DISSECTION OF A CATERPILLAR 53 



When the skin with its attached muscles has been reflected 

 two whitish and crumpled sheets are brought into view. 

 These conceal to a great extent the parts beneath. They 

 are loaded with fat, and are hence called the fat -bodies. 

 The fat which they contain is often used up to a great extent 

 during the pupal stage. It will be found that the fat-bodies 

 are held in place by a multitude of branching air-tubes or 

 traehesB. Displace the fat -bodies so as to expose the ali- 

 mentary canal beneath. In this and similar cases it is very 

 desirable to avoid breaking up the tissue which has to be 

 removed. When a dissection gets encumbered with fragments 

 which intercept the view, the parts beneath are often acci- 

 dentally injured. Raise the fat-body with a needle or forceps, 

 and carefully cut through its attachments with fine scissors or 

 a sharp scalpel. The alimentary canal will thus be exposed, 

 and on either side the longitudinal tracheae or main air-tubes 

 of the body will be found. 



The alimentary canal or digestive tube takes a straight 

 course from mouth to anus. It consists of three primary 

 sections: — (i) the oesophagus or fore-gut, (2) the stomacli or 

 mid-gut, and (3) the intestine or hind-gut. The appendages 

 of the alimentary canal are the salivary glands, the silk-glands, 

 and the MalpigMan tubules. The first and third sections are 

 lined by a thin chitinous cuticle. The oesophagus and intestine 

 are short, compared with the stomach, which in most cater- 

 pillars occupies more than half the length of the alimentary 

 canal. The cesophagus is narrow near the mouth, from which 

 it takes its origin. It dilates behind to form a pear-shaped 

 sac, the crop, which is often distended with food. In some 

 caterpillars, such as the wood-eating larva of the goat-moth, 

 the hinder part of the oesophagus may have its muscular wall 

 thickened, and form a kind of simple gizzard, which has 

 not, however, the chitinous armature found in the cockroach 

 and many other insects. The stomacli or mid-gut is long, 

 cylindrical, and of simple form, wanting the circles of radiating 

 tubules which in some insects act as digestive glands. In a 

 caterpillar which has recently fed, the stomach often bulges 

 outwards at a number of places, and appears to be beaded. 

 The intestine or hind-gut is short. It begins in many cater- 

 pillars with a narrow portion, which dilates and forms a short 

 colon, usually distended with the refuse of the food. Thi^ 



