302 



THE ESSENTIALS OF AGRICULTURE 



388. Insects undergo changes in form. Nearly all insects in 

 their development undergo very remarkable changes of form 

 expressed by the term metamorphosis. The egg of a grass- 

 hopper produces a creature which, except for the absence of 

 wings, resembles the adult. This form, known as the nymph, 

 feeds and grows, molts several times, and develops into the 

 adult. Grasshoppers, chinch bugs, and dragon flies have this 

 type of life history. 



On the other hand, the egg of a moth produces a caterpillar, 

 which is the active feeding stage, or the stage in which the in- 

 sect does its serious injury. The 

 caterpillar feeds, molts, and grows, 

 and when fully grown, it spins, 

 in many cases, a firm casing of 

 silk known as a cocoon. In this 

 protective case it is transformed 

 into a pupa, which is the inactive, 

 or dormant, stage of development. 

 The pupa takes no food. It re- 

 sembles neither the caterpillar nor 

 the moth. In many cases it passes 

 the winter in this stage, so that 

 the pupal stage varies from a few 

 days in summer to several months 



in winter. Finally, the pupal shell splits open and the adult 

 moth (Fig. 1 50) emerges with wings soft and limp but expand- 

 ing and hardening in a few hours. Butterflies, beetles, flies, 

 bees, and ants have this type of life history. 



Insects of the latter type are said to have a complete meta- 

 morphosis, while those of the former have an incomplete meta- 

 morphosis. The growing stage for all insects having complete 

 metamorphosis is the larva, known as the caterpillar of the 

 moth or butterfly and the maggot of the house fly. 



Three distinct types of injurious insects will be discussed 

 here one a pest on staple crops, another a pest on fruit, and 

 another a pest on the leading fiber plant. 



FIG. 149. Mouth parts of a squash 



bug adapted for piercing and 



sucking. (Enlarged.) (Redrawn 



from Comstock) 



