THE I NSECTS— ARTHROPODS 219 



is quite different from the adults. Respiration must also be different; 

 the adult breathes air by means of its spiracles, but the naiads suck 

 water in and out of their anus in order to absorb the needed oxygen. 

 When ready for its final molt the naiad will crawl up out of the water 

 and emerge with fully formed wings ready to take up its life in the air. 

 Finally, there are a few primitive insects that have no metamorphosis. 

 In these the egg hatches directly into a form very much like the adult 

 and merely increases in size without much change in body form. The 

 silverfish and some of the lice are examples of this method. 



Insect Wings 



The wings of insects show great variation, and wing structure is fre- 

 quently used as a basis for subdividing the class into orders. Some in- 

 sects, such as fleas and fishmoths, have no wings at all. Others, such as 

 the flies, have only one pair, but the second pair is represented by a rudi- 

 mentary projection at the place where the second pair should be. Some, 

 such as the ants, have generations without any wings at all, but eventu- 

 ally produce a winged sexual generation that flies out of the nest and 

 spreads the species. The majority, however, have two pairs of wings as 

 adults. 



Wings which are used for flying are thin and light with delicate veins 

 running through them. These are called membranous wings. In some 

 insects, such as beetles, the front pair of wings has been greatly thick- 

 ened and hardened to form horny wings. These are used for wing 

 covers to protect the delicate second pair of wings which are mem- 

 branous and used in flying. During flight the horny front pair of wings 

 are held out to the side like the wings of an airplane. In other insects, 

 such as the grasshopper, the front pair of wings become tough yet flexi- 

 ble and are called leather-like wings. They also act as wing covers and 

 are held rigid in flying. Some insects, like the butterflies, have mem- 

 branous wings covered with scales which rub off like a fine powder when 

 they are handled. Some, such as the bugs, have a leather-like front part 

 of a wing and a membranous hind part. With so many variations it is 

 often possible to tell the order of an insect by glancing at its wings. 



Insect Food and Feeding Habits 



Insects are adapted to feed on almost every conceivable kind of or- 

 ganic matter. A small pile of manure dropped from the body of a cow, 

 after the cow has extracted all possible nourishment from it, provides all 

 the food necessary for hundreds of maggots to live and grow into flies. 



