SENSILL^ 



105 



They differ in various ways from the simple trichoid type just 

 described, and may become either scale-Hke, peg-hke or conical ; 

 or, the hair shaft may be wanting and the cuticular structure 

 represented by plates, papillae, discs, or flattened domes. In 

 other cases, their only external manifestation is marked by a pit 

 or a mere cuticular thickening. Whatever their external form 

 may be, they are all j^resumably derived from originally simple 

 hairs. Furthermore, they all exhibit the same general similarity 

 of internal structure in that they consist of a trichogenous cell, a 

 membrane cell and a sense cell. In many cases the distal process 

 of the latter becomes differentiated at 

 its apex into a sense-rod or scolopale, 

 which is directly attached to the inner 

 side of the cuticular portion of the 

 sensilla. The exact nature of this 

 structure is not entirely clear, but its 

 staining reactions and optical appear- 

 ance seem to indicate that it is 

 cuticular in nature. This belief receives 

 support from Sihler's observation that 

 the scolopales in connection with 

 trichoid sensilla; on the cerci of the 

 Acridian Gomphocerus rufiis are cast at 

 each ecdysis. Structurally, scolopales 

 exhibit considerable diversity ; they 

 may be mere caps or cones over the 

 apices of the sensory processes, or they become elongate with 

 their walls thickened by a variable number of longitudinal ribs 

 (I'ig. 50). The extremity of such a rod often contains a deeply 

 staining apical body, which is connected with an axil fibre 

 extending through the sensory process into the nerve cell itself. 

 In other cases, there is no apical body, and the axial fibre 

 terminates at the extremity of the scolopale. 



Most sensillae, in their simpler forms, pertain to the general 

 type just described. The chief exception is met with in the 

 visual receptors whose evolution is probably traceable originally 

 from mere localised areas of light-sensitive pigment. Visual 



Fig. 50. Scolopalae or ter- 

 minations of sensory pro- 

 cesses of A, a trichoid sen- 

 silla of Gryllus (from 

 Sihler), and B, a campani- 

 form sensilla of Dytiscus 

 (from Hochreiither). a. 

 Apical body ; «/, axial 

 fibre ; r, rib ; sd, sense 

 dome or cuticular covering 

 of sensilla. 



