849 



reparation for the injuries they commit in some respects, by keeping within 

 bounds other annoying insects. In the larva and perfect states they are 

 predaceous, sanguinary, and carnivorous. Their food is aquatic animals, 

 such as the young fry of fish, tadpoles, some of the mollusca, Crustacea, and 

 worms, and the larvas of dragon-flies, gnats, and mosquitoes: and in their 

 turn they serve as nourishment to fishes, &c. The larvae live in the water 

 only, the pupae are terrestrial, and the perfect insects amphibious. I have 

 repeatedly found the latter remote from water, where they had transported 

 themselves by flight. It is at night that they usually ascend into the air, 

 to exercise their ample wings, or to seek for a better supply of food in more 

 distant waters. They sometimes enter our windows in the evening, being 

 attracted by the lights. The perfect insect swims with celerity by means of 

 its feet, particularly the hind ones which are flattened and densely fringed 

 with hairs: it rises from the bottom without effort in consequence of being 

 rather lighter than the water. 



The respiration of these insects is worth examination. In terrestrial an- 

 imals of the higher classes air is inhaled through the throat into the lungs, 

 where it produces an essential and vital change upon the blood. In fishes 

 the same effect is produced by the water, which always contains a large pro- 

 portion of air mixed with it, and which, entering by the mouth, passes out 

 again at the gills, a kind of lungs. Other aquatic animals have these gills, 

 or branchiaj as they are called, outside of the body, and hence constantly 

 bathed in water, and exposed to the action of the air that it contains. Some 

 aquatic larva? of insects have similar respiratory branchiae, and these are the 

 only ones to which pure or unmixed air is not necessary. The majority of 

 insects, in all stages, breathe air through lateral pores or spiracles, none 

 receive it through the mouth, and the lungs are replaced by longitudinal 

 tubes, their numerous minute branches, and air vesicles. The spiracles of 

 . insects, which open directly into the longitudinal tubes, are from two to 

 eighteen in number: through the posterior or anal and abdominal ones the 

 air is admitted or inspired, and it is expelled or expired through the anterior 

 ones or those of the trunk. The Dytisci, in the perfect state, have eighteen 

 spiracles, the abdominal pairs are situated upon the dorsuni, and are cov- 

 ered by the elytra. When it becomes necessary for the insect to take breath 

 it ascends to the surface of the water, reversed, the under side of the tail 

 being uppermost, and the body kept steady by means of the long oar-like 

 hind legs, which are extended at a right angle with the trunk. The abdo- 

 men is then slightly curved so as to separate it from the elytra, and elevate 

 it above the water, the air easily insinuates itself into the cavity thus 

 formed, and is inhaled at the pores, the last pairs of which arc larger than 

 the rest to admit it in this position with greater freedom. When inspira- 

 tion is completed the elytra are quickly closed upon the body, and the 



