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ENTOMOLOGY 



On account of the large amount of air that they carry about, most 

 aquatic imagines are lighter than the water in which they live, and 

 therefore can rise without effort, but can descend only by exertion, and 



can remain below only by clinging to 

 chance stationary objects. The mosquito 

 larva (Fig. 232, A) is often heavier than 

 water, but the pupa (Fig. 232, B) is lighter, 

 and remains clinging to the surface film. 



The tension of this surface film is suffi- 

 cient 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 insect, 

 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 collembo- 

 lans 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 swim- 

 ming, but use them alternately, like ter- 

 restrial species. 



The adaptations for swimming do not 

 necessarily affect the power of flight. 

 Dytiscus, Hydrophilus, Gyrinus, Notonect'a, 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 (holopneustic) type of respiration, characterized by the presence 



FIG. 232. Larva (A) and pupa 

 (JB) of mosquito, Culex pipiens. 

 r, respiratory tube; /, tracheal 

 gills. 



