CLASS INSECTS. 421 



superior animals, we observe a relation between the nature 

 of the food and the development the canal requires ; in car- 

 nivorous insects it is in general very short, whilst in 

 insects which are nourished on vegetable substances it 

 is in general very long. The food which reaches it is im- 

 bibed with saliva ; the apparatus which secretes this liquid 

 consists in a certain number of floating tubes, terminat- 

 ing sometimes by kinds of utricles, and communicating with 

 the pharynx by excretory canals. A multitude of villo- 

 sities with which the chyle-forming stomach is usually 

 furnished, seem to serve for the secretion of a gastric juice, 

 and it is also in this cavity that the bile is poured. There 

 exists no liver, properly speaking, in insects ; but this organ 

 is replaced by long and delicate tubes, which float in the 

 interior of the abdomen, and open superiorly into the chyle- 

 forming stomach (c, Fig. 443). These biliary vessels also 

 take the place of urinary glands, for it is here that the uric 

 acid is formed. By one of their extremities they always 

 open into the chyle-forming stomach, and the other extremity 

 is sometimes free, sometimes fixed to the intestine, whether 

 near the first opening or in the neighbourhood of the rectum. 

 Finally, we still find, towards the posterior extremity of the 

 intestinal canal, other secreting organs (e) serving to elabo- 

 rate particular fluids (such as the venom of the bee) which 

 several insects eject from the extremity of the abdomen when 

 they are disturbed. 



526. It would appear that it is by simple imbibition 

 that the chyle traverses the walls of the digestive tube, and 

 mingles with the blood. This last liquid is watery and 

 colourless ; it is not enclosed in vessels, arid is found spread 

 about in the interstices which the organs have between them, 

 or present in the substance of their tissue. Insects also have 

 no regular circulation. We perceive, indeed, in certain parts 

 of their bodies, currents even sufficiently rapid, but the 

 nourishing liquid does not describe a circle so as to return 

 constantly to its point of departure. There exists, in fact, in 

 these animals only the vestiges of the circulating apparatus 

 ( 112). We observe near the dorsal surface of the body a 

 longitudinal tube (Figs. 414 and 415), which performs alter- 

 nate movements of contraction and of dilatation, analogous to 

 those of the heart in the superior animals. But this dorsal 

 vessel seems to give off no branch. The nourishing liquid 

 penetrates into it by lateral openings, furnished with valvules 



