On Subaquatic Insects. 125 



to descend with its miniature diving-bell to the bottom of the water 

 in which it resides, has not yet been discovered. The observation of 

 spiraculated aquatic imagines will not very greatly assist us in this 

 inquiry, because we tind no uniformity existing in their mode of in- 

 spiration ; thus, when the perfect Dyticideous beetles ascend to the 

 surface of the water, they expose the extremity of the body, and thus 

 admit air into the space which exists between the upper surface of 

 the abdomen and the closed elytra ; whereas in the Hydrophilidae 

 the head is brought to the surface of the water, and then one of the 

 clavate antennse is projected, the club of these organs being cover- 

 ed with fine hair. This club is, however, so twisted that whilst the 

 base is exposed to the air the extremity is brought in contact with 

 the breast, which, as well as the whole under side of the insect, is 

 covered with short silky pubescence. " By this means," observes 

 Burmeister, " a communication is made with the external air and 

 that beneath the water covering both the clava of the antennae and 

 the whole under surface of the insect, to which it adheres by means 

 of the coating of down; and by this communication fresh air is trans- 

 mitted to the venter of the insect, and by the same means the ex- 

 pired air is also removed, and the air is likewise transmitted from 

 the ventral surface beneath the elytra, where it is in, and expired 

 by the spiracles there situated." * 



This distinction appears to me to result entirely from the pre- 

 sence or absence of the coating of plush or fine down, with which the 

 bodies of some of these insects are provided, because in the Dyti- 

 cidae, which do not respire by means of a supply of air coating the 

 underside of the body, we find the body not externally covered with 

 this coating of plush. 



The genus Nepa offers a still more remarkable modification in 

 the structure of its respiratory organs and mode of respiration. On 

 examining an insect of this genus, the spiracles appear at first sight 

 to be in the ordinary position and of the ordinary form ; but we 

 learn from M. Dufour's admirable Recherches Anatomiques sur les 

 Hemipteres, that these spiracles have no orifice and are quite use- 

 less, the only spiracles being two, which are placed at the base of the 

 anal setae. Thus it is only by thrusting these setae out of the water 

 that the insect can obtain a supply of air.t 



The insects to which we have directed our attention are en- 

 abled to swim Avith greater or less facility, and hence it is that they 

 can obtain fresh supplies of external air at pleasure ; moreover, for 



* Manual of Entomol. p. 392, Shuckard's translation, 

 t See Brit. Cyclop. Nat. Hist. Vol. ii. p. 870. fig. 150. 



