1910] Alimentary Canal of Psychoda Alternata 297 



The slender cells, of the region immediately caudad of the 

 caecal glands, have a coarse granular and finely vacuolate 

 structure. In some preparations a slight discharge of granular 

 secretions is to be seen although this is more frequently absent. 

 The striate border is only slightly developed. Between the 

 middle and distal margin of these cells are to be found vacuoles 

 of various sizes which inclose dark brown concretions (Figs. 

 19 and 47, con.) These concretions are spherical, almost com- 

 pletely filling the vacuoles and very dense. I have not found 

 them in any of the other epithelial cells of the mid-intestine 

 of this larva. They closely- resemble those found by Folsom 

 and Wells in the mid-intestinal epithelium of Collembola, and are 

 probably waste products which are secreted and held by these 

 cells until the molting period when they are discharged along 

 with the cells. If my interpretation of these concretions is 

 correct these cells have a definite excretory function. 



Passing forward to the anterior region of the mid-intestine 

 we find the cells which produce the peritrophic membrane. 

 These cells are especially active during feeding. They appear 

 long, club-shaped and become finely vacuolate on their inner 

 margin. From them is discharged a finely granular secretion, 

 which forms a narrow inner border to the cells (Figs. 18 and 47, 

 p. m.) It is difficult to tell just where the cell-body ends and 

 the layer of secretion begins. On the inner surface of this 

 border a portion of it hardens to form the thin uniform peri- 

 trophic membrane which closely resembles the chitinous intima 

 of the esophagus. From the reaction of these cells to stain it 

 is evident that they are secreting continuously; but the under- 

 lying border of secretion is especially prominent at certain 

 times. The course of most active secretion seems to pass along 

 the canal like a wave, following the entrance of a new supply 

 ■of food somewhat as found in dragonfly nymphs. By the time 

 the discharge of secretions in the posterior part of the mid- 

 intestine has reached a maximum it has begun to decrease in 

 the anterior portion. This is the case with the cells producing 

 the peritrophic membrane as well as those of the caecal glands. 



Changes at the Molting Periods. 



It is well known that, with each larval molt, the entire outer 



cuticula, as well as the intima of the fore- and hind-intestine and 



the tracheal system, are cast. Sharp ('95) maintains that the 



facilitation of growth by the casting of the chitinous coat is 



