Aquatic Adaptations of Insect Larvce 279 



There is another form of tracheal gills, sometimes 

 called "tube gills" developed upon the thorax of many 



dipterous pupae. Whatever their form 

 they are merely hollow bare chitinous 

 prolongations from the mouth of the 

 prothoracic spiracle. They are ex- 

 panded "respiratory trumpets" in 

 mosquito pupae, branching horns in 

 black-fly pupae, and fine brushes of 

 silvery luster in bloodworm pupae. 

 No pupae, save those of the caddis- 

 flies, have tracheal gills of the ordin- 

 ary sort. 



Gills are developed rarely on the 

 head, more often on the thorax, and 

 very frequently on the abdomen. 

 They grow about the base of the maxil- 

 lae in a few stonefly and mayfly 

 nymphs, about the bases of the legs 

 in most stonefly nymphs and almost 

 anywhere about the sides or end of 

 the abdomen in all the groups. They 

 are ventral in the spongilla flies, dorsal 

 in the mayflies, lateral in the orl-fly 

 and beetle larvae, caudal in the damsel- 

 flies, anal in most dipterous larvae, 

 and they cover the inner walls of a 

 rectal respiratory chamber in dragon- 

 flies. Such extraordinary diversity in 

 structures that are so clearly adaptive 

 is perhaps the strongest evidence of 

 the independent adaptation of many 

 insect larvae to aquatic life. 

 Propulsion by means of fringed swimming legs 

 occurs in a few insect larvae, such as the caddis-worm, 

 Triaenodes, and the "water-tiger" Dytiscus. The gill 



Fig. 171. Tube-gills 

 of Dipterous pupa: : 

 a, of a mosquito, 

 Culex ; b, of a black- 

 fly, Simulium ; c, 

 of a midge, Chiro- 

 nomus. (a and b 

 detached). 



