98 NATURAL HISTORY OF AQUATIC INSECTS 



Cll. 



Fig. 22. 



-Larva of Gnat (Cule.v), in 

 side view. 



larva ordinaril)- rcst.s in 

 .such a position that the tip ; 

 of the respiratory siphon 

 is flush with the surface of 

 the water, and, thus sus- 

 pended, it feeds inces- 

 santly, breathing uninter- 

 ruptedly at the same time. 

 It is of obvious advan- 

 tage to the larva that it 

 should in this way divide 

 the operations of breathing 

 and feeding between the 

 two ends of the bod}-, for 

 the breathing must be done 

 at the surface, and the 

 beneath the sur- 

 Observe that the 



feeding 

 face. 



arrangement requires one 



thing 



unusual in floating 



eanisms which inhabit 



objects — viz., that the body 

 shall be, in some parts at 

 least, if not altogether, 

 heavier than water. If it 

 were lighter than water 

 it could not maintain a 

 vertical position, but would 

 lie along the surface like 

 a stick, and would be quite 

 unable to sweep about in 

 search of the minute or- 

 the water beneath. When 



