INJURIOUS INSECTS, FUNGOUS DISEASES, ETC. 161 



times in greater numbers than others. Frequently when a 

 certain species becomes very numerous, natural enemies 

 and parasites develop that kill the greater part or an entire 

 brood. Sometimes during the metamorphosis of insects, 

 unfavorable climatic conditions arise that kill a great num- 

 ber. Again, an insect species may become so numerous as 

 to exhaust the food plant and then the race dies. It should 

 not be permitted, however, to adopt such heroic measures in 

 insect fighting. 



The transformations which insects undergo from the 

 time of their hatching from the egg to their maturity is 

 called their metamorphosis. The egg is the first stage in the 

 existence of any insect. Almost always the eggs are laid by 

 the mother insect on or near the plant food which gives 

 nourishment to the young. The larva is the second stage of 

 an insect's life and is the form that hatches from the egg. 

 Familiar examples of larvae are caterpillars, maggots, and 

 grubs. The larval stage is the feeding period in the life 

 history of most insects injuring shade-trees, and is of 

 supreme importance from an economic standpoint, for it is 

 during this condition that many insects commit their great- 

 est depredations. 



The third stage in the life of an insect is the change of 

 the larva into a pupa. Many larvae, especially those of 

 moths when fully grown, spin about their body silken cases 

 called cocoons, at the same time transforming themselves 

 into pupas. The pupa remains inactive in the cocoon for 

 some time, then breaks through the cocoon and emerges as 

 the adult or mature insect. Some insects pass the winter or 

 hibernating condition in the egg form, others as partly grown 

 larvae. Again, many hibernate in the pupal stage and still 

 others pass the winter in the adult condition. The transf or- 



