298 Annals Entomological Society of America [Vol. Ill, 



only of secondary importance. The discharge of the chitinous 

 coat is merely an outward sign that internal changes have been 

 going on. Eisig ('87) suggests that since chitin is a nitrogenous 

 substance, perhaps the casting of the chitinous coat is a means 

 of disposing of nitrogenous wastes, to which Sharp adds that 

 l)crhaps the same is true of carbonaceous wastes. If the casting 

 of the exuviae has an excretory function associated with it and 

 if the epithelium of the mid-intestine has an ectodermal origin, 

 as believed by some writers, then it will be easier to explain 

 certain morphological changes which occur in the mid-intestinal 

 epithelium at the time of molting. The striking morphological 

 changes undergone by insects at the time of pupation and emerg- 

 ing of the adult, are well known and have been given much 

 attention by workers, but there are similar although perhaps 

 less intensive changes at each larval molt, which have been 

 almost entirely overlooked. We shall now consider the changes 

 in the mid-intestinal epithelium at the time of molting. 



Folsom and Wells' ('06) work on epithelial degeneration, 

 regeneration and secretion in the mid-intestine of Collembola 

 deals primarily with the changes in the digestive epithelium at 

 the molting periods. The Collembola are among our most 

 primitive forms of insects, lacking Alalpighian vessels and devel- 

 oping without obvious metamorphosis. With this type of 

 development we should expect to find equivalent morphological 

 changes at the different molting periods. Conditions found in 

 the study of a form of this generalized type will not necessarily 

 be like those in higher forms but are suggestive for one making a 

 study of any more sjDecialized form. 



The normal epithelium of the mid-intestine of Collembola 

 was found to be composed of very large granular and finely 

 vacuolate cells. At the approach of an ecdysis the cells begin 

 elongating and the inner half becomes more vacuolate. As 

 this vacuolation continues, any concretions and gregarine 

 parasites present in the cells and about half of the nuclei migrate 

 from the more dense portions of the cells into the inner, more 

 vacuolate region. Then comes a cutting off of the inner half 

 of the epithelium. During these changes cell walls are obliter- 

 ated. A new striate border forms along the line of constriction 

 and the inner portion of the epithelium is cast into the lumen 

 where it is digested. To replace the nuclei lost by the sloughing 

 off, those remaining undergo mitotic division, cell boundaries 



