110 



THE ALIMENTARY AND EXCRETORY SYSTEMS 



[CH. 



be borne in mind that the normal state (A) represents a state of 

 fasting. This is broken at irregular intervals by large and rapid 

 meals, when the larva is fortunate enough to secure a victim. The 

 larva can fast for long intervals without any danger of starvation 1 . 

 In fact, as will presently be seen, the longer the larva fasts, the 

 bigger becomes its capacity for dealing with the next meal when it 

 does eventuate. 



ib 



m 



B. 



bfn 



Fig. 49. Part of epithelium of mid-gut in larva of Aeschna sp. ( x 330). A. During 

 period of rest. B. During period of preparation, c. During period of activity. 

 bm basement membrane ; b,t balls of secretion ; cm circular muscle ; ib internal 

 lamina of epithelium; lm longitudinal muscle; ns nests of developing cells; 

 nu nucleus of epithelial cell; pm peritrophic membrane; yp yellow pigment 

 granules. After Voinov. 



We may divide the process of digestion into three periods, (i) the 

 normal period, or period of rest, (ii) the period of preparation, and 

 (iii) the period of activity. 



(i) The period of rest. Fig. 49 A shews the appearance of the 

 cells of the mid-gut, after the completion of digestion of a heavy 

 meal. We have already described this state. We shall only add 

 that, here and there, just within the basement membrane, there 



1 I have kept larvae of Synthemis alive for three months without food. 



