140 ANATOMY. 



membrane, and were very bright and transparent. According to 

 Ramdohr's observations, the width of the colon is in proportion to 

 that of the pharynx (crop), for where the latter is broad so is also the 

 colon, and vice versa. 



The situation of the colon is always determinate, for it is always 

 found at the apex of the abdomen, surrounded by its last segments. 

 The evacuating opening, or ANUS, is found in the last segment itself; 

 it is covered above by a peculiar valve, and beneath this the anal 

 vessels, which we shall describe lower down, open themselves. The 

 corresponding lower valve conceals the sexual aperture, so that both 

 the anal and sexual apertures open into one cavity, which might be 

 called the CLOACA, and which are separated only by a fold if no other 

 organ, for example, an ovipositor, be present. The anus, as well as the 

 ilium and its correspondent the thick gut, are wanting in the larvae of 

 the bees, wasps (PL XVII. f. 9.), the Formicaleo, and of perhaps all 

 the internal parasites, for example, the Ichneumons ; their intestinal 

 canal consisting of the pharynx and stomach, and a small bag beyond it, 

 into which the biliary vessels open themselves ; it is here that the faeces 

 collect, which are evacuated upon the perfect insect quitting the pupa 

 state, when it is provided with an anus. 



110. 



THE CAECUM. 



In many insects we find, in connection with the colon, a blind, sack- 

 shaped appendage, or rather similarly shaped superior distension of it 

 which we call caecum (PI. XIX. f. 3 and 4 G, G). It originates at the 

 very commencement of the colon, contiguous to its connection with the 

 ilium, and extends anteriorly towards the stomach, in either larger or 

 smaller distension; it is consequently not separated from the colon by 

 any constriction or valve, but both cavities are in immediate connection 

 with each other. This, as well as their uniformity of structure, proves 

 that it must only be considered as a distension of the colon. In form 

 this caecum is sometimes nodose (Stlphu) and directed forwards, some- 

 times laterally distended (Necrop/wrus) , sometimes it is a long tubular 

 point ( Dyticus}, sometimes a shorter cylindrical process of equal width 

 with the colon (^Nepa), similar to this, but sometimes slightly con- 

 stricted at its commencement, we find it in the butterflies. It thence 

 appears that this portion of the intestine is more peculiar to the car- 

 nivorous tribes, as Ramdohr, somewhat justly, remarks ; yet its struc- 



