112 



AN INTRODUCTION TO ENTOMOLOGY 



The sequence of the different layers is as follows : a lining epithelium, 

 which is supported by a basement membrane, a layer of circular 

 muscles, a layer of longitudinal muscles, and a peritoneal membrane. 



The epithelium. — The epithelium of the mid-intestine is very con- 

 spicuous, being composed of large cells, which secrete a digestive fluid. 

 These cells break when they discharge their secretion and are replaced 

 by new cells, which are developed in centers termed nidi (Fig. 129, n). 

 The extent of the digestive epithelium is increased in many insects 

 by the development of pouch-like diverticula of the mid-intestine, 

 thes3 are the gastric coeca (Fig. 124, h). These differ greatly in num- 

 ber in different insects and are wanting in some. In some predaceous 

 beetles they are vilHform and very nimierous (Fig. 126, mi). 



The peritrophic membrane. 

 — In many insects there is a 

 membranous tube which is form- 

 ed at or near the point of union of 

 the fore-intestine and the mid- 

 intestine and which incloses the 

 food so that it does not come in 

 contact with the delicate epithe- 

 lium of the mid-intestine ; this is 

 known as the peritrophic mem- 

 brane (Fig. 128, pt). As a rule 

 this membrane is found in insects 

 that eat solid food and is lacking 

 in those that eat liquid food. It 

 is obvious that the digestive fluid 

 and the products of digestion 

 pass through this membrane. It 

 is continuously formed at its 

 point of origin and passes from 

 the body inclosing the excrement. 

 7 w 



d. THE HIND-INTESTINE 



The layers cf the hind-intes- 

 tine. — The layers of the hind-in- 

 testine are the same as those of 

 the fore-intestine described 

 above, except that a greater or 

 Jess number of circular muscles exist between the basement membrane 

 of the eoithelial layer and the layer of longitudinal muscles. The 



Fig 129. — Resting epithelium of mid- 

 intestine of a dragon-fly naiad; b, 

 bases of large cells filled with digestive 

 fluid; cm, space filled by circular mus- 

 cles, /m . longitudinal muscles ; n, nidus 

 an which new cells are developing (From 

 Needham). 



