INTERNAL ANATOMY OF INSECTS. 103 



other Classes, often an index of a less substantial kind 

 of nutriment ; while their shortness and slenderness in- 

 dicate that the insect lives by prey a . 



In considering therefore the parts connected with the 

 digestive functions of the insect world, it will not be amiss 

 to have reference to their food, and their mode of taking 

 it; but first it will be proper to state and define the parts 

 of this important organ. 



In general the alimentary canal b is composed of the 

 same essential tunicJcs as that of the vertebrate animals, 

 consisting of an interior epidermis,'apapillaryand cellular 

 tunick, and an exterior muscular one c . The first is 

 usually tender, smooth, and transparent; but not always 

 discoverable, perhaps on account of its tender sub- 

 stance d . Ramdohr does not notice the papillary and 

 cellular tunicks; they are probably synonymous with 

 what he denominates ihejlocky layer (Diejlockige lage\ 

 and which he describes, when highly magnified, as ap- 

 pearing to consist of very minute globules or dark points, 

 and as being of a cellular structure 6 . The exterior tu- 

 nick is thicker and stronger than the interior, and com- 

 posed of muscular fibres, running either longitudinally, 

 or transversely so as to form rings round the canal. 

 This tunick mostly begins at the mouth, and goes to the 

 anus, changing its conformation in different parts of the 

 above intestine. Sometimes however it originates only 

 at the beginning of the stomach f . With respect to its 

 general disposition, that canal in its relative length, in 



a Cuv. Anat. Comp. iv. 129. 



b PLATE XXI. FIG. 3. c, d, e, is the intestinal canal of the larva 

 of the Cossus. c Cuv. Ibid. 112. 



d Ramdohr Anat. dcr Ins. 6. c Ibid. 25. t Ibid. 6. 



