46 GNATS OR MOSQUITOES — CHAPTER III 



often obliterating that ot" the delicate, superjacet dorsal 

 vessel, is the stomach, or chylific ventricle, its opacity 

 varying with the fulness of its contents ; and on either 

 side may be seen the dilated thoracic portions of the 

 respiratory trachene, which are easily distinguishable by the 

 spiral thickening of their chitinous lining, while branches 

 of the tracheal system to the various organs can be traced 

 in various directions. 



The abdomen is between three and four times as long 

 as the thorax, but narrower, and consists of nine segments, 

 each of which is provided with a large tuft of bristles as well 

 as with numerous hairs, either single or grouped, on the 

 dorsal and ventral surfaces. The cardiac, intestinal, and 

 respiratory tubes can all be traced through the greater part 

 of its length, and the Malphigian tubes, and other intestinal 

 appendages, can all be made out, as the larva takes favour- 

 able postures. The nervous system, however, is completely 

 hidden, and owing to its transparency during life, cannot 

 be seen even if the animal be placed in the supine position. 



In the genera Cidcx and Mochlomjx, there springs from 

 the dorsum of the eighth segment a large process, at least as 

 wide and generally about two or three times as long as the 

 remaining segment of the abdomen, and into this the two 

 main respiratory trunks can easily be followed, and are seen 

 to open at its extremity by means of curiously guarded open- 

 ings. At the root of this breathing horn are a pair of rather 

 short but dense tufts of hairs. The last segment contains 

 the rectum and carries the anal tubercles, the anus being 

 placed almost at the very extremity of the body, but rather 

 towards its ventral aspect. Around the opening are two 

 pairs of delicate, leaf-like expansions, each furnished with 

 a branching twig of trachea, the lower pair being some- 

 what the larger. They probably act as gills, and subserve 

 respiration during the periods when the larva is completely 

 submerged, which, when the weather is cool, are often 

 somewhat protracted. On either side, too, but originating. 

 a little in front of these anal tubercles, are a pair of large 

 dense tufts of compound hairs, which are employed in 

 swimming nnich in the same way as a fish's tail, and are 

 so arranged as to form an expansion of similar shape. 



