an electron-lucid layer of cuticle, similar to the layer found in the uniporous pegs of the palp. Thin sections obtained from the ciliary regions showed that each of the pegs is innervated by a group of 5 neurons (Fig. 13). Distal to the ciliary region, one of the dendrites branches (Fig. 14). Electrophysiological evidence (unpublished) has shown that each peg is innervated by a mechanosensory cell. As was observed in the uniporous pegs of the maxillary palps the tip of one dendrite formed a simply structured tubular body (Fig. 14) and terminated at the base of the peg. A close association a m o n g the dendritic sheath, tubular body, and cuticle of the peg was also observed (Fig. 14). The remaining dendrites extend into the lumen of the pegs, and terminate a short distance below the pore. Although thin sections were not obtained through the tip of the pegs, thick sections revealed a pore with a diameter of approximately 100 nm. Three sheath cells and their sinuses are associated with each peg. As was observed in the uniporous pegs of the palp, the outer sheath cells formed an incomplete sensillar sinus, leaving the terminal portion open to the surrounding hemolymph. Aporous sensilla. The distal surface of the galea also bear a c a m p a n i f o r m sensillum, 2 small aporous papillae, and a small spike-shaped peg (Fig. 1, Table 1). The campaniform sensillum and the 2 small aporous papillae are each singly innervated. Thick sections revealed that the dendrites are encased by thick stellate-shaped dendritic sheaths and terminate in highly refractile bodies (presumably the tubular body) in the centre of the cap of the c a m p a n i f o r m sensillum and near the base of the papillae. Externally, the c a m p a n i f o r m sensillum appears as a round convex cap of cuticle. Thin sections show that the cap consists of an outer exocuticular layer, a middle electron-lucid layer containing numerous groups of small tubules, and an inner electron-dense layer. The ring of raised cuticle, which is typical of most c a m p a n i f o r m sensilla (Mclver, 1975), is not present. The small spike-shaped aporous peg is innervated by 2 neurons, which terminate in the lumen of the peg (Fig. 15). There are at least 2 sheath cells associated with the dendrites. These cells are unusual in that they were observed to extend well into the lumen of the peg.

264

B.D.

DEVITT a n d J. J. B. SMITH

FJ~s. 11 - 14 o n p a g e 265.

Morphology of Mouthpart Sensilla of Euxoa messoria

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3. Labrum The dorsal surface o f the labrum bears 12 a p o r o u s socketed hairs, ( 1 3 0 - 330 ~tm long) and the inner surface (or epipharynx) bears 6 large dorsoventrally flattened a p o r o u s hairs ( 9 0 - 150 I.tm long) and 2 c a m p a n i f o r m sensilla (Fig. 16, Table 1). Each o f the hairs is innervated by a single bipolar neuron, which terminates in the socket o f the sensillum. A p p r o x i m a t e l y 2/3 o f the epipharynx is covered by non-innervated microtrichia. On each side, there is a small area free o f microtrichia, which bears a single c a m p a n i f o r m sensillum. The structure and innervation o f this sensillum are like those of the c a m p a n i f o r m sensillum described on the galea. N u m e r o u s multiterminal and several bipolar neurons were observed to terminate along the edges and in the middle o f the labrum. There are no specialized surface structures associated with these dendrites. 4. Labial palps The distal surface o f the labial palps bears a small a p o r o u s cone (8 Ixm long and 5 ~tm wide) on the medial side and a large a p o r o u s cone (22 I~m long and 5 ~tm wide) with a bulbous base (27 ~tm long and 6 I.tm wide) on the lateral side (Fig. 17, Table 1). Closely apposed to the side o f the large cone is a peg-like protuberance (Fig. 17), which is not innervated. The cuticle o f the cones is structurally similar to the walls o f the labial palps, except for the addition o f small bundles o f tubules, which appear to be continuous with the sensillar channel (Figs. 18, 19). A layer o f electron-lucid cuticle was also observed at the base o f each peg. Each sensillum is innervated by a single bipolar nerve cell. The dendrites o f these cells form several branches, which extend into the lumen o f the pegs. Distal to the ciliary region, the dendrites are encased by a dendritic sheath. Within the lumen o f the sensilla, the dendritic sheaths are closely apposed to the cuticle o f the cones (Fig. 18). In the small cone a cuticular sleeve (Fig. 19) envelops the dendritic sheath At least 2 sheath cells are associated with each dendrite. Microvilli from these cells were observed to extend to the base o f each cone. DISCUSSION From the ultrastructural evidence, 10 types o f sensilla have been identified on the maxillary palps, galeae, labial palps, and labrum o f larval E. messoria. T w o o f t h e s e - - the uniporous pegs and the multiporous plate sensilla - - appear to be c h e m o s e n s o r y and the remaining 8 are p r o b a b l y m e c h a n o s e n s o r y . The chemosensilla are localized on the galeae and the maxillary palps and are innervated by approximately two-thirds o f the observed neurons. With the exception o f the multiporous plate sensilla and the campaniform-like sensiUa, the type and n u m b e r o f sensilla f o u n d are generally similar to those described in

FIG. 11. Cross-section through digitiform sensillum, showing wall of palp (pc), and lamellae of dendrite (dl) surrounded by cuticle of sensillum (c). x 21,029. FIG. 12. Cross-section through campaniform-like sensillum in wall of 2nd segment of palp (pc) showing tubular body (tb) encased within a dendritic sheath (ds) and surrounded by an electron-lucid lamellated layer of cuticle (s), x 8,960. Flc~. 13, Transverse section through base of lateral peg of galea proximal to ciliary region of dendrites showing 5 dendrites (1 -5) surrounded by inner sheath cell (is). × 15,642, F[(i. 14. Cross-section near base of papilla of medial peg of galea, In this section dendritic sheath encases 4 unbranched dendrites and 2 branches of 5th dendrite (large arrows). Note electron-dense material of tubular body in dendrite 1 and area of apposition between cuticle of peg (p). dendritic sheath and tubular body (arrows). × 18,560.

266

B . D . DLvtrt and J. J. B. SMITH

FI6. 15. Cross-section through small aporous peg of galea, showing 2 dendrites (d) encased by a dendritic sheath (ds), a sheath cell (sc), inner cuticular ring (ic) and small bundles of tubules (arrows) in cuticle of peg. × 12,800. FfG. 16. Scanning electron micrograph o f inner surface o f labrum, showing 6 aporous hairs (1 - 6 ) , and clear areas free of microtrichia bearing a single c a m p a n i f o r m sensillum (arrows). x 90. FIG. 17. Scanning electron micrograph of labial palp, showing large segmented (s) and small (c) aporous cones and non-innervated peg (arrow). × 2,000. FiG. 18. Transverse section through shaft o f large segmented cone o f labial palp showing distal segments of dendritic branches (db), small bundles of tubules (large arrow) associated with lumen (1) of cone, and dendritic sheath (ds) which is closely apposed to cuticle of cone. x 18,200. F~a. 19. Cross section through small aporous cone o f labial palp, above distal termination of dendritic sheath and dendritic branches showing cuticular sleeve (cs), small bundles of tubules in cuticle (arrows) and their association with lumen of peg (1). x 23,660.

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other lepidopterous larvae (Dethier, 1937; Schoonhoven and Dethier, 1966; Albert, 1980; Reese and Carlson, 1974). The uniporous epipharyngeal sensiila, which are a characteristic feature of the chemosensory systems in many lepidopterous larva (Ma, 1972, 1976; De Boer et al., 1977; Albert, 1980), are not present in this species. The multiporous plate sensilla have not been described in other lepidopterous larvae. The presence of pores and pore tubules in the cuticular cap, the multiple innervation pattern, and the branching of the dendrites, suggest that they are olfactory sensilla. Unlike other insect chemosensory structures, these sensilla do not project above the surface of the surrounding cuticle but are instead completely embedded within it. The inner lamellated layer of the cap also appears to be unique to these sensilla and may represent an adaptation that serves to stabilize or strengthen the thin cuticular cap. In the SMPS, the terminal portions of the dendrites remain encased by the dendritic sheath. Although this is an unusual feature for insect chemosensory structures, it has been reported in at least one other insect, the cotton stainer Dysdercusfasciatus (Peregrine, 1972). A similar situation has been observed in some aquatic insects where certain chemosensilla are covered with an aporous cuticle that appears to allow penetration of stimulating molecules by a mechanism that does not require the aid of pores (Jez and Mclver, 1980; Slifer, 1970). The changes in thickness and density in the terminal segments of the dendritic sheath in the SMPS of E. messoria possibly reflect changes in the character of the sheath which would a c c o m m o d a t e a similar poreless-transport mechanism. The fine structure of the multi-innervated uniporous pegs of the galeae and maxillary palps are similar to uniporous contact chemosensilla of other insects (Zacharuk, 1980). In E. messoria, each of the medial and lateral uniporous pegs of the galea is innervated by 5 bipolar neurons. Four terminate below the apical pore and resemble the gustatory neurons of other insects and one (presumably mechanosensory) terminates in a simply structured tubular body at the base of the peg. A similar arrangement has been observed in these pegs in Manduca sexta, Philosinira cynthia ricine, Hyalophora brassicae, Galleria mellonella (Schoonhoven and Dethier, 1966), Mamestra brassicae (Wieczorek, 1976) and Pieris brassicae (Ma, 1972), while lshikawa (1965) determined that in B o m b y x mori the lateral peg was innervated by 4 and the medial peg by 5 chemosensory neurons and that no mechanosensory neurons were present. A detailed description of the ultrastructure of these sensilla is available for only one other lepidopteran larva, P. brassicae (Ma, 1972). The fine structure differs slightly in this species in that the chemosensitive neurons remained unbranched, the presumed mechanosensory neuron did not terminate in a tubular body, there was a ridge of cuticle observed to extend from the wall of the papilla into the lumen and abut the side of the dendritic sheath, and the outer sheath cells completely enclosed the dendrites within a closed sensillar sinus. Schoonhoven and Dethier (1966) noted that division of the chemosensory dendrites innervating these sensilla may occur in the distal portions of the outer dendritic segments. However, in E. messoria branching was observed to begin near the ciliary region. Scanning electron micrographs have indicated morphological differences among the pegs at the tip of the maxillary palps in Choristoneura fumiferana (Albert, 1980), lsia isabella (Hanson and Dethier, 1973), Manduca sexta (Schoonhoven, 1972), and Malacosoma americana (Dethier and Kuch, 1971). There were no differences observed between these pegs in E. messoria. Information about the innervation and fine structure of these sensilla in other species is not available. Electrophysiological responses observed

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in Hyalophora gloverii and M. sexta (Schoonhoven and Dethier, 1966), morphological evidence in lsia isabella, (Hanson and Dethier, 1973) M. americana (Dethier and Kuch, 1971) and M. sexta (Schoonhoven, 1972), and extirpation experiments with Porthetria dispar (Dethier, 1937) and B. mori (lshikawa, 1965) have indicated that the sensilla on the maxillary palps respond to olfactory stimuli. A contact chemosensory role has also been suggested (Dethier and Kuch, 1971; Schoonhoven, 1972; Albert, 1980) and is supported by the ultrastructural evidence presented here. In considering the number of gustatory neurons observed (8 in each galea and 28 in each maxillary palp) it appears that the maxillary palps represent a significant part of the gustatory system in these larvae. Sensilla similar to the digitiform sensillum in the wall of the maxillary palp appear to be present in at least 4 other lepidopteran species. Albert (1980) described a digitiform sensillum in Choristoneura fumiferana and scanning electron micrographs of the maxillary palps of P. brassicae (Ma, 1972) and Spodoptera exempta (Ma, 1976) revealed, but did not identify, sensilla, which closely resemble the digitiform sensillum described here for E. messoria. In Agrotis ipsilon, Reese and Carlson (1974) also observed a sensillum of this type but classified it as a campaniform sensillum. This sensillum is also similar to the digitiform sensillum found on the maxillary palps of several coleopteran species (Zacharuk et al., 1977; Honomichl, 1980; Guse and Honomichl, 1980) and to the protuberance found on the antennal cone of a larval mosquito, Toxorhynchites brevipalpis (Jez and Mclver, 1980). The digitiform sensillum of E. messoria differs from these in that there was no evidence of a pore or exuvial canal found (Zacharuk et al., 1977; Jez and McIver, 1980) and the dendrite did not extend to the tip of the sensillum (Zacharuk et al., 1977; Guse and Honomichl, 1980; Jez and Mclver, 1980). In addition, modifications to the terminal portions of the dendrites innervating these sensi[la are different. In E. messoria the dendrite is lamellated, while in coleopteran species the dendrite branches (Zacharuk et al., 1977; Guse and Honomichl, 1980), and in T. brevipalpis the tip of the dendrite is folded (Jez and McIver, 1980). Zacharuk et al. (1977) demonstrated that the digitiform sensilla of Ctenicera destructor are sensitive to mechanical forces such as contact or vibratory stimuli, while Jez and McIver (1980) suggest that the protuberance in T. brevipalpis is a chemosensory structure and Guse and Honomichl (1980) suggest that the sensilla are hygro- and thermosensory structures. The absence of any pores and the lamellated structure of the dendrite suggest that the digitiform sensillum of E. messoria is not chemosensory. Lame[lated dendrites have been postulated to function as infrared radiation receptors, photoreceptors and as internal chemosensory structures (Mclver and Siemicki, 1975; Corbibre-Tichan6, 1977) and have been demonstrated to be sensitive to temperature (Altner et al., 1978). Bipolar neurons terminating in a tubular body are characteristic of most insect cuticular mechanosensilla (McIver, 1975). This feature was observed in neurons associated with singly innervated sensilla, including the campaniform sensilla, the campaniform-like sensilla, the aporous papillae, and the hairs, and with multi-innervated uniporous pegs on the galeae and palps. Two forms of tubular bodies were observed. The first type is associated with the singly innervated sensilla and is characterized by an electron dense appearance. The second type were the simply structured tubular bodies, which appeared in neurons associated with the uniporous chemosensilla of the galeae and maxillary palps. These are characterized by the electron-dense patches in the cytoplasm and do not have the dense accumulation of microtubules common to most insect mechanosensory neurons (Mclver, 1975). Rice et al. (1973) suggested that the more highly organized and rigid the

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structure of the mechanoneuron the more highly sensitive it will be. The differences noted here could therefore reflect differences in the sensitivity a n d / o r response characteristics of the respective sensilla. In the c a m p a n i f o r m sensilla of the galeae and labrum, the construction of the trilayered cap and its connection with the tubular body are similar to these features described in other insect c a m p a n i f o r m sensilla (Mclver and Siemicki, 1975; Mclver, 1975). A characteristic feature of most c a m p a n i f o r m sensilla is the presence of a cuticular ridge around the cap (Mclver, 1975), although it may not always be present (Jez and Mclver, 1980); it was absent in the c a m p a n i f o r m sensilla on the mouthparts of E. messoria. The presence or absence of this ridge may be related to the flexibility of the cuticle (Jez and Mclver, 1980) or could also reflect differences in the sensitivities of the sensilla. Although the campaniform-like sensilla observed embedded within the walls of the palp lack any external cuticular features, they have several features c o m m o n to campaniform sensilla (i.e. the presence of a tubular body at the tip of the dendrite and the insertion of this body into specialized layers of cuticle). C a m p a n i f o r m sensilla are known to be sensitive to cuticular stress causing deformation of the cuticular cap (Thurm, 1964; C h a p m a n et al., 1973). Stimulation of the neuron may result from deformation of the tubular body (Thurm, 1964; Rice et al., 1973) or from shearing forces associated with the different densities in the layers of the cap (Mclver and Siemicki, 1975). The 2 innervated sensilla on the distal surface of the labial palps have structural features similar to both mechanosensory and chemosensory sensilla. Like mechanosensilla, the cones are singly-innervated, aporous and the dendrites are completely encased within the dendritic sheath. Along with these are features such as the branching of the dendrites, the extension of the dendrites into the lumen of the sensilla and the absence of a tubular body, which are structural characteristics of chemosensilla. Sensilla with similar features in other insects have been suggested to function as mechanosensory structures which may be sensitive to distortions over the entire length of the sensillum and not just deflection at the base (Jez and Mclver, 1980), or to changes in haemocoelic pressures (Zacharuk et al., 1977), and have been shown to be sensitive to mechanical stimuli (Zacharuk et al., 1977). The sensilla of the labial palps of one lepidopteran larvae, C. f u m i f e r a n a , have been shown to be mechanosensory (Albert, 1980). The small multi-innervated peg on the galeae also has features c o m m o n to both mechano- and chemosensilla. The location of this structure between the 2 large uniporous pegs suggests that it is unlikely that it would contact the leaf surface. This, along with the aporous character of the cuticle, indicates that it is probably not a chemosensory structure. The double innervation pattern and the absence of tubular bodies in the dendrites also suggest that this sensillum is probably not involved in mechanoreception. .4cknowledgements--We thank Dr. Susan B. Mclver of the Department of Zoology, University of Toronto, for her critical review of the manuscript and the National Science and Engineering Research Council of Canada for financial support, REFERENCES ALHErT, P. J. 1980. Morphology and innervation of mouthpart sensilla in larvae of the spruce budworm, Choristoneurafumiferana (Clem.) (Lepidoptera : Tortricidae). Can. J. ZooL 58: 8 4 2 - 51. AI1NEr, H., H. TICHY and t. ALTNEr. 1978. Lamellated outer dendritic segments of a sensory cell within a poreless thermo- and hygroreceptive sensillum of the insect Carausius morosus. Cell Tissue Res. 191 : 287 - 304.

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CHAPMAN, K. M., R. B. DucKrow and D. T. MORAN. 1973. Form and role of deformation in excitation of an insect mecbanoreceptor. Nature (Lond.) 244:453 - 54. CORBIERE-TICHANE, G. 1977. Donn6es et hypoth6ses sur la fonction du r+cepteur sensoriel en lamelles chez les col6opteres cavernicoles. Bull. Soc. Zool. Fr. 102: 3 1 - 8. DE BOER, G., V. G. DETHIEr and L. M. SCnOONHOVEN. 1977. Chemoreceptors in the preoral cavity of the tobacco bornworm, Manduca sexta, and their possible function in feeding behaviour. EntomoL Exp. Appl. 21:287 - 98. DETHtER, V. G. 1937. Gustation and olfaction in lepidopterous larvae. Biol. Bull. (Woods Hole) 72:7 - 23. DETr~IEr, V. G. and J. H. KUCH. 1971. Electrophysiological studies of gustation in lepidopterous larvae. I. Comparative sensitivity to sugars, amino acids, and glycosides. Z. Vgl. Physiol. 72:343 - 63. DeVITT, B. D., B. J. R. PmUOC;ENE and C. F. HIN~S. 1980. Effects of veratrine, berberine, nicotine, and atropine on developmental characteristics and survival of the dark-sided cutworm Euxoa messoria (Lepidoptera : Noctuidae). Phytoprotection 61: 8 8 - 102. GovIno, C. K. and J. W. T. D,~NDV. 1970. The innervation of the thorax of the adult milkweed bug Oncopeltus fasciatus (Dallas). I. The prothorax. Can. J. Zool. 48: 1 0 7 3 - 77. GUSE, G.-W. and K. HONOMICHL. 1980. Die digitiformen Sensillen a u f dem Maxillarpalpus von Coleoptera 11. Feinstruktur bei Agabus bipustulatus (L.) und Hydrobius fuscipes (L.). Protoplasma 103: 5 5 - 68. HANSON, F. E. and V. G. DeTHIER. 1973. Role of gustation and olfaction in food plant discrimination in the tobacco hornworm, Manduca sexta. J. Insect Physiol. 19: 1 0 1 9 - 34. HONOMICHL, K. 1980. Die digitiformen Sensillen a u f dem Maxillarpalpus von Coleoptera 1. Vergleichendtopographische Untersuchung des kutikularen Apparates. Zool. Ariz. 204:1 - 12. ISHI~AWA, S. 1965. Maxillary chemoreceptors in the silkworm, pp. 7 6 1 - 7 7 . In T. HAYASHI (ed.) Proc. Int. Syrup. Olfaction and Taste H. Pergamon Press, Oxford. JEz, D. H. and S. B. MClVER. 1980. Fine structure of antennal sensilla of larval Toxorhynchites brevipalpis Theobald (Diptera : Culicidae). Int. J. Insect Morphol. Ernbryol. 9:147 - 59. MA, W-C. 1972. Dynamics of feeding responses in Pieris brassicae Linn. as a function of chemosensory input: A b e h a v i o u r a l , u l t r a s t r u c t u r a l and electrophysiological study. Meded. Landbouwhogesch. Wageningen. 72 (11): I - 162. MA, W-C. 1976. Mouthparts and receptors involved in feeding behaviour and sugar reception in the African armyworm Spodoptera exempta (Lepidoptera : Noctuidae). Syrup. Biol. Hung. 1 6 : 1 3 9 - 51. MclvER, S. 1975. Structure of cuticular mechanoreceptors of arthropods. Annu. Rev. Entornol. 20:381 - 9 7 . M c l v e r , S. and R. S|EM|C~I. 1975. C a m p a n i f o r m sensilla on the palps of Anopheles stephensi Liston (Diptera : Culieidae). Int. J. Insect MorphoL Embryol. 4: 1 2 7 - 30. MclvEr, S., and R. Slemlcr~l. 1978. Fine structure of tarsal sensilla of Aedes aegypti (L.) (Diptera : Culicidae). J. Morphol. 155: 1 3 7 - 5 6 . PErEgrINE, D. J. 1972. Fine structure of sensilla basiconica on the labium of the cotton stainer Dysdercus fasciatus (Signoret) (Heteroptera : Pyrrhocoridae). Int. J. Insect MorphoL Embryol. I: 241 - 5 1 . REESE, J. C. and S. D. CARLSON. 1974. Scanning electron microscopy of final-instar larval mouthparts of the black cutworm, Agrotis ipsilon. Ann. Entomol. Soc. Amer. 67: 3 0 1 - 08. RicE, M. J., R. GALUN and L. H. F|NtAYSOn. 1973. Mechanotransduction in insect neurons. Nature (Lond.) 241: 2 8 6 - 88. ROSSI~nOL, P. A. and S. B. MclvEr. 1977. Fine structure and role in behavior of sensilla on the terminalia of Aedes aegypti (L.)(Diptera : Culicidae). J. Morphol. 151: 4 1 9 - 4 3 8 . SCHOONHOVEN, L. M. 1972. Plant recognition by lepidopterous larvae: pp. 8 7 - 99. In H. F. VAN EMDEN (ed.)

Insect~Plant Relationships. Syrup. R. Entornol. Soc. Lond. SCHOON~tOVEN, L. M. and V. G. DETHIEr. 1966. Sensory aspects of hostplant discrimination by lepidopterous larvae. Arch. Neerl. Zool. 16:497 - 530. SLtFE¢, E. H. 1970. The structure of arthropod chemoreceptors. Annu. Rev. Entomol. 15:121 - 4 2 . THURM, U. 1964. Mechanoreceptors in the cuticle of the honey bee: Fine structure and stimulus mechanism. Science tWash., D.C.). 145:1063 - 6 5 . WIECZOrE~, H. 1976. The glycoside receptor of the larvae of Mamestra brassicae L. (Lepidoptera : Noctuidae). J. Comp. Physiol. 106: 1 5 3 - 7 6 . ZACH,~ruK, R. Y. 1980. Ultrastructure and function of insect chemosensilla. Annu. Rev. EntomoL 25:27 - 4 7 . ZACHArUI,:, R. Y., P. J. ALBerT and F. W. BELl_AMY. 1977. Ultrastructure and function of digitiform sensilla on the labial palp of a larval elaterid (Coleoptera). Can. J. Zool. 55: 5 6 9 - 78.