UTILITY OF BIRDS IN NATURE. 



15 



pupal form. This stage it passes without food and while 

 fixed to some object. The pupa? or nymphs of some other 

 insects, however, move about freely, as is the case Avith 

 locusts, grasshoppers, and like insects ( Orthoptera) . ^ 



The pupa finally throws 

 off its outer shell, and 

 emerges a fully developed 

 or perfect insect or imago 

 with win OS ; althouoh some 



Fig. 8. — Pup;v or chrvsiiliris. 



insects which, like some 



birds, have lost the use 



of their wings, never fly.^ 



After the union of the sexes 



the female insect eventually 



deposits the eggs for the 



next generation. Thus we have four forms which insects 



assume : (1) the Qgg, (2) the larva, (3) the pupa or nymph, 



(4) the imago or perfect winged insect. 



Practically all living animals of appreciable size, as well 

 as most plants that are visible to the unaided eye, furnish 

 food for certain insects. Other insects feed on dead animals, 

 dead trees, or other decaying animal or vegetaljlc matter. 

 A certain larva has been known even to tunnel into marble. 

 Those insects Avhich feed on live vegetation or living animals 

 are capable of doing great harm if they increase unduly ; 

 while those that feed only on dead animals or dead and 

 decajdng vegetation can do on^y good in nature, although 

 they may be injurious to man by destroying hides, furs, pre- 

 served meats, or clothing. 



It is ditficult to perceive the usefulness of those so-called 

 injurious species which feed on the different parts of plants ; 

 still, the larvte that eat the buds, the caterpillars that feed 



^ In tlie Ortlaoptera the transfomiations are imperfect; the larvae of grass- 

 hoppers, for example, are provided with well-developed legs, and much resemhle 

 the imago or perfect insect, but are without wiiigs. In this stage they are usually- 

 called nymphs. As they approach maturity they enter what is virtually an im- 

 perfect pupal stage, but retain their shape, limbs, and activity. They now show 

 rudimentary wings, but it is only at maturity that they are capable of flight. 



^ The Thysanura, or lowest order of insects, including " bristle tails," " spring 

 tails," " fish moths," and the like, never become winged or develop any trace of 

 wings. 



