NO. 8 MORPHOLOGY OF INSECT SENSE ORGANS SNODGRASS 



31 



that some invertebrate animals possess sense cells in the skin with 

 no accessory structures. The common earthworm, for example, has 

 specialized innervated cells in its hypodermis, which are regarded as 

 being the receptive organs through which the creature receives a 

 stimulus from light suddenly thrown upon its body. Insects are not 

 known to have sense organs of so primitive a nature, though the 

 photoreceptive tissue of eyeless Dipteran larvae that respond to light 

 has not been specifically determined. The chitinous parts of many 



SCls 



^ Nlm' 

 Jd Nv 



Fig. 14. — Showing the relation of the sense cells of a sensillum to 

 the nerve and to the cuticular part of the sense organ. 



A, two sense cells in an organ of the antenna of a wasp, stained by 

 the Golgi method, each cell with a distal process (d) to the cuticular 

 part of the organ, and a proximal process (p) continuous with a sen- 

 sory nerve liber (Vogel, 1923). 



B, diagram of a sensillum containing a group of sense cells (SCls), 

 the distal processes {d) ending in sense rods {SR) attached to cuticu- 

 lar part by terminal filaments (f), the proximal processes (p) con- 

 tinuous with nerve fibers. 



insect sense organs, however, are reduced to mere points of the 

 cuticula where the cellular elements are attached. 



The sensory cells in the sense organs of adult insects are probably 

 in all cases specialized liypodermal cells ; that this is the usual case, 

 at least, has been amply proven by the observations of many investi- 

 gators on the development of diverse types of sense organs. Whether 

 the sensory nerve fiber, however, is a product of the sense cell, or 

 the axon of a nerve cell located elsewhere, may be regarded as still 

 an open question, though the evidence, presented in the last section 

 of this paper, appears to favor the second possibility, at least in the 



