I*ESSON 37.] NUTRITION IN INSECTS. 129 



591. The intestinal canal of Insects consists of three membranes 

 the innermost membrane is smooth and textureless, and variously 

 armed with horny lines, ridges, or teeth. It is most distinct in the 

 pharynx, crop, and proventriculus, or gizzard. 



The second layer is white and smooth, and, although usually thin, 

 is sometimes thick and spongy. 



The third (outer) layer is a compact, firm, fleshy, muscular coat ; 

 it is not equally observable in all parts of the intestinal canal, but is 

 readily seen in the pharynx, stomach, and colon, but it is lost in the 

 crop, or craw. 



592. The length of the intestine in Insects appears to be governed 

 by the same law that regulates it in other animals ; those that feed 

 on vegetable matter usually possess a longer and more distended in- 

 testine than those which feed on animal matter, in which it is shorter 

 and narrower. 



593. In this class the intestinal canal is divided into the pha- 

 rynx, the oesophagus, the crop, or craw, the proventriculus, or giz- 

 zard, the stomach, the duodenum, the ileum, the ccecum, and the 

 colon. 



594. The appendages are the salivary glands, the liver, and cer- 

 tain vessels supposed to constitute a kidney. 



595. The several parts enumerated are never all present in the 

 same individual ; sometimes one part is wanting and sometimes 

 another. Thus, insects with a suctorial mouth have no need of a 

 pharynx ; neither do they possess a gizzard, for, feeding upon fluid 

 food only, they have no need of such an organ. The part most usu- 

 ally deficient is the duodenum, which has only been found in some of 

 the beetles ; after this the ccecum is less usually found, as it belongs 

 only to those insects which feed upon animal matter. 



596. The biliary vessels are seldom wanting, the salivary glands 

 frequently, and the kidneys very generally. 



597. THE PHARYNX is the funnel-shaped commencement of the 

 oesophagus, and is only found in mandibulate insects ; it is of large 

 size in grasshoppers, cockroaches, and most caterpillars. 



598. THE (ESOPHAGUS extends from the pharynx to the stomach ; 

 it is distinguished from the pharynx by its smaller capacity, and from 

 the stomach, by its different structure. In the Lepidoptera (butter- 

 flies and moths) it is double for a space, and then united into one tube. 



599. The separation of cesophagus from stomach is sometimes ef- 

 fected by a constriction ; sometimes it passes insensibly into it, and 

 sometimes the crop intervenes between them. 



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