112 THE ALIMENTARY AND EXCRETORY SYSTEMS [CH. 



taking the place of the lost cells. These new cells quickly develop 

 a new internal lamina to take the place of the lost peritrophic 

 membrane. The latter surrounds the food-mass, and remains 

 around the faecal pellets after all the nourishment has been extracted. 



The Process of Absorption. Voinov [187] shewed conclusively, 

 not only that it is the cells of the mid-gut alone which produce the 

 digestive ferments, but that these same cells can and do carry on 

 the process of absorption of the resulting products at one and the 

 same time. The cells of the mid-gut mature in longitudinal bands 

 separated by nests of younger cells. By feeding the larva with 

 methylen blue, the mid-gut was made to shew blue longitudinal 

 stripes along the lines of the mature cells, with clear areas in between. 

 By use of the same reagent, Voinov also shewed that, if the blood 

 becomes overloaded with nourishment, the cells of the mid-gut can 

 absorb nourishment from without, and pass it back into the lumen for 

 temporary storage. It may be inferred that they can deal similarly 

 with impurities in the blood, if the latter becomes surcharged with 

 them. Voinov also shewed, by a clever use of various colouring 

 matters, that the cells of the mid-gut can exercise the power of choice, 

 absorbing certain kinds and rejecting others. 



The Peritrophic Membrane. This peculiar formation has given 

 rise to much discussion. According to Voinov, the cytoplasm of 

 each separate cell elaborates an independent internal filament. 

 When secretion begins, these filaments fuse into a single complete 

 lamina, which is cast off, as already described, in the form of the 

 peritrophic membrane. The cells then proceed to form a new set 

 of filaments. Thus we see that the food pellets of the larva are only 

 at first slightly shorter than the length of the mid-gut, each being 

 enclosed in a complete peritrophic membrane, as the result of a 

 single act of digestion. After the extraction of nourishment, a 

 considerable contraction, of course, sets in, so that the ejected faecal 

 pellets are considerably shorter. 



It should be added that other observers deny the formation of 

 the peritrophic membrane in the manner described by Voinov, and 

 hold that it is formed entirely from the anterior annular zone, in 

 the form of a gradually descending sleeve. 



Mid-gut of the Imago. This differs from the mid-gut of the 

 larva in being exceedingly long (fig. 44) and in varying, during the 



