UTILITY OF BIRDS IN NATURE. 15 
pupal form. This stage it passes without food and while 
fixed to some object. The pups or nymphs of some other 
insects, however, move about freely, as is the case with 
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 wings; although some 
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 Fig. 8.—Pupe or chrysalids. 
next generation. Thus we have four forms which insects 
assume: (1) the egg, (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 vegetable matter. 
A certain larva has been known even to tunnel into marble. 
Those insects which 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 
decaying vegetation can do only good in nature, although 
they may be injurious to man by destroying hides, furs, pre- 
served meats, or clothing. 
It is difficult to perceive the usefulness of those so-called 
injurious species which feed on the different parts of plants ; 
still, the larve that eat the buds, the caterpillars that feed 
1 In the Orthoptera the transformations are imperfect; the larve of grass- 
hoppers, for example, are provided with well-developed legs, and much resemble 
the imago or perfect insect, but are without wings. 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. 
