108 THE PESTS OF CULTIVATED PLANTS 



stage the insect does no damage, but remains quiet while 

 internal changes are taking place, and an entire reorganiza- 

 tion of its body goes on. When the transformation period 

 is completed the insect emerges from the pupa case or the 

 cocoon as a full-grown insect, as, for example, a butterfly, a 

 moth, or a beetle. These forms are called the adults. By 

 far the greatest damage which is done by most of the insects 

 is in the larva or the worm stage, but a few adult insects do 

 great damage. It is because of this change in the form of 

 an insect that it becomes imperative for the grower to know 

 the life history of an insect before adequate measures of 

 control can be used. 



Incomplete metamorphosis is, as the term signifies, an 

 incomplete change in the development of an insect. The 

 adult form is reached without going through the four stages 

 necessary in complete metamorphosis. In incomplete 

 metamorphosis the insect completes its life history without 

 radical changes in its form. The insect hatches from the egg 

 into a form resembling the adult, although considerably 

 smaller and without wings. It begins to feed at once on its 

 food plant and continues to feed for a longer or a shorter 

 period of time, going through several moults, in which it sheds 

 its skin when it gets too large for the old one. After passing 

 through several of these moults, the number being determined 

 by the species and the food supply, the nymph becomes an 

 adult. Such insects as the common grasshopper, the cock- 

 roaches, the bugs, and several more belong to this class. 



The growth of insects with incomplete metamorphosis is 

 somewhat different from those with complete metamorphosis. 

 It is a known fact that the skin of an insect hardens, due to 

 the presence of a horny substance known as chitin. This 

 hardening usually occurs to a greater extent in the adults 

 than in the young. However, in all insects with incomplete 

 metamorphosis the skin soon becomes so firm that it cannot 

 stretch, and consequently this hardening prevents any 

 more growth from taking place. The result is that the skin 

 becomes too small, and it must be shed before the insect can 

 grow. As the old skin grows hard a new skin forms under it 

 and the old hardened skin splits and bursts open, permitting 



