INSECTS IN GENERAL 7 



The second and third segments of the thorax may 

 each carry a pair of wings. These are unique struc- 

 tures, since they are not possessed by any other 

 invertebrate animals. Moreover, whereas the wings 

 of a bat or a bird are really modified fore-limbs, those 

 of an insect are expansions of the outer case r 

 exoskeleton. They consist of two layers in close 

 contact, supported upon a tubular framework of 

 " veins," which are called "nervures." Only adult 

 insects have functional wings, and no insect has 

 more than two pairs. 



In some insects, such as beetles, the fore-wings are 

 more or less hardened and thickened, and act as pro- 

 tective covers, 1 beneath which the more delicate hind- 

 wings are folded when they are not in use ; in others 

 (e.g., bluebottles and house-flies), the second pair of 

 wings are reduced to small, stalked knobs called 

 " balancers," or " halteres," useless for flight, but 

 perhaps serviceable in assisting the insect to main- 

 tain its equipoise in the air. In other insects, again, 

 we find that the fore-wings are very small or absent, 

 while some insects have no wings. 



The probable origin of insects' wings is a much- 

 debated question which need not concern us here ; but 

 it is well to remember that the wings are very im- 

 portant organs from the standpoint of the systematic 

 naturalist. A knowledge of their various peculiarities, 

 and of the different types of mouth-parts, enables him 

 to sort up insects, so to speak, into their proper tribes, 

 clans, or family groups. 



Behind the thorax of the insect there is frequently 

 a kind of stalk, or " waist/' usually very short, and 

 often quite hidden from view, which connects with 

 1 Called elytra. 



