24 THE PHYSIOLOGY OF INSECT SENSES 



forming cell; (3) the bipolar neuron; (4) the reniform neurilemma cell 

 (Haffer, 1921; Wigglesworth, 1933; Hsu, 1938). This group of cells 

 may be surrounded on all sides by the epithelium of the integument 

 or may lie sunken in a subepidermal position. In any case, the base- 

 ment membrane of the cells is continuous with the basement 

 membrane of the epidermis, so that the whole unit is walled off from 

 the body cavity except where the neuron enters. 



The different types of sensilla depart from this general picture in 

 details, and considerable variation exists among any single type from 

 one insect to another or from one part of an insect to another. The 

 most significant variations occur with respect to the number of 

 neurons and their relations with other parts of the sensillum. The 

 number of neurons associated with a sensillum may range from one 

 to more than fifty (Fig. 13) (Slifer, Prestage, and Beams, 1959). The 

 distal process, or dendrite, of the neuron may terminate at the 

 base of a seta or extend variable distances up the shaft. The exact 

 relations between the dendrites of the neurons and the cuticular 

 portions of the sensillum is imperfectly known despite extensive 

 studies (Snodgrass, 1926; Hsli, 1938; Vogel, 1923; Sihler, 1924; 

 Eggers, 1928; Debauche, 1935; Slifer, 1961). In almost all cases the 

 dendrite enters a sheath (not of neural origin) whose optical and 

 staining properties resemble in many respects that of cuticle. The 

 sheath, best described as a cuticula-like tube, has been variously 

 termed sense rod, scolopale, Stift, Stiftkorperchen, corps scolopoid, 

 Hiille, Chitineurium, etc. The exact relation of the dendrite to this 

 tube differs in the various sensilla. In chemoreceptors especially the 

 top (distal) end of the tube may open to the shaft of the hair or peg 

 so that the dendrites can extend the length of the tube and into the 

 lumen of the external process (Fig. 14). Or it may open to the outside, 

 in which case the dendrites pass through openings in the side of the 

 tube and continue in the lumen of the hair or peg (Fig. 13). 



In many mechanoreceptors the top of the tube is capped with 

 a dark-staining apical body. In some cases the dendrite terminates 

 within or just beneath this cap. This is the case with many tactile hairs 

 (Fig. 16) and campaniform sensilla (Figs. 9, 10, 30) and with most 

 chordotonal sensilla (Figs. 12, 48). In some cases, especially in 

 chordotonal organs, a long filament may extend from the cap to the 

 cuticle (Fig. 12). There are, however, chordotonal organs in which the 

 terminal filament is absent (Figs. 47, 48). 



Recent studies with the electron-microscope (Gray and Pumphrey, 

 1958; Gray, 1960; Slifer, 1961; Adams, 1961; Larsen, 1962; Dethier, 



