i68 NATURAL HISTORY OP^ AQUATIC INSECTS CH. 



of the body. A large tracheal tube is connected with 

 each spiracle, and runs along the body to the head, 

 giving off many branches to the 

 various organs. The spiracle itself 

 consists of a solid cone received 

 within a wider tube, or vestibule. 

 The cone is long and tapering, and 

 apparently impervious. The be- 

 haviour of the living larva would 

 seem nevertheless to indicate that the 

 spiracle is actually employed in re- 

 spiration. When it comes to the 

 surface of the mud, it commonly pro- 

 trudes the hinder end of its body. 

 This is furnished with the three pairs 

 of prominences already mentioned, 

 which are probably tracheal gills, and 

 also with spiracles. We can therefore understand 

 how it is that the larva seems to be indifferent whether 

 its tail projects into air or water. It will lie for a 

 long time in either condition without signs of un- 

 easiness. When it re-enters the water, after its tail 

 has been exposed to the air, a bubble can often be 

 seen attached to each spiracle. 



The pupa, as in most other aquatic Dipterous 

 Insects, is provided with a pair of respiratory 

 trumpets placed just behind the head. These are 

 flattened from before backwards, and have a rather 

 sharp edge. Within each trumpet is an expanded 

 tracheal tube, which blends with the outer integument 

 along the edge. The line of junction is perforated 

 by a regular row of small oval apertures, which no 



Fig. 54. — Spiracle of 

 larva of Dicranota, 

 seen in side-view 

 as a transparent 

 object. 



