104 THE SENSE ORGANS AND REFLEX BEHAVIOUR 



with the hypodermal sense cells. In the light of this finding he 

 concluded that the cytons related with this nerve are located in 

 the deiitocerebrum, but no investigator has yet revealed the 

 existence of any sensory cytons within the central ganglia. The 

 subject is more fully discussed by Snodgrass (1926), to whose 

 paper the reader is referred. 



Trichoid Sensillae. The simplest type of sensilla is hair-like in 

 form, it is secreted by a special trichogenous cell and innervated 



by a bipolar sense cell (Fig. 49). The 

 sense cell, it will be observed, lies within 

 the hypodermis, and its distal process, 

 after penetrating the trichogenous cell, 

 ,sb passes to the base of the hair ; both 

 the sense cell and its distal process 

 are invested by neurilemma. Trichoid 

 "^ sensillge of this type are well described 

 by Sihler (1924) in his paper on the 

 sense organs of the cerci of various 

 insects. In many cases the tricho- 



^ ^ o- , 1 • I J ojenous cell contains a large central 



Fig. 49. Simple trichoid sen- *=" ^ 



silla from the cercus of vacuole in which is lodged the sensory 



Gryllus campestris (integu- process of the nerve cell. It would 



mental parts deep black). ^ i ^ . . 



c, chitogenous cell ; h, hypo- appear that the vacuolar fluid extends 



dermis ;>i, neurilemma ; ?w ^^-^^^ |.j^g cavity of the hair, and certain 

 nerve hbre ; sc, sense cell n - i- ■ i • i i 



and its process sp ; t, base kmds of stmiuli received outside the 



S-hwV '' ^''^"''^*'* ^^^""^ latter would be transmitted through 



the fluid to the sensory process. More 

 usually a triclioid sensilla exhibits a more elaborate structure in 

 that the hair is attached to its socket by an articular membrane 

 which is the product of a special membrane cell, distinct from 

 the principal trichogenous cell. Membrane cells, which are little 

 more than individualised hypodermal cells, may also be detected 

 in association with ordinary covering hairs of a non-sensory 

 character. 



Other Types of Sensillae. In the well-known classification of 

 Schenk most of the prevailing types of sensillae are grouped 

 according to characters afforded by their external cuticular parts. 



