38 AN ECONOMIC ENTOMOLOGY. 



in the water, yet get all their air from above it without any power 

 of storing. This is the case with creatures like the mosquito 

 lar\'ae, which are unable to breathe without coming to the surface. 

 They have only a single spiracle at the tip of the body, and are 

 compelled to rise to the surface at intervals to breathe, and, 

 having done so, are able to sustain life beneath it for only a short 

 time afterward. 



This much for the general characteristics of the respiratory 

 system ; it remains to show how it is important from the eco- 

 nomic stand-point. It has been previously said that the insect 

 crust is of chitine, which is impervious to a great variety of 

 ordinarily penetrating substances. It resists alcohol, chloroform, 

 ether, the light mineral oils, benzine, turpentine, kerosene, and 

 other similar substances ; acetic acid penetrates slowly and so 

 does carbolic acid. This accounts for the difficulty which we 

 find in destroying many insect eggs, for there are no openings in 

 them that are sufficient for the penetration of these substances, 

 and they are unable to act through the egg-wall itself To get a 

 liquid insecticide to kill an insect by contact, it must be forced 

 into the body through the spiracles, or they must be so gummed 

 or clogged as to close them completely, thus preventing the in- 

 sect from breathing at all. Kerosene is the most reliable of the 

 penetrating liquids, and even where the spiracles are well pro- 

 tected this material penetrates into the smallest openings. It 

 has the advantage of actually wetting everything ; that is to say, 

 it is not repelled by small hairs, as globules of water would be, 

 and therefore, as it really comes into direct contact with them, is 

 enabled to work its way through the spiracles. That is what 

 makes the kerosene so much better than any watery mixtures 

 alone ; for whatever the material extracted by or dissolved in 

 water, it may perhaps be repelled by the insect body, and never 

 have the opportunity of exercising an effect. The various soaps 

 act in another way ; as the moisture evaporates, a film of sticky 

 or gummy material remains which clogs the spiracles, and thus 

 chokes the insect by depriving it of air. Soapy mixtures also 

 wet and stick well to insects through the caustics they contain, 

 and which have the power of cutting the repellent material of 

 their bodies. 



In some kinds of insects the spiracles are so feebly protected 



