ADAPTATIONS OF AQUATIC INSECTS 



155 



quito larva (Fig. 230, A) is often heavier than water, but the pupa (Fig. 

 230, B) is lighter, and remains clinging to the surface film. 



The tension of this surface film is sufficient to support the weight of 

 an insect up to a certain limit, provided 

 the insect has some means of keeping its 

 body dry. This is accomplished usually by 

 hairs, set together so thickly that water 

 cannot penetrate between them. As the 

 legs and body of Gerris are rendered water- 

 proof by a velvety clothing of hairs, the in- 

 sect, though heavier than water, is able to 

 skate about on the surface. Gyrinus, by 

 means of 'a similar adaptation, can circle 

 about on the surface film, and minute col- 

 lembolans leap about on the surface as 

 readily as on land. 



The modifications of the legs for swim- 

 ming have often impaired their usefulness 

 for walking, so that many aquatic Coleop- 

 tera and Hemiptera can move but awk- 

 wardly on land. When walking, it is inter- 

 esting to note, Cybister and some other 

 aquatic forms no longer move their hind 

 legs simultaneously as they do in swimming, 

 but use them alternately, like terrestrial 

 species. 



The adaptations for swimming do not 

 necessarily affect the power of flight. Dy- 

 tiscus, Hydrophilus, Gyrinus, Notonecta, 

 Benacus and many other Coleoptera and 

 Hemiptera leave the water at night and fly 

 around, often being found about electric 

 lights. 



Respiration. Aquatic insects have not 

 only retained the primitive, or open (holo- 



pneustic), type of respiration, characterized by the presence of spiracles, 

 but have also developed an adaptive, or closed (apneustic) , type, for 

 utilizing air that is mixed with water. 



Through minor modifications of structure and habit, many Kolo- 

 pneustic insects have become fitted for an aquatic life. In these in- 



FIG. 230. Larva (A) and pupa 

 (B) of mosquito, Culex pipiens. r, 

 respiratory tube; /, tracheal gills. 



