96 GENERAL ZOOLOGY 



incomplete metamorphosis, is illustrated by the grasshopper 

 (Fig. 5) and the squash bug (Fig. 23). Throughout their 

 development the young, or nymphs, are active and with few 

 exceptions pass through no inactive resting period. 



The most marked change of form is seen in the higher 

 insects, the Coleoptera (Fig. 35), Lepidoptera (Figs. 38, 39), 

 Diptera (Fig. 47), and Hymenoptera (Fig. 59). In all of these 

 the egg hatches into a worm-like larva totally unlike the 

 adult. After several molts and great increase in size the 

 larva stops feeding and becomes inactive. This resting 

 stage, which is usually very different in appearance from 

 the larva, is called the pupa. From this pupa the adult, or 

 imago, finally emerges. This cycle of stages, passing from 

 egg through larva and pupa to the adult, is called complete 

 metamorphosis. 



Along with the changes in body form in insects having 

 complete metamorphosis there are even more striking 

 changes going on inside the body. The internal organs of 

 the larva rarely become the organs of the imago. The tis- 

 sues of the larva become reconstructed to form the organs 

 of the adult. This process of reconstruction is carried on very 

 largely by the white blood cells, or phagocytes, which destroy 

 the larval organs and aid in building up the new organs for 

 the adult. In the fly maggot the wings and legs do not 

 become evident until the adult emerges from the pupa. 

 But even in the larva they begin to develop as small buds, 

 or "imaginal disks," inside the body. 



Insect Behavior. Insects are provided with a nervous sys- 

 tem and sense organs that enable them to be affected by 

 conditions that exist outside their own bodies. These sense 

 organs are of various kinds, and for some of them we 

 have no idea as to the functions they may perform. The 

 other sense organs more or less closely resemble those found 

 in the human body, and we therefore assume that they serve 



