292 Insects and their Surroundings 



expanded mouths can be thrust out of the clogging 

 surroundings of the mud or refuse into the fresh air, 

 while the grub remains concealed and continues to 

 feed. A similar suppression of most of the spiracles, 

 with the development of a tubular process at the tail- 

 end of the body in connection with the tracheal system, 

 is the adaptation by which many aquatic larvse breathe 



Fig. 159. — a. Floating egg-mass of Gnat ; b. two eggs, magnified, with newly- 

 hatched larvas below ; c. larvae hanging from the surface film, diving, and 

 rising, magnified; rt'. larva, highly magnified. From Howard, F5ull. 4(0.5.), 

 Div. Ent. U.S.A. Dept. Agr. 



— for example, the grub of the Gnat (figs. 72, 159) 

 and the " rat-tailed maggot" of the Drone-fly. The 

 families of insects nearly related to these have larvae 

 which live in mud and damp earth, and this suggests 

 that it was from the shores that the waters were 

 invaded by these insect-hosts. 



But there is another division of aquatic larvae still 



