DHiKSTION IN INSKf'TS 



3-25 



\\'hen digestion in the crop is accomplished, the matters are sub- 

 jected to an energetic pressure of the walls through peristaltic con- 

 tractions, and then, guided by the furrows and chitinous teeth, pass 

 along or gradually filter through the valvular apparatus or proven- 

 triculus, whose function is that of a strainer. 



At the beginning of the " chyle-stomach " (mesenteron) of Orthop- 

 tera are glandular caeca which secrete a feebly acid fluid. This fluid 

 emulcifies fats, and converts albuminoids into peptones. It passes 

 forwards into the crop, and there acts upon the food. 



In the mesenteron (mid-intestine) the food is acted upon by an 

 alkaline or neutral fluid, never acid, either secreted, as in Orthop- 

 tera, by local special glands, or by a multitude of minute glandular 

 cseca, as in many Coleoptera, or by a simple epithelial layer. It has 

 no analogy with the gastric juices of vertebrates ; its function differs 

 in insects of different groups; in carnivorous Coleoptera it actively 

 emulsionizes greasy matters ; in the Hydrophilidse it continues the 

 process of transformation of starch into glucose, begun in the oesoph- 

 agus. In the Scarabaeidse, it also produces glucose, but this action 

 is local, not occurring elsewhere ; in caterpillars, it causes a produc- 

 tion of glucose, and transforms the albuminoids into soluble and 

 assimilable bodies analogous to peptones, and also emulsionizes 

 greasy matters. Finally, in the herbivorous Orthoptera there 

 does not seem to be any formation of sugar in the stomach itself, 

 the production of glucose being confined to the crop (jabot). 



When digestion in the crop is finished, the proventriculus relaxes, 

 and the contents of the crop, now in a semifluid condition, guided 

 by the furrows and teeth, passes into the mesenteron, which is with- 

 out a chitinous lining, and is thus fitted for absorption. 



The contents of the mid-intestine (chylific stomach) then slowly 

 and gradually pass into the intestine, the first anterior portion of 

 which, usually long and slender, is the seat of an active absorption. 

 The epithelial lining observed in certain insects seems, however, to 

 indicate that secondary digestion takes place in this section. The 

 reaction of the contents is neutral or alkaline. 



The second and larger division of the intestine only acts as a 

 stercoral reservoir. (The voluminous caecum occurring in Dytici- 

 dae, Nepa, and Ranatra, whether full or empty, never contains gas, 

 and it is not, as some have supposed, a swimming-bladder.) The 

 liquid product secreted by the Malpighian tubes accumulates in this 

 division, and, under certain circumstances, very large calculi are 

 often formed. In his subsequent paper on the digestion of the cock- 

 roach, Plateau states that in the intestine are united the residue of 



