The mouth is situated deep inside the preoral cavity, in the region of 

 the clypeus; it leads into the small oral cavity and through the short pharynx 

 into the esophagus, the proventriculus and the cardiac sphincter. The 

 esophagus is a thin-walled tube which is narrow inside the head and then 

 becomes broader; it functions as a crop before passing into the proventri- 

 culus. 



The proventriculus is the posterior thick-walled part of the stomodaeum; 

 it is not always well marked, but in some groups (Hydropsyche, 

 Phryganea) it is well marked also on the outside of the intestine 

 (Figure 28). The stomodaeum is lined internally with flat epithelium with 

 indistinct boundaries and a thick intima; at the base of the epithelium lies 

 a membrane surrounded in the esophagus by longitudinal musculature, a 

 layer of circular muscles and on the outside by longitudinal muscles. The 

 inner layer of longitudinal muscles of the proventriculus contiguous with 

 the membrane is absent and in the walls of the proventriculus, unlike in 

 other insects, the longitudinal musculature is situated outside the circular 

 musculature (Figure 28) (Noyes, 1915:38,40; Branch, 1922:266; Snodgrass, 

 1935:355). The musculature of the proventriculus is better developed than 

 that of the esophagus, and the intima is thicker. 



The intima of the anterior part of the esophagus is smooth; that of the 

 posterior part is covered with posteriorly directed denticles which prevent 

 the food moving back (Figure 29). In species of Hydropsyche, the 

 intima of the denticulate part of the esophagus is divided into six longi- 

 tudinal stripes, each of which bears six secondary parallel folds covered 

 with longitudinal rows of dense, fine denticles. These folds are retained in 

 the proventriculus but the denticles are absent; the surface of the folds is 

 strongly sclerotized, especially on the combs, forming 36 hard, dark brown 

 plates (Figures 30 — 32) called the teeth of the proventriculus; these teeth 

 are arranged in six groups and each group consists of five large and one 



35 small tooth; in profile, a large tooth appears triangular with a long base 

 and a low apex; the small teeth have the form of a rod (Figure 31 ). The 

 intima of the posterior part of the proventriculus of Hydropsyche 

 contains a ring of several rows of anteriorly directed fine denticles 

 (Figure 30); the anterior part of the proventriculus with its large teeth and 

 strong musculature crushes the food; its posterior part, with its fine 

 denticles acts as a sieve preventing the passage of large food particles into 

 the midgut. The cardiac sphincter or proventricular valve (valvula 

 cardiaca) is a fold of the inner wall of the stomodaeum projecting into the 



37 lumen (Figure 28). The intima of the sphincter is without denticles; the 



epithelial cells (those contiguous with the midgut) are higher than those of the 

 stomodaeum, and have distinct boundaries; there are circular muscles and 

 a layer of longitudinal muscles in the fold; when these muscles contract, 

 the sphincter closes the posterior end of the proventriculus; in the larvae 

 of Hydropsychinae this is affected by two ridges in the lumen of the 

 proventriculus, anterior to the cardiac sphincter (Noyes, 1915:35—36,40). 



The midgut forms a cylindrical sac or is wider in the middle; it 

 constitutes about half of the intestine (Figure 27). The midgut is lined 

 inside with columnar glandular epithelium; an intima is absent and there 

 is a peritrophic membrane (Figure 33) which envelops the food; according 

 to Martynov (1903) the peritrophic membrane of the larvae of caddis flies 

 is formed by the surface of the epithelium of the midgut. Among the 



31 



