NO. 



MORPI]OLOCY OF INSECT SENSE ORGANS SNODGRASS 



^3 



sue layers surrounding the epithelium and uniting the muscles. The 

 rectal sac has no sensory innervation, but the posterior straight part 

 of the rectum is provided with sensory networks of varicose fibers, 

 arising from multipolar cells, lying between the epithelium and the 

 muscular coat, and on the outer surface of the latter. On the ter- 

 minal part of the rectum, Orlov notes, besides these cells of Type 

 II, also bipolar cells of Type I, the distal processes of which go to 

 sense organs in the rectal wall (fig. 22). 



The brief descriptions of the sensory nerves given in this section 

 summarize practically all that is known of the sensory innervation of 

 insects. It is evident that much must yet be done in this field before 

 we can pretend to have anything approximating a comprehensive 



Fig. 9. — Various types of sensory innervation of muscles of ali- 

 mentary canal of Lamellicorn beetle larvae by nerves of stomatogastric 

 system (Orlov, 1924). 



A, B, from oesophagus of larva of Oryctcs. C, from ventriculus of 

 larva of Melolontha. 



knowledge of the subject, and we should feel grateful to the few 

 workers who have contributed what information we now possess. 



III. THE GENERAL STRUCTURE AND CLASSIFICATION OF 

 INSECT SENSE ORGANS 



The true sense organs of insects are more or less complex struc- 

 tures formed, in all cases, from a part of the body wall. Usually there 

 is a specialized cuticular element having the shape of a hair, peg, dome, 

 plate, or lens, and there are always cellular elements consisting of 

 specialized hypodermal cells, of which one at least is connected with 

 the end of a sensory nerve. Any study of the structure of insect 

 sense organs must, therefore, be based on an understanding of the 

 general structure of the body wall and the particular structure of its 

 cuticular appendages. 



