1910] Alimentary Canal of Psychoda Alternata 295 



since but little real nourishment can be absorbed from the 

 already thoroughly digested content of the canal. 



With larvae that had fasted less than four days no marked 

 changes were found in the mid-intestinal epithelium. The food 

 content of the canal is sufficient, apparently, to sustain the 

 larva and to stimulate the cells of the epithelium to normal 

 activities for this period so that when a fresh supply of food is 

 taken in normal digestion is continued (Fig. 41). From the 

 results of these expreiments, which comprised upwards of fifty 

 separate lots of larvae, it was found necessary to turn to the 

 preparations of the normal feeding larvae, for the determination 

 of the type of secretion and the relation of its discharge to the 

 intake of food. 



For some time, the changes in the mid-intestinal epithelium 

 just preceding the casting of the larval skin were mistaken for 

 signs of an excessive production and storing of secretions, but 

 when series of larvae preparing to molt were studied the mistake 

 was detected. In this larva there are four sources from which 

 the digestive secretions of the mid-intestine are derived. 



As previously noted, the gastric caecae, usually present in 

 the higher Diptera, are replaced in Psychoda alternata by a 

 greatly reduced caecal gland which girdles the mid-intestine 

 in the region of the esophageal valve. The large cells of this 

 gland produce a yellowish digestive secretion similar to that of 

 the gastric caecae of other insects. During most active secre- 

 tion of these caecal glands, the cells become swollen and finely 

 vacuolate (Figs. 18 and 47, c. gl.) The pouch, into which the 

 secretion is poured, becomes so completely filled that its neck 

 appears to flare, leaving a wide exit of the secretion into the 

 lumen. During less active secretion and excretion the cells 

 become somewhat reduced, the vacuolation less distinct and the 

 pouch less distended with secretion or even almost empty. This 

 is one source of digestive secretion. 



A second source is the long stretch of very large cells caudad 

 of the esophageal valve (Fig. 2, ep.) These cells during active 

 secretion and excretion give off a finely granular secretion. 

 This secretion is not given off as globules or droplets but simply 

 oozes through the thin striate border of the large finely vacuolate 

 cells and comes to fill all the space between the epithelium and 

 the peritrophic membrane (Fig. 42). This is perhaps the most 

 abundant supply of digestive secretion found in the mid-intes- 

 tine of this form. 



