292 AfDKils Entomological Society of America [Vol. III^ 



with that of the four slightly eversible anal papillae, where it 

 becomes much thicker and the cells more vacuolate. The 

 chitinous intima of the rectum is much heavier than that of the 

 colon and its thickness increases toward the anus where it is 

 continuous with the chitin of the anal papillae (Fig. 2, a. p.) 



\|()KI'll()T.Or,T( \1, ( HANG lis IN Till;: I■:I■^rF^I•:I.I^^r OF THE 



MID-INTESTINE. 



In insects the changes, which the epithelium of the mid- 

 intestine undergoes, are associated either with the pouring out of 

 digestive secretions at the time of feeding or with the larval 

 molting periods. These changes are marked by total or partial 

 epithelial degeneration and regeneration. In the present work, 

 special efforts have been made to get as complete series of 

 larvae as possible in order to show the changes accompanying 

 feeding and molting. By means of carefully timed starvation 

 and feeding experiments, all desired stages associated with the 

 feeding period were easily obtained. But it was much more 

 difficult to get all the stages at the molting period, since the 

 morphological changes in the mid-intestine are largely completed 

 before the skin is cast and there is practically no forewarning 

 as to just when the transformation begins. Only by means of 

 a large series of specimens taken during the inactive period 

 preceding and following the casting of the skin was it possible 

 to get ^11 the desired stages of transformation. 



Changes Accompanying Feeding. 



The extent of degeneration and regeneration of the epithe- 

 lium of the mid-intestine, at the time of feeding depends upon 

 the type of secretion found in the epithelial cells. Where the 

 holocrine type of secretion prevails, partial or even total degen- 

 eration and renewal of the epithelium will follow each full 

 feeding, while with the merocrine type only slight or no renewal 

 will be found since the e])ithclial cells are not destroyed by the 

 discharge of secretions. 



The nymphs of dragonflies and adult water beetles, in which 

 may be found the holocrine type of secretion, are excellent 

 examples for the study of epithelial regeneration following 

 feeding. Needham ('97) found that, in case of dragonfly 

 nymphs, after prolonged fasts of two months, the cells became 

 so gorged with digestive secretions that the epithelium became 

 twice the height of the normal and when food was taken in 



