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Ultrastructure of the rostral sensory organs of the water bug, Cenocorixa bifida (Hungerford), (Hemiptera) Lo, S. Esther 1967

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THE ULTRASTRUCTURE OF THE ROSTRAL SENSORY ORGANS OF THE WATER BUG, CENOCORIXA BIFIDA (HUNGERFORD), (HEMIPTERA). fey S. ESTHER LO B. Sc. (General) The University of Hong Kong, 1964 B. Sc. (Special) The University of Hong Kong, 1965 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in the Department of Zoology We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA .June, 1967 In p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f the r e q u i r e m e n t s f o r an advanced deg ree a t the U n i v e r s i t y o f B r i t i s h C o l u m b i a , I ag r ee t h a t t h e L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . I f u r t h e r ag r ee t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by the Head o f my Depa r tmen t o r by h i s r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l no t be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . Depar tment The U n i v e r s i t y o f B r i t i s h C o l u m b i a V a n c o u v e r 8, Canada Da te 3ltM«. ( < U 9 " - i -Abstract The sensory organs in the transverse grooves of the dorsal labium of the water bug, Cenocorixa bifida, (Hungerford) (Hemiptera) were studied with the electron microscope. It was found that each sense organ i s supplied by a single, bipolar neuron, which, together with i t s sheath c e l l , forms a sensory unit. The dendrite of the neuron i s modified into various structures along i t s length; i t has a root system, two basal bodies, and an axial filament complex. These structures are characteristic of many mechanoreceptors and chemoreceptors in insects. The sheath c e l l surrounding the dendrite possesses many characteristic fine structures, such as the desmosomes and the microtubules. According to their ultrastructure and their location near the mouth opening, i t i s most likely that these sensory organs are chemoreceptors. The significance of the presence of the c i l i -ary regions in the dendrites of these organs i s suggested to be related to the regeneration of the distal portion of the dendrite which may be torn off during the process of moulting. The axial filament complex may also serve as an internal support in the dendrite. Table of Contents page Abstract i List of Diagrams i i List of Figures i i i Introduction 1 Materials and Methods 5 Results 1. Terminology (a) The Integument 7 (b) The Sensory Unit 9 2 . Ultrastr.ucture of the sensory unit (a) The soma and axon 14 (b) The root system 14 (c) The basal bodies 15 (d) The c i l i a r y region 16 (e) The sheath c e l l 17 (f) The neurofilament region 19 (g) The end organ 20 Discussion 23 Summary 39 Literature cited 40 List of symbols used in the figures and diagrams 47 Figures IA to 14c with explanations 48 - i i -List of Diagrams Diagrams page IA Dorsal view of a typical heteropteran head..... 2 IB Dorsal view of the labium of C. bifida 2 IIA & B Distribution of the sensory organs in the rostrum (after Benwitz 1956) 4 III Diagrammatic longitudinal section through a group of sensory neurons 9a IV Diagrammatic longitudinal and transverse sections through the sensory unit in the epi- and exo-cuticle. 11 V Diagrammatic longitudinal and transverse sections through the sensory unit in the endocuticle 12 VI Diagrammatic longitudinal and transverse sections through the sensory unit in the epidermis..... 13 - i i i -List of Figures Figures page IA. Cuticular structure of the dorsal labium 48 IB. Cuticular structure of the ventral labium 4-9 2A. Longitudinal section of a sensory unit in the epidermis 50 2B. Section through a dendrite emerging from the epidermis into the cuticle 51 3A. Section through a group of neurons 52 3B. Section through a group of axons 53 4A. Longitudinal section through the dendrite 54 4B. Section through the rootlet region 55 4C. Transverse section through the rootlets 56 5A & B Transverse sections through the distal basal body 57 5C. Longitudinal section through the proximal basal body 58 5D. Transverse section through the proximal basal body 59 6A. Transverse section through the cilium 60 6B & C Transverse sections through the cilium 61 7A. Section through the nucleus of the sheath c e l l .. 62 7B. Section through the sheath c e l l enclosing neurofilaments 63 7C Section through sheath c e l l enclosing c i l i a 63 8. Transverse section through the neurofilaments ... 64 9A. Transverse section showing characteristic structure of the cuticular sheath 65 9B. Longitudinal section showing characteristic structure of the cuticular sheath 66 - i v -Figures page 10A. Dendrites emerging from epidermis into the endocuticle 67 10B. Longitudinal section of neurofilaments in endocuticle 68 IIA. Longitudinal section through the terminal portion of the sensory organ 69 IIB. Section showing the structure at the base of the peg 70 IIC. Longitudinal section through the end of the sensory organ 71 IID. Transverse section through the end organ ....... 71 12A. Longitudinal section through the end organ ..... 72 12B. Transverse section through the cuticular sheath. 73 12C. Transverse section through a labial sensory groove, showing rows of dendrites in the exocuticle 74 13A. Section through the ci l i a r y region of a sensory unit near the mouth opening 75 13B. Section through the basal body of a dendrite in the same region 75 Ikk. Transverse section through a sense organ made up of a group of three dendrites 76 l4B. Transverse section through a similar type of sense organ 76 l4C. Transverse section through a sense organ made up of a group of five dendrites 77 Acknowledgment I would like to thank Dr. A.B. Acton for his encouragement and assistance during the course of this study and in the fi n a l thesis preparation. Thanks are also extended to Dr. G.G.E. Scudder for his generous supply of living specimens of the water bug, Cenocorlxa bifida, and for his advice in this study. I would also like to express my appreciation and gratitude to Mr. L. Veto for his guidance, advice and suggestions in the techniques in electron microscopy throughout this project. -1-Introduction Cenocorixa bifida i s a corixid which i s truly aquatic a l l through i t s l i f e . Most members of the aquatic heteropteran families, or Hydrocorisae, are predacious, possessing raptorial forelegs. However, the corixids differ considerably from the typical Hydrocorisae: they are detritus feeders, ingesting both f l u i d and particulate matter, their head and mouthparts being modified accordingly. The mouthparts of insects are composed of three gnathal somites, the mandibular, maxillary and labia l segments. In biting insects the jaw-like mandibles and maxillae are paired, while the labium, serving as a lower l i p , i s unpaired, broad and f l a t , and formed from the union of two maxilla-like appendages. In the Hemiptera the mouthparts are modified for a flu i d diet. The mandibles and maxillae form long, slender, un&egmented stylets. The stylet bundle, which i s supported and directed by a highly modified, beak-like labium (Diag. IA), pierces the tissue of the plant or animal upon which the insects feeds. The corixid labium i s much shortened and broadened, lacking a distinct four-part segmentation. Dorsally i t bears a deep, closed stylet groove flanked, in C. bifida, by a series of transverse sclerotized bands and grooves (Diag. IB). In the Corixidae the rostrum i s densely covered with sensory organs. Weber (1930) was among the f i r s t to describe it 6 o m e of these organs. Hsu (1937) described the sensory organs in the dorso-lateral lab i a l grooves in greater detail. Benwitz (1956) 2 a -Diagram IA Dorsal view of a t y p i c a l heteropteran head (after Parson, 1966). Four segments ( I, I I , I I I , IV) are present, a median s t y l e t groove (SG) i s also present. Diagram IB Dorsal view of the labium of C. b i f i d a . The labium i s shortened and broadened, lacking a d i s t i n c t 4-part segmentation. Dorsally there i s a deep s t y l e t groove (SG) flanked by a series of transverse s c l e r o t i z e d bands (TG). Two fleshy lobes (FL) surround the d i s t a l a p i c a l plate (DA). distinguished four or five different sensory organs in the grooves, and ten or more near the protruded stylets and other areas on the rostrum. However, a l l of the work6 mentioned above deal only with the terminal portion of the sensory organ in the cuticle of the insect. Because of the limited resolving power of the light micro-scope, i t i s d i f f i c u l t to study the structure of the rest of the sensory organ in the epidermis, and hence relate it to the cuticular portion. With the electron microscope i t i s now possible to study the ultrastructure of the entire sense organ, both cuticular and epidermal regions. Without accurate determination of the structure of the sense organs, l i t t l e or no progress i s possible on the studies of their function. In the present study, special attention was directed to the sensory organs in the labi a l grooves. Those in the other areas of the rostrum w i l l be only briefly reviewed. Each transverse band on the dorsal lab i a l surface i s in fact a much thickened sclerotized ridge with an associated groove of thin, soft cuticle. Each of these grooves has three to four rows of sensory organs (Mags. IIA & B). Since they are located near the mouth opening, the suggestion of their role as chemoreceptors by Benwitz (1956) i s an easily understandable one. However, their orderly arrangement in transverse rows may suggest that they have other functions besides that of chemoreception related to feeding. It was thus intended that by studying the ultrastructure of these sensory organs, i t would be possible to add new information to that we already possess and to support or correct earlier interpretations. - 4 a -Diagram II D i s t r i b u t i o n of the sensory orttans i n the rostrum (after Benwitz, 1956) A. Sensory organs around the mouth M, and those i n the f i r s t and second transverse grooves (1 & 2) are shown. B. Sensory organs i n one of the transverse grooves. -4-II B -5-Materials and Methods Cenocorixa bifida (Hungerford) was collected from White Lake, in the B.C. interior — the Green Timbers Plateau. They were transported to the laboratory in one-gallon thermos jugs half f u l l of lake water. They were then kept at 10°C. in constant temperature cabinets until needed. In a l l cases, specimens were used within three weeks of capture. The head of the water bug was fixed in 6% glutaraldehyde in phosphate buffer (pH 7.5)• This was followed by fixing in OsO^ (pH 7.3)• The fixed material was rinsed in 70% ethanol and dehydrated rapidly with ethanol. It was then embedded in Spon 812 according to the method of Luft (1961). Some heads were al&o embedded in Araldite. Sections were cut with a Porter-Blum micro-tome, and stained with uranyl acetate and lead citrate, each for a period of 3-5 minutes. Some sections were cut at a thickness of 0.5 to l.Ou, and were studied under the phase contrast microscope. The main di f f i c u l t y in studying the rostral sensory organs i s in sectioning, since there i s l i t t l e a f f i n i t y of the cuticles for the embedding medium, and the two always break apart. In studying the rostrum of C. bifida, there has been many attempts to overcome this problem: a very hard embedding medium has been used, and also the period of i n f i l t r a t i o n at lower temperatures has been greatly-lengthened. After dehydration and passing through propylene oxide, the tissues were embedded in Epon. These were le f t at room tempera-ture overnight, and then for another day at 37°0. The tissues were fina l l y transferred to 5o C. until polymerization was complete. However, this procedure did not completely overcome the problem -6-of separation between cuticle and embedding medium, though the chance of separation was decreased. Carbon-coated grids were used a l l through the study and they prevented, in most cases, the drifting apart of the already separated cuticle from the embedding medium. Owing to the separation, i t can be easily seen that i f hairs or fine projections were present in the insect rostrum, cutting through one of these when no embedding medium surrounds i t will be quite impossible. Thus in the following study, only a few sections through the part of the hair-like structures terminating the sense organs were obtained. Interpretation of these few sections, however, gave a clearer picture of the structure of the terminal portion of the sensory organ. -7-Results 1. Terminology (a). The integument Before describing the structure of the sensory organs i t i s important to clarify the terminology used for the insect integument. According to Lower (1956), the term integument denotes the unicellular layer with i t s internal limiting membrane and i t s outer secreted covering. The internal limiting membrane i s termed the basement membrane, and the cellular layer i s the epidermis or hypodermis. The external secreted part of the integument is the cuticle. The latter can be divided into two principal layers — an inner, relatively tMck, procuticle and a thin outer epicuticle. The procuticle i s composed essentially of protein and chitin. The epicuticle does not contain chitin but consists of lipoprotein, polyphenol and wax, and a cement layer (Hackman 1964). Protein in the epicuticle, and sometimes also in the outer layer of the pro-cuticle, i s sclerotized, that i s , i t becomes after each moult hard, dark and insoluble in water. When the outer region of the procuticle becomes sclerotixed, the hard, dark layer so formed i s known as the exocuticle, and the remaining inner soft portion i s known as the endocuticle. The endocuticle i s made up of a number of lamellae whose appearance i s due to the orderly arrangement of chit.in-protein f i b r i l s . However, the endocuticle next to the epidermis has no lamellate structure. Schmidt (1956) called this endocuticle the subcuticular layer and thought of i t as a cement holding the cuticle to the ce l l s . But according to Locke (1961) i t i s more probably newly secreted endocuticle in which fibres are not yet arranged in -8-order, and thus have a granular appearance under the electron microscope. The cuticle i s not uniform over the entire insect. Various parts of i t differ from one another i n appearance and in other physical properties. Generally, unsclerotized procuticle and soft cuticles have only two regions, the epicuticle and endocuticle. But in the regions where i t i s thick and hard, the cuticle shows distinctly three layers, the epi-, exo- and endo-cuticle. The dorsal wall of the labium of C. bifida has the three layers present (Fig. IA). In this region the exocuticle i s thick and homogeneous in appearance, with no evidence of lamellae. Only pore canals and sensory extensions through the cuticle interrupt this layer. This i s true of a l l insects in regions where greater mechanical strength i s necessary, for example, areas of the limbs, antennae, furcula, and area in the head capsule. A fully sclerotiged exocuticle i s absent from the ventral wall of the labium of C. bifida (Fig. IB). Below the dorsal lab i a l wall in C. bifida, the epidermis i s much thicker than the ventral epidermis of the labium. This thickening of epidermis appears to be associated with the presence of transverse ridges on the dorsal labium. I have also studied with the light microscope the heads of two other corixide Hesperocorixa  laevigator and Callicorixa audeni which have sensory grooves on their labial surfaces. Both were found to have a thickened epidermis dorsally. Cymatia americana, which lacks transverse rostral grooves, was also examined by me under the light microscope, and sections show few sensory organs and no thickening of the epidermis, that i s , the dorsal epidermis i s virtually identical with the ventral rostral epidermis. - 9 a -A 4 DIAG. IV PEG JTEPICUTICLE EXOCUTICLE CUTICULAR SHEATH ENDOCUTICLE -EPIDERMIS -f-EPf DERMAL v-ELL SHEATH CELL DENDRITE A GROUP O F S E N S i L L A - 9 -This examination suggests that the presence of rostral grooves i s correlated with the presence of a thickened epidermis. Further, there are more sensory organs present in these insects with rostral grooves, (b). The sensory unit Though a number of structure which appear to be sensory are f<3und near the mouth, most of the work which i s reported below concerns the sensillae of the transverse l a b i a l groove. The reasons for choosing these from the variety available were their preponder-ance and uniformity of structure. A detailed description of the ultrastructure of the sensillum w i l l be found below, but to appreciate the general arrangement of the parts, one should refer to Diag. I l l , which gives only the l/frarest outline. For convenience of description, the sensillum i s divided into three regions: (A) the terminal region lying in the exo- and epi-cuticle; (B) the central region lying in the endocuticle; and (C) the epidermal region in the epidermis. A more detailed structure of each of the three regions can be found in Diags. IV, V & VI. Each sensory unit in the labial groove i s made up of a single bipolar neuron with a short distal process and an axon which enters the central nervous system (Diag. III). The c e l l body or soma, that part of a neuron containing the nucleus and surrounding cytoplasm, l i e s in the epidermis. It i s thus termed a superficial sensory nerve c e l l , in contrast to the deep-lying c e l l body and long branching distal process of the higher invertebrates and the vertebrates. The axon i s a process of a neuron specialized to distribute or conduct nerve impulses, generally over a considerable -10-distance (Bullock and Horridge, 1965). The distal process extends out through the cuticle to enter into the terminal cuticular struc-ture. It i s termA«Si ;<J. the dendrite and i s specialised for receiving an excitation. Since the nerve c e l l i s i t s e l f a receptor, i t is also known as a primary sensory neuron. Along i t s length the dendrite i s differentiated into various regions. The portion of the dendrite lying in the exo- and endo-cuticle has neurofilaments along the whole length (Mags. IV & V). The part of the dendrite lying in the epidermis has four dis-tinguishable regions; a rootlet region, a basal body region, a c i l i a r y region and a neurofilament region (Diag. VI). Rootlets extend from the proximal end of the basal body down into the c e l l and end freely in the vicinity of the nucleus (Diag. VI). When examined in longitudinal sections the fibres of the rootlets haow prominent cross-striation of definite periodicity. There are two basal bodies, a proximal and a distal one. They are cyclindrical structures formed by nine triple outer f i b r i l s . The distal end of the basal body i s continuous with the axial filament complex. The latter has nine pairs of peripheral f i b r i l s , and two or more f i b r i l s may be present in the central region. As the cilium enters-the open inner end of a tubular cuticular sheath, the periph-eral and central f i b r i l s are replaced by numerous neurofilaments. The c e l l body and the portion of the dendrite in the epidermis are surrounded by additional sheath c e l l s . The sensory unit terminates in a peg, which i s a cuticu-lar specialization. This peg l i e s in a sunken pit in the transverse groove on the dorsal labium of the insect and surrounds the dendrite of the receptor neuron. The structure of the cuticle where the peg 11a Diagram IV Diagrammatic longitudinal sections and transverse sections through the distal dendrite in the cuticle. The end organ i s composed of a short, stumpy peg, with a modified cuticular base struc-ture. The socket consists of a rim (R) and pad (P) enclosing an inner cylinder of cuticle which in turn surrounds the cuticular sheath. Lamellae of soft cuticle (LS) connect the socket rim to the inner cylinder. (Terminology adopted from Noble-Nesbitt 1963). -11-D I A G J I V -12a-Diagram V Diagrammatic longitudinal and transverse sections through the sensory unit i n the endocuticle. Neurofilaments (NF) are enclosed i n the c u t i c u l a r sheath (CS). Around the sheath, the orderly arrangement of the f i b r i l s i n the lamina breaks up and the sheath i s surrounded by granules and some longitudinal f i b r i l s ( F l ) . Fin g e r - l i k e projections of the epidermis (F) are present, j u t t i n g into the Schmidt layer (SL). -12-D S A G . V _ -13a-Diagram VI Diagrammatic longitudinal and transverse sections through the sensory unit i n the epidermis. Transverse sections: A—through neurofilament region (NE) B—through c i l i a r y region (CI) C--through the d i s t a l basal body (B) D—through the root apparatus (RA) Desmosomes (D) between the plasma membranes of the neuron and the sheath c e l l (S) are charact-e r i s t i c of t h i s region. E—through rootlet region (RL) showing r o o t l e t s of the dendrite enclosed i n the sheath c e l l , whose nucleus (NUS) i s also shown. The axon (AX) and the nucleus (NU) of the neuron are also shown i n the drawing. -OA-i s located i s different from the remainder; the dendrite i s surround-ed by an inner cylinder and an outer rim of cuticle, with a pad in between the two (Diag. IV). The following i s a detailed description of the various regions of the sensory unit. 2. Ultrastructure of the sensory unit (>a). The soma and axon The soma or perikaryon i s that part of a neuron contain-ing the nucleus. The c e l l body i 6 limited by a thin membrane and is surrounded by the sheath c e l l (Fig. 3A). The c e l l cytoplasm shows a matrix of low electron density. Usually groups of nerve c e l l nuclei can be distinguished from the rest of the epidermal c e l l s . The axons l i e in groups below the epidermis. Each axon contains a large number of microtubules, and i s ensheathed by inter-locking cells (Fig. 3B). (b). The root system From the inner end of the basal body, rootlets extend down to near the nucleus of the neuron (Fig. 4A)« A root system i s found in many invertebrate and vertebrate motile c i l i a . In a l l cases they have been observed to have cross striations with a periodicity of 500-700 A*. In C. bifida there i s a major periodicity of 600-700X. The root region i s enclosed in a much folded sheath c e l l which has a characteristic structure. The apposed c e l l membranes have septate desmosomes throughout. Islands of cytoplasm surrounded may b$ c e l l membranes are exceptionally abundant and" group together to form lamellae (Fig. kB). These structures of the sheath c e l l w i l l be described later. - 1 5 -(c). The basal bodies The distal basal body i s a cylindrical structure at the inner end of the cilium (Fig. 4 A ) , about O.fyi long and 0.2JU in diameter. The wall i s composed of nine triple f i b r i l s arranged parallel to i t s longitudinal axis (Fig. 5A). From the triple f i b r i l s spoke-like structures radiate out (Fig. 5B). It i s from the inner end of this basal body that the rootlets arise. At a short distance below their origin, the rootlets surround another cylindrical structure, which closely resembles the distal basal body in size, shape, and structure. No rootlets, however, originate within this cylinder. This, according to S l i f e r and Sekhon ( 1 9 6 4 ) in their work on the antennal olfactory sense organs of the grasshopper, i s the proximal basal body. In longitudinal section (Fig. 5 C ) electron-dense substances are located outside the cylinder of the proximal basal body, with the rootlets closely surrounding them. In cross section (Fig. 5D) the inner cylinder is surrounded by an outer ring of electron dense material which i s in tarn encircled by the rootlets. The two basal bodies are in fact very similar to those in the auditory units in the locust (Gray, 1961). However, instead of the distal and proximal basal bodies, they are termed the c i l i a r y base and the root apparatus respectively, since no triple tubules are found in either structure. Two basal bodies are also found in the antennal sense organs of the milkweed bug (Slifer & Sekhon I963), but the proximal basal body l i e s not directly below the distal one but to one side. In the dendrite of C. bifida a distal basal body i s evident, but i t i s d i f f i c u l t to decide whether the term proximal basal body or root apparatus should be used for the second structure below the distal one. Triple f i b r i l s are never found in cross -16-sections of t h i s region, although i t has a very s i m i l a r astructure to the d i s t a l basal body i n longitudinal section. This w i l l be discussed l a t e r . (d). The c i l i a r y region The c i l i a r y region i s about l.l/i long and the diameter increases from base to apex. In the a x i a l filament complex one can i d e n t i f y nine peripheral double f i b r i l s , each containing two sub-f i b r i l s . At the base the diameter i s approximately 0.2u and the nine pairs of peripheral f i b r i l s appear to be joined up i n a ring of electron dense substance (Figs. 6A & B). Two or more f i b r i l s may occupy the cen t r a l region of the a x i a l filament complex. It i s i n t e r e s t i n g to note that departure from the usual number of f i b r i l s i n c i l i a and f l a g e l l a has been reported i n many cases, both i n vertebrates and invertebrates. Near the apex of the c i l i a r y region the diameter increases to 0.3yu by the spreading out of the peripheral f i b r i l s ( F i g . 6C). At the junction of the a x i a l filament complex and the d i s t a l basal body, the c e l l membrane of the dendrite i s closely applied to the cylinder formed from the peripheral f i b r i l s , so that a structureless region i s formed between the plasma membranes of the nerve c e l l and the surrounding sheath c e l l . Nearer to the base of the dendrite, the two membranes are apposed to each other with a gap of approximately 75- 1008. D i s t a l l y , t h i s widens out to the structure-l e s s region which, however, i s f i l l e d with fine granules (Figs. 6A, B & C). The c u t i c l e which forms the c u t i c u l a r sheath i s deposited i n this granular region close to the membrane of the sheath c e l l . At the lower l e v e l of the c i l i a r y region, the c u t i c l e i s discontin-uous, and thus seems to be the region where the c u t i c u l a r sheath -17-just begins (Fig. 6B). Higher up in this region, the cuticle i s thicker and the wall of the sheath i s almost continuous in outline (Fig. 6C). Groups of microtubules are abundant just outside the wall in the cytoplasm of the sheath c e l l (Figs. 6B & C). (e). The sheath c e l l The soma of the neuron as described above i s surrounded by a simple sheath c e l l , but the dendrites, on the other hand, are bounded by a loosely folded system which may represent the membranes of a single rolled-up sheath c e l l folded as in vertebrate myelinated nerve, but with many indentations. In Fig. 7A, a single dendrite containing rootlets is wrapped around by a sheath c e l l . In some cases, two dendrites, both containing c i l i a or neurofilaments, may be enclosed in a common sheath c e l l . (Figs. 7A & B). In sections the membranes are seen to be double. Due to the fact that when the membrane rol l s i t s e l f around the single nerve the plasma membranes become apposed to form pairs. Each unit membrane shows] the characteristic triple layer described by Robertson (1959)i that i s , two parallel dense lines and a clear intervening layer. Each pair of membranes i s separated by a space containing material of higher density that the surrounding cytoplasm of the sheath c e l l . The relationship between adjacent units of the invaginated membrane is essentially as has been described by Edwards and his co-workers and also by Gray (1961), Similar membrane structure i s also found in the peripheral nerves of Tenebrio (Smith, i960). Very often the spaces between the adjacent membranes are transversed by fine bars. These are the septate desmosomes f i r s t described by Wood (1959) in a study of Hydra. In C. bifida, septate desmosomes are found in great abundance in the rootlet region (Figs. -18-kB &4-C). They are not only found between the apposed membranes of the sheath c e l l , but also between membranes of the nerve c e l l and i t s surrounding sheath c e l l . The distribution of the septate desmosomes suggest that they are structures primarily concerned in adhesion. In a l l sections through the dendrites and the sheath cells there are present also small islands of cytoplasm surrounded by c e l l membranes. Some of these may be arranged in lamellae. These are most abundant in the root region as mentioned previously. They are in fact sheet-like and finger-like extensions of the sheath cell s , so that two neighbouring sheath cells become interdigitated. These are presumed to provide sites of firm attachment between the loose folds of the sheath c e l l and also between two neighbouring sheath c e l l s . Another aspect of the relation of sheath c e l l to nerve c e l l that deserves comment i s the presence of desmosomes on their apposed surfaces. This i s restricted to the region of the basal bodies (Figs. 5C & D). Each desmosome consists of localized areas of thickening of the c e l l membrane, on the cytoplasmic sides of the limiting membranes. The most profuse sheath c e l l inclusion consists of micro-tubules, which can be seen in varying concentrations in a l l regions of the c e l l , including the dendrite. They have an overall diameter of 2 2 0 - 2 7 0 X ; the wall i s about 55&, and the hollow core about 1 0 0 -lkO% thick. Their lengths are indeterminate, some probably well in excess of lyu. These microtubules are oriented primarily along the long axis of the nerve fibres, but others may be arranged in any direction. -19-( f ) . The neurofilament region The c i l i a r y structure of the dendrite finally loses i t s peripheral f i b r i l s and the only elements now recognisable within the dendrite are delicate neurofilaments. These neurofilaments are enclosed by an irregularly shaped cuticular sheath, which i s in turn enclosed by a much folded sheath c e l l (Fig. 8). Where the axial filament complex changes into neurofilaments, the structure-less region f i l l e d with fine granules is s t i l l evident outside the plasma membrane of the dendrite. But at a greater distance above this, the granular substance disappears, and the c e l l membrane of the dendrite i s now closely apposed to the cuticle of the sheath (Fig. 9A ) . The latter has a very characteristic structure, with a much folded wall. These finger-like projections may give the cuti-cular sheath a firmer grip in the cytoplasm of the sheath c e l l . When the neurofilaments begin to emerge from the epi-dermis to the Schmidt layer, the sheath c e l l disappears and the neurofilaments are enclosed in the smooth cuticular sheath only (Fig. 10A). As the cuticular sheath traverses the endocuticle (Fig. 10B), the orderly arrangement of the f i b r i l s in the lamina! breaks up and the sheath i s here surrounded by granules. This region becomes more pronounced in the exocuticle, where a large area of these gran-ules surrounds the cuticular sheath containing the neurofilaments (Figs. 11A & 12C). The cuticular sheath i s believed to be an invagination of the cuticle from the peg of the sensory organ. This i s present in a l l of the antennary sense organs of many insects described by Slif e r and co-workers. -20-(g). The structure of the sensory end organ The cuticular sheath in the outermost region of the exocuticle bordering the epicuticle becomes very narrow and the neurofilaments inside i t are reduced to a few, and run up into the peg (Fig. 11B). The structure of the cuticle where the peg is inserted i s modified. A very similar structure has been seen in the setal insertions of the insect, Podura aquatica (Noble-Nesbitt, 1963). The terminology used in the following description is adopted from that used by Noble-Nesbitt (1963). In C. bifida a socket i s present which consists of a rim surrounding an inner pad, overlain by an epicuticular 'articular membrane'. Structurally the rim i s made up of hard cuticle which resembles exocuticle. It has a tapering connection to the base of the inner cylinder which enclosed the cuticular sheath. This gives a solid foundation from which the peg juts out. The pad consists of lamellated cuticle running from the socket rim to the inner cylinder. In Fig. 11C these lamellae have a different pattern from those of the endocuticle. According to Noble-Nesbitt (1963), the lamellae in the socket of the seta stain differently in Mallory when compared to the lamellae of the endo-cuticle. This suggests a chemical difference between these two types of lamellae in the cuticle. The structural las well as chem-ic a l differences could well be associated with different mechanical properties such as elasticity. The rim i s further connected to the peg by means of the epicuticle which presumably i s a tough, inelastic membrane giving an additional suspension at this level between the peg and the socket rim (Fig. 12A). -21-In spite of many attempts to section the mouth parts of the water bug, i t proved d i f f i c u l t to obtain a longitudinal section through one of these pegs which includes at the same time the walls, the cuticular sheath, and the neurofilaments enclosed within. Thus i t i s impossible to describe how nerve endings are arranged and their subsequent fate in the peg. The following account i s an attempt to guess the possible structure from the micrographs. In Fig. 12A, i t i s evident that a peg has been present and that the cuticular sheath opens into i t . In other sections not shown here, a broken and indistinct peg whose length is about that of the narrow cuticular sheath i s often seen in the region of the end organ. In the thin-walled basiconic peg of the grasshopper (Slifer and Sekhon 1964), the cuticular sheath opens to the outside by a single pore at the base of the peg. Numerous neurofilaments enter into the lumen of the peg, pass through openings in the wall of the cuticular sheath and open to the outside through many small pores in the peg wallL distal to the single opening through which the cuticular sheath opens. This arrangement i s unlikely in the pegs of the water bug, since enough detail can be seen to make i t clear that they are dissimilar. The lumen of the cuticular sheath i s so narrow that only one or a few of the neurofilaments can pass through (Fig. 12B). It seems to be more likely that the cuticular sheath runs into the lumen of the peg and opens to the outside through a pore at the tip. This situation i s present in the thick-walled basiconic peg of the grasshopper (Slifer et. a l . , 1957)• The structure of the sensory unit described above 22-represents a t y p i c a l sense organ found i n the transverse grooves. Each sensory organ i s innervated by a single bipolar neuron, with i t s own sheath c e l l . However, i n the region anterior to the grooves at the t i p of the labium near to the mouth opening, other v a r i e t i e s of sensory organs are found. B a s i c a l l y t h e i r dendrites have the same structure, with a root region, basal bodies, and an a x i a l filament complex (Figs. 13A & B), but each sense organ may be innervated by two or more neurons. In F i g s . Ikk & B, two or three neurons are present. They have a d i f f e r e n t c u t i c u l a r sheath s t r u c t -ure and m i c r o v i l l i are present i n the sheath c e l l . In general, dendrites i n t h i s region are much larger i n diameter. In F i g . l4C, groups of f i v e dendrites are enclosed i n a common cu t i c u l a r sheath. A l l these sensory organs at the t i p of the labium w i l l be known as the a p i c a l r o s t r a l sensory organs, as d i s t i n c t from those sensory organs located i n the l a b i a l grooves. Benwitz (1956) can i d e n t i f y ( l i g h t microscope) ten or more diffe r e n t types of a p i c a l sensory organs i n Corixa punctata. -23-Discussion E a r l i e r workers regarded the sensory organs found i n the r o s t r a l area as olfactory organs (Benwitz, 1956). Receptor c e l l s s ensitive to chemicals are among the most important components of the insect's sensory system. Such c e l l s are designated as chemo-receptors, and the physiological processes which occur i n these c e l l s upon chemical stimulation are termed chemoreception. Insect chemoreception i s generally divided into two categories, analogous to o l f a c t i o n and gustation i n vertebrate animals (see Hodgson 1964). Olfactory sense i s defined as that mediated by chemical stimuli i n a gaseous state at r e l a t i v e l y low concentrations, and gustation, or contact chemoreception, as that mediated by chemical st i m u l i acting as l i q u i d s or solutions at r e l a t i v e l y high concentrations son;contact (Dethier & Chadwick 1948). However, t h i s d i s t i n c t i o n into o l f a c t i o n and gustation must be regarded as a matter of convenience, rather than a c r i t i c a l one. There i s s t i l l a t h i r d chemical sense, involved i n reactions to high concentrations of' chemicals, such as the so-c a l l e d i r r i t a t i n g compounds, and which mediate ; avoidance reactions. The p r i n c i p a l methods for investigating the chemical senses are many. They can be studied from observations on the behav-iour of intact animals i n the f i e l d , or under simulated f i e l d condi-tions i n the laboratory. The studies of von F r i s c h (1938) are an example of t h i s approach. The behavioural experiments reached a new l e v e l during the 1940's when Dethier and Chadwick (1948) carried out large-scale experiments defining the relationship between molecular structure and the effectiveness of chemicals applied to the t a r s a l chemoreceptors of the blowfly, Phormia regina. During the 1950's, a more precise measurement of the -24-responeee of chemoreceptor cells using electrophysiological tech-niques was achieved. The f i r s t sucessful application of the elect-rophysiological methods was an analysis of the function of single chemoreceptor cells in the labellar lobes of the blowfly, Phormift  regina (Hodgson et_al.,1955)• Electrophysiology now provides an indispensible aid in the study of single receptor cells, and even large populations of cells (Schneider, 1957). Another method of studying insect chemoreceptor i s by the detailed anatomy of the sensory cells involved. Development of the electron microscope and other superior histological tech-niques gave more detailed information about the anatomy of chemore-ceptors. In this present study, f u l l attention i s given to the anatomy of the receptor organs in the rostrum of the water bug. It i s apparent that anatomy alone cannot provide us with a definite function for a particular sensory organ. However, the structure of a sense organ can be compared with that whose function has already been established, and a probable function can be arrived at for this new organ. In C. bifida, the rostral sensory organs have a very similar structure to the typical gustatory receptors in insects. The best known gustatory receptors are those found in the longer labellar hairs of the f l i e s . These have unbranched, tapering dendrites about O.lyu in diameter at their d i 6 t a l end. Receptor cells thought to function as chemical sensilla have dendrites of relatively simple structure (Slifer et al.,1957).. Dendrites of olfactory receptors are more complex in structure. Receptors for the detection of food and water by the grasshopper (Slifer & Sekhon 1964) have dendrites which divide into several branches. Sutton (1951) has studied the behaviour of feeding -25-corixids. The f i r s t pair of legs of the corixids bear upon their surfaces a variable number of s t i f f setae, or palar pegs. These create a current of water containing detritus which passes ventral-ly , in an antero-posterior direction. When a large piece of food reaches the rostral apex, the palae hold i t there for a few seconds, after which the current i s retarded. Food i s held close to the mouth, thus f a c i l i t a t i n g the insertion of the stylets. Frequently after detrital feeding has ended, the palae clean the rostrum by removing small food particles which have been trapped by the rostral hairs. These are scraped by the palae and are then held against the mouth and the adhering food particles sucked into the alimentary canal. From the above account, i t i s highly probable that the api-cal rostral sense organs of the water bug are truly gustatory re-ceptors. However this i s unlikely to be the function of the sensory organs placed further up on the head and somewhat protected within the grooves. The association of the abundance of thses sense organs with the presence of the rostral grooves and the possession of an elaborated dorsal epidermis would seem to indicate some alternative function. At the present time this function i s unknown. There i s some indication that the thickened dorsal epi-dermis i s associated with osmotic and ionic regulation (Scudder & J a r i a l , unpublished). If this i s the true nature of the rostral epidermis in C. bifida, i t then seems quite likely that the sensory organs in the rostral grooves have an associated function. The labellar hairs of the blowfly, Phormia regina, are the most intensively investigated and well-known receptors. At the -26-present time i t i s certain that there are four neurons supplying each hair: one neuron i s specifically sensitive to sugars, one i s sensitive to monovalent salts, a third one i s sensitive to mechani-cal stimuli, and a fourth one i s a water receptor (Dethier, 1963). It i s possible that the sensory organs located in the labial grooves may be sensitive to different cincentrations of various compounds. By the lengths of the cuticular end organs of the sen-s i l l a in the labial grooves of Corixa punctata, Benwitz (1956) distinguished three or four types of sensilla. In studying the ultrastructure of the sensilla located in the same area in C.bifida there i s no distinction into various types, but the terminal por-tions are d i f f i c u l t to study with the electron microscope. This does not necessarily mean that they have identical function. Dethier (1961) shows that some labellar hairs of Phormia respond more vigor-ously to bending or to water than the others. He concludes that even though the labellar hairs may be supplied with the same four or more neurons they are not equally sensitive to a l l compounds. It follows that the sensilla in the l a b i a l grooves of C. bifida, though of the same structure, may react defferently to various ionic concentrations. It i s now generally agreed that chemoreceptor cells of insects are modified epithelial c e l l s , and that the central axon of the receptor i s formed by the ingrowth of an extension process to-wards the central nervous system, that i s , they are primary sensory c e l l s . Wigglesworth (1953) studied the sensory neurons in Rhodnius  prolixus (Hemiptera) and found that the four cells which together make up a sensory hair, the tormogen, trich'gen, neuron and sheath c e l l , are the granddaughter cells of a single epidermal c e l l . The inwardly growing process from the sense c e l l forms the axon which joins the f i r s t sensory nerve i t meets so that impulses can now -27-pass to the central nervous system. However, in C. bifida, of the four cells which usually make up a sensory unit in other insects, only the neuron and i t s sheath c e l l are clearly evident. The presence of the cuticular sheath surrounding the distal dendrite i s common to a l l cuticular sense organs in insects. According to S l i f e r et_al.(1959), this cuticular sheath i s secret-ed by the trichogen c e l l . The principal function of this sheath i s suggested to be a mechanical one. It serves to protect the dendrite from sudden movement of the internal organs which might tear i t loose from the body wall to which i t i s attached. And in the more complex olfactory sense organs, where numerous neurons innervate a single sense organ, the sheath serves to hold the distal dendrites together. There i s general agreement in the literature that the so-called cuticular sheath is of a cuticular nature. This i s based on the observation that i t i s shed with the cuticular exuvium at ecdysis, that i t stains the same way as does the cuticle, and that i t appears to be continuous with the cuticular covering of the sen-it sillum. Hsu (1933), based on the fact that i t resists treatment with sodium hydroxide, concluded that the cuticular sheath i s of a chitin-ous nature. However, in more recent studies, i t has been found that the cuticular sheaths of the wireworms resist treatment with a weak solution of potassium hydroxide (Zacharuk 1962). In this latter work the cuticular sheaths of the wireworm are observed to be strongly chromophilic to methylene blue when administered intra-vitally, un-like the surrounding layer of the cuticle. In the sensory cells of C. bifida this difference in property of the cuticular sheath from the ordinary cuticle i s con-firmed. In fingures showing these sheaths (Figs. 10B, 11A, B & D), -28-i t can be seen that they stain more darkly than the surrounding cuticle. The sheath just outside the apex of the c i l i a r y region, where i t originated, shows a characteristic, much folded outline (Figs. 6A, B & C). These characteristic sculptures of the wall of the sheath have not been described by any other authors studying insect sensilla. The cuticular sheath, besides having the mechani-cal function of holding the dendrite in place, may also serve as a selectively permeable membrane, which 'insulates' each neuron, or units of neurons, from the other, and also from the other tissues and fl u i d in the body. During the process of moulting, the sense c e l l retains i t s connection with the sensillum on the old cuticle. It i s suggest-ed that the filaments enable the old sense organ to function until an advanced stage in moulting. Richard (1952) observed in termites a period shortly before moulting during which the sensitivity of the insect i s greatly reduced, and this may be ascribed to the rupture of the distal filament. It i s also seen that the sensory c e l l areas are more resistant to moulting f l u i d . This may be due to the pro-tection given by the cuticular sheath. Although the process of moulting of C« bifida has not been studied, i t i s probable that in this insect the cuticular sheath surrounding the dendrite protects the latter from the action of the moulting fluid until late in the moulting cycle. A surprising finding of several recent studies of sense organs i s that the dendrite of the neuron frequently contains a ^  modified c i l i a r y structure. These are now known in many different kinds of receptors, such as photoreceptors, mechanoreceptors and chemoreceptors. In a hydromeduean, Polyorchis penicillatus, studied -29-by Eakin and Westfall (1962), the photoreceptor c e l l has a c i l i a r y structure, with nine pairs of peripheral f i b r i l s and two single ones at the centre. In the vertebrate eye, a cilium pushes outward at the growing point of the embryonic rod c e l l and develops a row of vesicles along i t s side. Eakin and Westfall (1959) also concluded that the reptilian third eye, or the parietal eye, i s evolved from a c i l i a r y structure. Sensory organs for the reception of vibration or press-ure, generally known as mechanoreceptors, show various c i l i a r y modification too. Illustration of this can be cited in the auditory sense organ of the locust (Gray 1961). The chordotonal sensillum in the xantenna of Drosophila melanogaster (Uga & Kuwabara 1965) has a structure very similar to that of the locust auditory sense organ. Many chemoreceptors in insects show c i l i a r y modifications also, e.g. in the plate organ of the honey bee antenna (Slifer 8c Sekhon i960), in sensory organs on the antennal flagella of the milk-weed bug (Slifer & Sekhon 1963), in grasshopper (same authors, 1964), and i n aphid (1964), as well;..as in the fleshfly (1964). The presence of a cilium in the dendrite in sensory organs of C. bifida indicates that i t also belongs to this category of typical sensory structure. In a l l these sensory organs, nine pairs of peripheral f i b r i l s are always present. However there i s some variation in the presence or absence of the central pairs and their number. Barnes (1961) reviewed types of c i l i a and.concluded that many sensory c i l i a , in particular light receptors, lack the two central f i b r i l s common to motile c i l i a . This type is usually associ-ated with two centrioles or basal bodies. The 9+2 system i s usually associated with a single basal body. However i t i s now obvious that -30-not a l l sensory dendrites can be included in the above c l a s s i f i -cations only. In the photoreceptors of ctenophores (Horridge 1964), c i l i a of both 9+0 and 9+2 patterns are found. There i s only one basal body present. In the study of the aesthetasc hairs (chemo-receptors) of the decapod crustacean Panulirus argus (laverack & Ar d i l l 1965), the number of the central f i b r i l s varies between 1 and 4, and these may be single or paired. In many of the insect chemoreceptors mentioned above, the central two f i b r i l s are absent, but in the fleshfly, some den-drites have two single f i b r i l s and an extra paired f i b r i l s in the centre, in addition to the nine peripheral pairs. In the thin-walled peg of the grasshopper antenna, vesicles of various sizes are present in the centre. In the present study, the dendrites in the sensory organs have 1-4 central f i b r i l s in the c i l i a (Fig. 6B). In some sections, two f i b r i l s are present in the central areas of the c i l i a , however, these f i b r i l s are smaller in diameter than the peripheral ones (Fig. 6C). A&sensory dendrite with a 9+0 c i l i a r y pattern could characterize a structurally primitive cilium, which, having failed to develop the structure associated with motility, s t i l l retains a primitive sensory or conducting capacity. Equally well, this pattern could characterize a structurally degenerate form, which, having lost i t s abi l i t y to move, has been functionally modified in the direction of sensory reception. The latter seems more probable. Those sensory neurons which have a 9+2 axial unit system, such as those of C. bifida may represent the fact that the central pair, for some reason or other, has not degenerated. -31-Cili a r y structure in the dendrite i s generally associat-ed with structures such as the root system and the basal bodies, In C. bifida, these two structures are present. The rootlet in this insect has a major periodicity of 600-700 X . This i s similar to that found in the sensory organ of the locust ear (Gray 1961) with a periodicity of 650-700 as well as that of the antennal sensory organ in the milkweed bug (Slifer & Sekhon I963), the periodicity of which i s 750 8. As suggested by Fawcett (1954), the striated character of these rootlets places them in the category of protein fibres, and they may perform a mechanical or contractile role in the c e l l . The variation in the period length could conceivably result from different degrees of stretching. In studying c i l i a r y movement in a protozoan, Roth (1958) suggested that rootlet f i b r i l s play a major role in c i l i a r y co-ordination by functioning as electrical conductors. In C. bifida, two structures similar to the basal bodies are present. The distal one, at the inner end of the cilium, has triple f i b r i l s in the wall of the cylinder, as typical of other basal bodies. It seems justified to c a l l this structure the distal basal body. Triple f i b r i l s , on the other hand, are never found in the proximal structure, but in longitudinal sections this i s very similar to the distal one. Two similar structures are present in the auditory sense organ of the locust (Gray 1961), and in the chordotonal sensillum in the antenna of D. melanogaster( Uga & Kuwabara 1965), and the authors named them the c i l i a r y collar and the root apparatus respectively. It i s l e f t undecided whether the proximal structure in the dendrite of C. bifida should be termed the root apparatus or the proximal basal body. In most cases in this -32-study, the term root apparatus i s used. In C. bifida the sheath c e l l surrounding the dendrite in the epidermis has many characteristic structures, as found in ; the sheath cells in other insects. Of these structures, the most pronounced one6 are the double membrane system, the normal desmo-somes, the septate desmosomes, and the cytoplamic microtubules. As described above, the double membrane i n the sheath c e l l of C. bifida i s formed when the sheath c e l l membrane rol l s i t s e l f around the nerve, and the plasma membranes become apposed to form pairs. Very often the spaces between the; adjacent mem-branes are traversed by fine bars. These, according to Wood (1959), are the septate desmosomes. These are also found in a large number of insect tissues: they are present in the plasma membranes of the epithelium below the cuticle of the greater wax moth, the yellow mealworm, and the boney bee (Locke I 9 6 I ) . Similar structures are also noted in the dendrites of the milkweed bug and the grasshopper (Slifer & Sekhon 1963, 1964). Septate desmosomes are also found in annelids (Hama 1959)» anemones (Grimstone e_t_al. 1958), and in echinoderm embryos (Balinsky 1959). In fact they are probably universally present in the invertebrates. Wood (1959) defines them as an adherent region between two plasma membranes which are joined together by parallel arrays of lamellae arranged at right angies to the surface. But by means of sections tangential to the surface of these desmosomes, i t i s now suggested (Locke 1965) that the septa are arranged in a hexagonal network. The walls pf the spaces are formed from three arrays of septa oriented at 120° to one another, giving an almost perfectly symmetrical pattern. Wiener (1964) studied the epithelial c e l l junctions of Drosophila salivary glands -33-and found that the junctional surfaces or septate desmosomes of these epithelial cells may be concerned with the permebility pro-perties of the c e l l attachment area. Another structure present in the sensory unit of C. bifida, which i s also present in a large number of invertebrates, i s the desmosome in the region of the basal bodies of the dendrite (Figs. 5C & D). Desmosomes are present in the dorsal giant fibres of the earthworm Eisenia foetida (Hama 1959)i and in the sheath cells encapsulating the neurons in the eighth nerve ganglion of the goldfish (Rosenbluth & Palay 1961). They are also present in the interdigitating neuroglial cells in barnacle photoreceptors (Fahr-enbach 1965). Such specialization of the surface has generally been interpreted as a device for maintaining cohesion of adjacent ce l l s . Since they usually join cells of the same type, i t has been speculated that they might be the morphological basis for the well known selective cohesion of like cells, which has been extensively studied by embryologists (e.g. Overton 1962). So :the formation of a desmosome seems to involve the co-operation between cojoined cell s , and i t has been considered likely that initi a t i o n of a half desmosome in one c e l l would induce^the formation of the complement-ary half in an adjacent c e l l of the same kind (Fawcett 1962). In the study of a leech nervous system (Coggeshall & Fawcett 1964), desmosomes are found to be present on the apposed surfaces of the gl i a or sheath cells and the neurons, as in those of C. bifida. This fact makes i t necessary to modify our views as to the type specificity of this mechanism of c e l l attachment. In C. bifida, microtubules are abundant in the sheath c e l l , as wellaas in the dendrite. Electron microscope studies of a -34-variety of invertebrate and vertebrate c e l l types have supported the postulate that the microtubules i s a universal cellular organ-el l e . Microtubules are found to occur in the axoplasm of many ani-mals, including coelenterates, annelids, arthropods, and vertebrates. When present in the axoplasm, microtubules are termed neurotubules by Bullock and Horridge (19-65). Cytoplasmic microtubules in both animal and plant cells have the same dimensions as the flagellar and c i l i a r y tubules. The spindle fibres of the mitotic apparatus (Kane 1962) also are tubules with a diameter of 180-220 2^  very close to that of the microtubules in C. bifida, 220 Studies on the fine structure of both the f i b r i l s of c i l i a and flagella (Pease 1963, Phillips 1966), and the cytoplasmic microtubules (Gall 1966, Ledbetter & Porter 1964), indicate that subttttits are present in the two kinds of tubules. This suggests that these two different organelles may have a similar function, that i s , they are contractile organelles. Another possible function of the microtubules i s that they may serve for intracellular conduction. Rudzinska (1965),studied microtubules in the feeding tentacles of a suctorian, and suggested that these organelles form a structural basis for the passage of microwaves of contraction which serve to conduct food material down the tentacle. Fukuda and Koelle (1959) suggested that acetylcholinesterase in the neuron i s synthesiz-ed within the endoplasmic reticulum, then transported by means of the microtubules to the surface of the c e l l . It i s s t i l l not certain i f microtubules are hollow structures (Winston et a l . 1966), and i t i s possible that intracellular conduction takes place in the manner . described by Rudzinska. A third possible function i s that micro-tubules may serve as intracellular supports . Since in C. bifida, -35-microtubules are abundant in the sheath c e l l cytoplasm, they may serve a l l the different functions. From the above discussion, i t i s obvious that the sheath c e l l in C. bifida serves to separate or to protect the epidermal region of the dendrite from the surrounding tissues or fluids. The characteristic structures of the sheath c e l l , such as the septate and conventional desmosomes and the microtubules serve various other functions in supporting or promoting the activity of the dendrite. Horridge and Bullock (I965) suggested that there i s a linear relat-ionship between the number of the sheath cells and the length of the nerve axon or dendrite. The data were interpreted as indicating that the sheath cells are indeed 'nurse cells' or 'auxiliary meta-bolic units'. It was proposed that the neuron, being incapable of hypertrophy, recruits sheath cells as auxiliary metabolic units whenever the demands for maintenance of the c e l l processes are i n -creased by reason of their length, ramification, or excessive acti-vity. Recent studies on the neurons and sheath cells suggested that the two components should be thought of as forming a single function-a l metabolic unit. After discussing the ultrastructure of the sensory organ °^ C. bifida, I shall now speculate on i t s possible function. In i960 when Gray gave a detailed description of the fine structure of a c i l i a r y region in the dendrites of the locust ear, i t seemed logi-cal to suppose that dendrites with a c i l i a r y process would be restrict-ed to structures which were receptors of vibration or pressure. Since both receptors would be directly concerned with movement or stretch-ing of the neurons, this would account for the presence of the cilium. Ciliated dendrites were then reported for many other sensory -36-organs in insects, which by previous experimental work were proved to be chemoreceptors. It i s thus possible for the dendrites of insect chemoreceptors also to have a c i l i a r y region. The presence alone of a cilium in a sensory neuron cannot therefore suggest the function of the organ. It i s now certain, at least in the sensory organs of some species of Hemiptera, Isoptera and Diptera ( a l l works by Sl i f e r and co-workers), that the dendritic endings of chemoreceptors are freely exposed to ai r . Whether a chemoreceptor may function when completely covered by cuticle, even specialized types of cuticle, is s t i l l not certain. Thus, opening of dendrites to the outside seems a more reliable criterion for identifying chemoreceptors. Hence in C. bifida, a c i l i a r y structure in the dendrite provided no evidence about i t s function as a chemoreceptor or a mechanoreceptor. A longitudinal section through one of the sensory pegs which includes the walls, the cuticular sheath and the dendrit-i c endings i s very d i f f i c u l t to obtain. However, a possible structure can be constructed from the electron micrographs. In Figs. 11B, 11C and 12A, i t i s obvious that the cuticular sheath continues to pass into the peg, and as shown in the results section (page 21), i t i s most probable that this opens at the tip of the peg. However, whether the dendritic endings continue into the narrow cuticular sheath and become exposed to the outside at the tip of the peg i s doubtful. In most sections (Figs. 11B & 12A-> i t appeared that the dendrite stops at the region where the cuticular sheath becomes narrow, and a few supportive or connective fibres serve to hold the dendrite close to the outer opening. -37-Now that the presence of c i l i a in nerve cells of both vertebrates and invertebrates i s now well established, i t i s obvious that one question w i l l be asked. What i s the function of the c i l i -ary structure in some of the neurons? The role of the cilium in the rods and cones of the vertebrate eye has been discussed by Brown, Gibbons and Wald (1963). They suggest that the ci l i a r y pro-cess may be mainly concerned with the development of the outer seg-ments pf the embryo rod c e l l , and their regeneration in the adult. The basal body i s necessary for the regeneration of the axial f i l a -ment complex, while the latter i s required for the regeneration of the outer segment of the rod. According to Sl i f e r and Sekhon (1964), need for a centre from which regeneration could start would be appar-ent when applied to the process of moulting in insects. During the earlier stages of the moulting cycle the distal processes of the dendrites may be retracted into the sheath, and they w i l l grow out later to make contact with the new sensory peg. On the other hand, i t i s possible that the dendrites are not retracted at the time of moulting, and are torn off. New processes would be required to grow out from the stumps of the old ones. Since in the sensory unit of C. bifida, the cuticular sheath originates from around the ci l i a r y region, i t may be possible that during moulting in the nymphal stages, the axial filament com-plex isftbrn of f. eTheT-basai bodyi.would then regenerate a new complex and the neurofilaments distal to i t . If only the neurofilaments are lost during moulting, the cilium sould be necessary for the regener-ation of the distal process of the dendrite. For species of insects which do not moult again after acquiring their sensory pegs, the -38-basal body and cilium would be needed for the development of the dendrites i n the f i r s t place. Another p o s s i b i l i t y i s that c i l i a i n neurons may serve as s k e l e t a l or supporting elements. The dendrites are delicate f i b r e s , and to have them stand erect i n the c u t i c l e , even i f the c u t i c u l a r sheath i s present, some sk e l e t a l supports would be needed. With the presence of c i l i a , these delicate structures might be able to remain intact during the violent changes i n i n t e r n a l pressure which occur i n an insect during locomotion, r e s p i r a t i o n , moulting and other normal a c t i v i t i e s . Since i t i s apparent that a c i l i u m i n the dendrite does not i n i t s e l f lend any wW/eight to the acceptance of a p a r t i c u l a r function for the sense organ, can these organs which are found i n the rostrum of C. b i f i d a be mechanoreceptors? According to Horridge and Bullock (1965) each mechanoreceptor has only one bipolar sensory neuron. This i s just the case observed i n the r o s t r a l sense organs. However, from the structure of the short, stumpy peg, i t would be a very i n e f f i c i e n t receptor for pressure or v i b r a t i o n . So, i f they are chemoreceptors, what i s the functional significance of t h e i r arrange-ment i n rows on the rostrum? No adequate answer i s available to t h i s question at present, but the abundance of a single type of sense organ i n a certain area i n insects may be used to explain the s e n s i t i v i t y of these insects to chemical s t i m u l i , since they may be the anatomical basis for summation e f f e c t s . -39-Summary The sensory organs in the transverse grooves of the dorsal labium of the water bug, Cenocorixa bifida were studied with the electron microscope. It was fo»nd that each organ i s innervated by a single bipolar neuron. The dendrite of the latter i s modified into various structures along i t s length; i t has a root system, two basal bodies and an axial filament complex. The last structure is unusual, consisting of nine pairs of peripheral f i b r i l s and one to four central f i b r i l s , as compared to the 9+0 or 9+2 system in the dendrites of other insect sense organs. Neurons are surrounded by their own sheath cel l s , which possess many characteristic fine structures, such as 'normal' and septate desmosomes, and microtubules. Other sensory organs having a modified c i l i a r y region in the dendrites are reviewed. Since the dendrite appears to open to the outside through a pore in the sensory peg, i t seems most likely that the sense organs in the grooves are chemoreceptors. Sensory organs, other than the type found in the trans-verse grooves, are located at the tip of the rostrum nearer to the mouth opening. These apical rostral sense organs are likely to be gustatory organs in the feeding of the insect. These structures are only briefly reviewed. -40-Literature cited Balinsky B. 1959 An electron microscope investigation of the mechanism of adhesion of the cells in a sea urchin blastulas and gastrula. Exp. Cell Res. 16: 429 Barnes B. G. 1961 Ciliated secretary cells in the pars distalis of the mouse hypophysis. J. Ultra. Res. 5: 453-67 Bensitz G. 1956 Der kopf von Corixa punctata 111. (Hemipter) Zoll. Jahrb. 75: Heft 3 Blatchley W.S. 1926 Heteroptera of Eastern North America. Nature Pub. Co. Indianapolis. Brown P.K., I.R. Gibbons & G. Wald 1963 The visual cells and visual pigments of the mud puppy. J. Cell Biol. 19: 79-106 Dethier V.G. 1954 The physiology and histology of olfaction in insects. Ann. N.Y. Acad. Sci. 58: 139-58 Dethier V.G. 1955 The physiology and histology of the contact chemo-receptirs of the blow f l y . Quart. Rev. Biol. 30: 348-71 Dethier V.G. 1961 Behavioural aspects of protein ingestion by the blowfly. Biol. Bull. 121: 456-70 Dethier V.G. 1963 The physiology of insect senses. Methuen, London. Dethier V.G. & L.E. Chadwick 1948 Chemoreception in insects. Physiol. Rev. 28: 220-58 Dethier V.G. & M.L. Wolbarsht 1956 The electron microscopy of chemosensory hairs. Experimentia 12: 335-37 Doolin P.F. & Bridge W.J. 1966 Ultrastructure of c i l i a and basal bodies. J. Cell Biol. 29: 333 -41-Dorey A.E. 1965 The organization and replacement of the epidermis in acoelous turbellarians. Quart. J. Micr. Sci. 106: 147-72. Duvall A.J., Flock A. & Wersall J. 1966 Ultrasturcture of sensory hairs. J. Cell Biol. 29: 497 Eakin R. M. & Westfall J. A. 1962 Fine structure of photoreceptors in Hydromedusan, Polyorchis penicillatus. Proc. Nat. Acad. Sci. 48: 826 Farquhar M.G. & Palade G.E. 1963 Junctional complexes in various epithelia. J. Cell Biol. 17: 375 Fawcett D.W. 1961 jIntercellular bridges. Exp. Cell Res. suppl. 8: 174-87 Fawcett D.W. 1962 Physiologically significant specializations of the c e l l surface. Circulation 26: 1105-25 Fawcett D.W. & Porter K. R. 1954 Fine structure of ciliated epithelia. J. Morph. 94: 221 Gall J.G. 1966 Microtubule fine sturcture. J. Cell Biol. 31: 639-43 Gray E.G. I960 The fine structure of the insect ear. Phil. Trans. Roy. Soc. Lond. B 243: 75-94 Grimstone A. V., Home R.W., Pantin C.F.A. & Robson E.A. 1958 The fine structure of the mesenteries of the sea anemone. J. Micr. Sci. 99: 523 Hama K. 1959 Some observations on the fine structure of the giant nerve fibres of the earthworm. J. Biophys. Biochem. Cyto. 6: 61-66 Hilton W.A. 1933 Nervous system and sense organs. J. Ent. Zool. 23:31. Fahrenbach W.H. 1965 The morphology of some simple photoreceptors. Z. Zellforsch. 66: 233-54 -42-Hodgson E.S. 1955 Problems i n i n v e r t e b r a t e chemoreception. Quart. Rev. B i o l . 30: 331-4? 1958 Chemoreception i n ar t h r o p o d s . Ann. Rev. Ent. 3: 19-36 1964 Chemoreception i n the P h y s i o l o g y of I n s e c t a . M. R o c k s t e i n (ed.) Academic P r e s s . N.Y. Horridge G.A. 1964 Presumed p h o t o r e c e p t i v e c i l i a i n a ctenophore. Quart. J . M i c r . S c i . 105: 311-17 H o r r i d g e G.A. & B u l l o c k T.H. I965 S p e c i a l r e l a t i o n s and components of nervous t i s s u e i n S t r u c t u r e and f u n c t i o n i n the nervous systems of i n v e r t e b r a t e s . Freeman & Co. Kane R.E. 1962 The m i t o t i c apparatus J . C e l l B i o l . 15: 279-83 L a n s i n g A.I. & Lamy F. 1961 F i n e s t r u c t u r e of the c i l i a of r o t i f e r s . J . Biophys. Biochem. C y t . 9: 799-812 Laverack M.S. & A r d i l l D.J. 1965 The i n n e r v a t i o n of the ae s t h e t a s c h a i r s of P a n u l i r u s  argus. Quart. J . M i c r . S c i . 106: 45-60 fcdbetter M.C. 8c P o r t e r 1964 Morphology of mi c r o t u b u l e s of p l a n t c e l l s S c i . 144: 872 Locke M. 1961 Pore c a n a l s and r e l a t e d s t r u c t u r e i n i n s e c t c u t i c l e . J . Biophys. Biochem. Cyt. 10: 589-618 1964 The s t r u c t u r e and formation of the integument of i n s e c t s , i n the P h y s i o l o g y of I n s e c t a Academic P r e s s , N.Y. 1965 The s t r u c t u r e of s e p t a t e desmosomes. J . C e l l B i o l . 25: 166-69 Loewenstein W.R. & Kanno Y. 1964 Study on an e p i t h e l i a l c e l l j u n c t i o n . J . C e l l B i o l . 22: 565 -43-Lower H. F. 1956 Terminology of the i n s e c t integument. Nature 178: 1355-56 — 1957 A comparative study of the c u t i c u l a r s t r u c t u r e of th r e e female mealy bugs. B i o l . B u l l . 113: 141-59 L u f t J.H. 1961 Improvement i n epozy r e s i n embedding method. J . Biophys. Biochem. Cyt. 9: 409 M i l l e r N.C.E. 1956 The b i o l o g y of the H e t e r o p t e r a Leonard H i l l L t d . Lon. No b l e - N e s b i t t J . 1963 The f u l l y formed i n t e r m o u l t c u t i c l e and a s s o c i a t e d s t r u c t u r e of Podura a q u a t i c a ( C o l l e m b o l a ) . Quart. J . M i c r . S c i . 1C4: 24~3-70 1963 The c u t i c u l a r and a s s o c i a t e d s t r u c t u r e s at the moult. Quart. J . M i c r . S c i . l O J K 369-91 Overton J . 1962 Desmosome development i n normal and a s s o c i a t i n g c e l l s i n the e a r l y c h i c k blastoderm. Develop. B i o l . 4: 532-48 1963 I n t e r c e l l u l a r c o n n e c t i o n s i n the outgrowing s t o l o n of Cordylophora. J * C e l l B i o l . 17: 661 Parson M.C. 1966 L a b i a l s k e l e t o n and musculature of the Hydr o c o r i s a e ( H e t e r o p t e r a ) . Can. J . Z o o l . 44: 1050-84 Pease D.C. 1963 The U l t r a s t r u c t u r e of f l a g e l l a r f i b r i l s . • J . C e l l B i o l . 18- 313 P h i l l i p s D.M. 1966 S u b s t r u c t u r e of f l a g e l l a r t u b u l e s . J . C e l l B i o l . 31: 635 Reese T.S. 1965 O l f a c t o r y c i l i a i n the f r o g . J . C e l l B i o l . 25: 211 Ric h a r d s A.G. 1952 S t u d i e s on Arthropod c u t i c l e 111. The antennal c u t i c l e of the honeybee. B i o l . B u l l . 103: 201 -kk-Robertis E. 1956 Morphogenesis in the retinal rods. J. Biophy. Biochem. Cyt. suppl. 2: 209-18 Roberton J. D. 1959 The ultrastructure of c e l l membranes and their derivatives. Biochem. Soc. Sym. 16: 3 Rosenbluth J. & Palay S. L. 1961 The fine structure of nerve c e l l bodies and their myelin sheaths in the eithth nerve ganglion on the goldfish. J. Biophy. Biochem. Cyt. 9: 853-77 Roth L.E. 1958 Ciliary co-ordination in Protozoa Exp. Cell Res. supp.,5: 573-85 Rudzdnska M. A. 1965 Tentacle structure in suctoria. J. Cell Biol. 25: 459 Satir P. 1962 On the evolutionary stability of the 9+2 pattern. J. Cell Biol. 12: 181 — 1965 Studies on c i l i a J. Cell Biol. 26: 805 Schmidt. E. L. 1956 Observations on the subcuticular/ layer in the insect integument. J. Morph. 66: 211 Shigekazu Uga & M. Kuwabara On the fine structure of the chordotonal sensillum in the antenna of D. melanogastefc J. of E. M. 14: 173-81 Slautterback D.B. 1963 Cytoplasmic microtubules I. Hydra J. Cell Biol. 18: 367-88 Slifer E.H., Prestage J.J. & Beams H.W. 1957 The fine structure of sensory pegs of grasshopper. J. Morph. 101: 359 — 1959 The chemoreceptors and other sense organs on the antennal flagellum of the grasshopper. J. Morph. 105: 1^ 5 -45-S l i f e r E.H. & Sekhon S.S. I960 The f i n e s t r u c t u r e of the p l a t e organs on the antenna of the honeybee. Exp. C e l l Res. 19: 410-14 1961 The f i n e s t r u c t u r e of the sense organs of the antennal f l a g e l l u m of the honey bee. J . Morph. 109: 551 1965 Sense organs on the ant e n n a l f l a g e l l u m of the s m a l l milkweed bug. J . Morph. 112: 165 1964 F i n e s t r u c t u r e of the sense organs on the antennal f l a g e l l u m of a f l e s h f l y . J . Morph. 114: 185-208 _ 1964 O l f a c t o r y d e n d r i t e s of the grasshopper J . Morph. 114: 393-410 g. Lees A.D. 1964 The sense organs on the antennal f l a g e l l u m of aphi d s . Quart. J . M i c r . S c i . 105: 21-29 S l i f e r E.H. 1961 The f i n e s t r u c t u r e of i n s e c t sense organs. I n t e r n . Rev. Cyt. 11: 129-59 Smith D. S. i960 I n n e r v a t i o n of the f i b r i l l a r f l i g h t muscle of an i n s e c t . J . Biophys. Biochem. C y t . 8: 487 Southwood T.R.E. & Leston D. 1959 Land and water bugs of the B r i t i s h I s l e s . F r e d e r i c k Warne & Co. L t d . London & N.Y. Sutton M.S. 1951 Food and f e e d i n g i n C o r i x i d a e Z o o l . S o c i e t . Lond. P r o c . 121: 465 T r u j i l l o - C e n o z 0. 1959 S t u d i e s on the f i n e s t r u c t u r e of the c e n t r a l nervous system of Pholus l a b u r s c o e ( L e p i d o p t e r a ) . Z. Z e l l f o r s c h . 49: 432-446 Wiener J . , S p i r o D. & Loewenstein W. 1964 S t u d i e s on an e p i t h e l i a l c e l l j u n c t i o n J . C e l l B i o l . 22: 587 - 46-Wigglesworth V.B. 1953 The origin of sensory neurons in an insect Quart. J. Micr. Sci. 94: 93-112 1959 The histology of the nervous system of an insect Rhodnius prolixus (Hemiptera). Quart. J. Micr. Sci. 100: 285-98 Winston A.A., Weissman A. & E l l i s R.A. 1966 A comparative study of microtubules in some vertebrate and invertebrate c e l l s . Z. Zellforsch. 71: 1-13 Wood R.L. 1959 Intercellular attachments in the epithelium of Hydra J. Biophy. Biochem. Cyt. 6: 343 Zacharuk R.Y. 1962 Sense organs of the head of larvae of some Elateridae (Coleoptera). J. Morph. I l l : 1-35 - 1962 Exuvial sheaths of sensory neurons i n the larvae of Ctenicera destructive J. Morph. I l l : 35-48 -47-List of symbols used in the diagrams and figures AM articular membrane AX axon B basal body C Cuticle CI c i l i a r y region CS cuticular sheath D desmosome ED epidermis EPI epicuticle EXO exocuticle F finger-like projections of the epidermis F l f i b r i l s in endocuticle outside the cuticular sheath FS fibrous structure in exocuticle G granular cylinder surrounding cilium IE inner cylinder IS island of cytoplasm enclp'8dd!ilxi.^:deuble;wimembj?aiie^:L-.lL'» L lamellae of sheath c e l l LS ^lamellae of soft cuticle at the base of the sensory peg MT microtubules NE neurofilaments NU nucleus of sensory neuron NUS nucleus of sheath c e l l P pad PC pore canal PG sensory peg R rim RA root apparatus RL rootlet S sheath c e l l SD septate desmosome SE sensory end organ SL Schmidt layer ~48a-Fig. IA Cuticular structure of the dorsal labium: Epi-, Exo- and Endo-cuticle are present. A sensory end organ (SE) i s present in the exocuticle. Pore canals (PC) traverse this region. x 17,600 - 4 9 a -Fig. IB Cuticular structure of the ventra labium. A very thin epicuticle i s present: there i s no exocuticle. Pore canals (PC) are present, x 12,500 -49--50a. Fig. 2A Longitudinal section of a sensory unit. This l i e s in the epidermal region, and the dendrite distal to the nucleus (A) i s differentiated into a root region (B), a c i l i a r y region (C), and a neurofilament region (D) -5 l a -F i g . 2B Dendrite in the form of neurofilament (NF) emerging from the Schmidt layer (SL) into the endocuticle (END). The neurofilaments are enclosed in a. cuticular sheath (CS)» In the immediate vicinity of the sheath, the orderly arrangement of the f i b r i l s into endocuticular laminae changes, so that the sheath i s either surrounded by 6ome cytoplasmic granules or long f i b r i l s running parallel to the axis of the dendrite (FI). x34,000. -51--52a-Fig. 5A Section through a group of neurons. The nuclei (NU) of the neurons are grouped together as distinct from the surrounding spidermal cells* X&750. -52-~53a-Figo 3B Section through a group of axons. A group of axons (AX) enclosed in sheath cells close to the soma of the neuron in the epidermis of the insect. NU i s the nucleus of the neuron. x 50,0006 -53--54a-Fig. 4A Longitudinal section through the dendrite, showing the origin of the rootlets from the inner end of the distal basal body (B). They then encircle the proximal basal bddy or.root apparatus (RA). Just above the distal basal body the dendrite assumes the structure of a cilium (Cl), and the latter i s soon broken down into neurofilaments (NF)» Desmosomes (D) are characteristic of the region of dendrite containing the basal bodies. x 58,000. -55a-Fig. kB Section through the rootlet region showing a much folded sheath c e l l . Septate desmosomes (SD) and microtubules (MT) are abundant. Lamellae (L) representing interdigitation with neighbouring sheath c e l l are also present. x 50,000. -55--56a-Fig. kC Transverse section through the :rootlet region. A double membrane system i s evident in this section. Each i s formed from a pair of unit membranes closely apposed to each other, leaving a gap of about 75-100 A* in between. This gap i s f i l l e d with material of higher density than the surrounding cytoplasm. A double membrane i s not only formed between the membranes of the folded sheath c e l l , but also between the latter and that of the nerve c e l l . x 57*600. - 5 6 --57a-Fig. 5A Transverse section through the distal basal body. Triple f i b r i l s are clearly distinguishable. This section also cut through the structureless region f i l l e d with very fine granules (G). This shows that i t i s cut through a region near the axial f i l a -ment complex, x 62, 400. Fig. f?B Transverse section through the distal basal body, at the level slightly below that in Fig. 5A. Dense materials are condensed outside the f i b r i l s and radiate out like the spokes of a wheel. The cuticular sheath (CS) i s not continuous in outline, x 55,000 -58a-Fig. 5C Longitudinal section through the proximal basal body or root apparatus (RA). It has a structure and sixe very similar to that of the distal one. However no rootlets arise from i t s inner end* Desmosome" (D) i s present between the apposed . membrane of the dendrite and the sheath c e l l * x 57,000. -58--59a-Fig. 5D Transverse section through the proximal basal body or root apparatus (RA). There i s a dense inner ring which i s surrounded by an outer ring of electron dense material. RL represents one of the rootlets encircling the root apparatus. x 50,000. -59--60a Fig. 6A Transverse section through the c i l i a r y regiin. This shows different regions of the insect integument, the epidermis (ED), the finger-like projections (F) from the epithelium into the Schmidt layer (SL) and the endocuticle (END) above i t . The cilium l i e s in a structureless region f i l l e d with fine granules (G), and the cuticular sheath (CS) i s discontinuous. x 36,500. -6 l a -Fig. 6B Transverse section theough the c i l i a r y region. Nine peripheral paired f i b r i l s and three central ones are evident. Of the three at the centre, two are together to form a pair and the third one i s a single one by i t s e l f . Microtubules are abundant around the cuticular wall. x 78,000. Fig. 6C Transverse section through the c i l i a r y region a short distance above the one in Fig. 6B. Here the diameter of the axial filament complex i s much increased, by the spreading out of the paired peripheral f i b r i l s . Two central f i b r i l s are present in this section, smaller in size than the outer. In both sections, the cuticle (C) i s deposited in the granular structureless region (G), next to the membrane of the sheath c e l l . x 78,000. -62a-F i g . 7A Section through the nucleus of the sheath c e l l (NUS). The dendrite containing rootlets (R) i s wrapped around by a protrusion of the sheath c e l l . The dendrite is lodged in the notch of the nucleus of the sheath c e l l . x 40,000. -63a-Fig. 7B Transverse section through the neurofilament region of the dendrite, showing two dendrites enclosed in a common sheath c e l l . x 50,000. Pig. 7C Transverse section through the ci l i a r y region of the dendrite, showing two dendrites enclosed in a common sheath c e l l . x 32,400. -63--64a-Fig. 8 Transverse section through the neurofilaments. The cuticular sheath becomes smoother in outline. x 72,000. -64-- 6 5 a -Fig. 9A Transverse section through the neurofilament region. In the upper one the structureless region containing fine granules has completely diaappeated." The wall of the cuticular sheath (CS) has many finger-like projections. x 5 4 , 8 0 0 . - 6 5 --66a-Fig. 9B Longitudinal section showing the cuticular sheath with characteristic structure. Microtubules are abundant. x 45,600. -66--67a-Fig. 10A Longitudinal section showing the emerging of dendrite from the epidermis (ED) into the Schmidt layer (SL). Note that sheath c e l l i s absent when the dendrite is in the Schmidt layer. r:;33t6oo. -67--68a-Fig. 10B Longitudinal section showing neurofilaments in endocuticle (END). The formation of endocuticular laminae i s inhibited, so that the cuticular sheath is surrounded by unorientated fibres or granules. x 28,500. -69a Fig. 11A Longitudinal section through the teeminal portion of the sensory organ. The cuticular sheath becomes harrow when i t enters the inner cylinder (IE). The socket consists of a rim (R) surrounding the inner pad. The rim has a tapering connection to the base of the inner cylinder. Lamellae of soft cuticle (LS) run from the socket rim to the inner cylinder. • x 15»500. -69-70a-Fig. 11B Section to show the structure of the ease of the peg, Ai.rim (R) surrounds an inner pad (P), The narrow cuticular sheath i s surrounded by an inner cylinder (IE), Articular membrane (AM) i s present. x 50,000. -71a-Fig. 11C Longitudinal section through the end of the sensory organ, showing lamellae of soft cuticle running from the socket rim to the inner cylinder. x 13,000. Fig. 11D Transverse section through the end organ,showint that the innermost ring of the cuticular sheath is encircled, by the inner cylinder (IE), the latter i s in turn surrounded by the socket rim (R), with the pad (P) lying between them. x 30,000. -72a-Fig. 12A Longitudinal section through the end organ. An articular membrane (AM) of epicuticle over-lies the rim (R) and pad (P). The cuticular sheath opens into the peg (PG) which i s broken off in this section. The size of the peg can be visualized. x 40,000. -73a-Fig. 12B Transverse section through the cuticular sheath. The lumen of the sheath into the peg i s so narrow that only a few neurofilaments would.be able to pass through i t to enter into the peg. Refer to Fig. 11B for symbols. x 42,000. -73--74a-Fig. 12C Transverse section through a labial sensory groove, showing rows of dendrites (in the form of neuro-filaments NF) surrounded by granular substance in the exocuticle (EXO). The section also cut through a portion of the peg near the epicuticle (EPI). In this a cuticular sheath (CS) i s surrounded by the peg wall. x 18,000. -74--75a-Fig. 13A Fig. 13B Section through the c i l i a r y region of a sensory unit near the mouth opening. Two c i l i a (CI) are present in a common cylinder of fine granules (G) surrounded by a cuticular sheath (CS). x 64,000. Section through the basal body of a dendrite in the same region as above. It has a structure similar to the type found in the dendrite of the sensilla within the transverse groove. Microtubules are abundant both in the dendrite and in the sheath c e l l . x 76,000. -76a-F i g . IkA Transverse section through a sense organ made up of a group of three dendrites. The wall surround-ing the dendrites i s quite d i f f e r e n t from the cuticul a r sheath of the groove sensillum. Micro-v i l l i are abundant i n the surrounding sheath c e l l . x 16,000. F i g . 14B Transverse section through a similar type of structure. x 50,000. -76--77a-Fig. IkC Transverse section through a sense organ made up of a group of five dendrites. These are enclosed in a common cuticular sheath (CS). 

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