1910] Alimentary Canal of Psychoda Alternata 303 



These conditions of degeneration and regeneration of the 

 mid-intestinal epithehum are found in larvae previous to the 

 casting of the larval skin. Changes at this point take place 

 with very great rapidity so that it is easy to see how former 

 workers came to overlook, almost completely, this degeneration 

 and regeneration of the mid-intestinal epithelium at the larval 

 molting periods. 



In larvae, taken immediately after the casting of the skin, 

 the new epithelium of the mid-intestine is already developed. 

 At first the cells are low and stain deeply with iron haematoxy- 

 lin (Fig. 52, ep.). The nucleoli are large and stain deeply, 

 while the chromatin material is not distinctly made out. The 

 cytoplasm is coarsely granular and has the appearance of being 

 striated next to the lumen. The striate border is developed 

 early and appears even more distinct than in later stages when 

 the cells have begun to secrete (Figs. 52 and 53, st. b.) 



The contents of the mid-intestine, not cast off before meta- 

 morphosis began, is still found to be enclosed in the resistant 

 peritrophic membrane and occupies the central portion of the 

 lumen. Between this central mass and the new epithelium, 

 there is still to be found a great quantity of the finely granular 

 yellowish product from the degenerate epithelium. Occasion- 

 ally the sloughed off intima of the esophagus is found coiled 

 about in the mid-intestine, but the degenerate epithelium 

 of the mid-intestine is so completely dissolved that only the 

 slightest traces of it are to be found after the skin has been cast. 



In this way the epithelium of the entire mid-intestine, 

 including the caecal glands and the group of cells which produce 

 the peritrophic membrane, is renovated at each larval molt. 

 As soon as the metamorphosis is completed and the skin cast 

 off the larva resumes activity and begins to feed again. The new 

 cells of the mid-intestinal epithelium at once respond to the 

 demand made upon them for secretions and the larva enters 

 upon the next stage of its development. 



CONCLUSIONS. 



The alimentary canal of the larva of Psychoda alternata is a 

 generalized one in conformity with the habits of the larva. 

 Both the fore- and hind-intestine are relatively short while 

 the mid-intestine makes up at least two-thirds of the entire 

 length of the canal. 



