124 BRANCH ARTHROPOD A 



The psychology of insects is a very interesting study. Whether 

 the activities of insects are due to reflex action, instinct, or 

 intelligence can be better determined when studying the various 

 species, but one will surely find that insects, as well as being the 

 most numerous and various, are also the most interesting and 

 wonderful of all the classes of invertebrates. 



Multiplication is by eggs, of which many are deposited in 

 various places. Some are placed on or in another animal's 

 body, others on leaves or stems of plants, which serve as food 

 for the young. However, some insects, as the Aphides, show 

 parthenogenesis, i. e., they are supposed to produce young 

 from unfertilized eggs. 



Metamorphosis. Insects pass through a more or less com- 

 plete series of changes, called metamorphosis. The larvae, 

 whose business it is to feed and grow, are called by various names, 

 as caterpillars, grubs, nymphs, and maggots. Since the larvae 

 are wingless they are placed in different relations to their 

 environment from those of the adult, and hence often have 

 special larval organs. The larval stage is followed by a quiet 

 stage called the pupa (Fig. 89). In this condition many in- 

 sects pass the winter and come forth in the spring as adults 

 or imagoes, the reproductive stage. Others remain in the 

 pupa stage but a few weeks, thus giving time for two or more 

 broods in a season. 



Parasitism is common in insects. Parasites may be ex- 

 ternal or internal. The natural consequence of a parasitic life 

 is degeneration, as is seen in lice and fleas, whose ancestors were 

 winged insects. 



Environmental Influences. Insects are affected by tempera- 

 ture. They become active with the rise of temperature in the 

 spring, and some become dormant or hibernate as the tempera- 

 ture declines in fall and early winter. Most of them die with 

 the advent of frost. The direction and velocity of the wind 

 is a factor in insect life, especially in its distribution. 



The amount of precipitation will influence the amount 

 and kind of vegetation, which determines to a large extent the 

 number and kind of insects. Certain kinds of precipitation, 

 as hail, for example, or floods, would destroy large numbers of 

 insects. Any environmental factor would increase or decrease 



