FOOD SUPPLY 51 



while still other types deposit large numbers of minute eggs 

 or larvae on the foliage upon which the host insects are feed- 

 ing. In the latter cases the eggs or maggots are ingested with 

 the food, whereupon the eggs hatch and the larvae, perforat- 

 ing the wall of the alimentary tract, reach the visceral cavity. 

 One or many larvae may live at the expense of a single host 

 according to their relative sizes, and on attaining full growth, 

 the larvae usually quit the host through orifices they make in 

 its body wall, and enter the soil for pupation, later to emerge 

 as adult flies. 



It seems probable, at least in regard to the Tachina flies, 

 that the parasitic habit has been derived as a modification of 

 the habit of feeding upon recently dead insects, which has 

 been transferred to living ones and later developed its most 

 remarkable adaptations as seen in the group at the present 

 time. With the parasitic Hymenoptera the derivation of 

 the parasitic habit is far more obscure, and doubtless of 

 much more ancient origin. 



Ever since scientific methods of investigation were applied 

 to the problems of economic entomology the importance of 

 parasites has been recognized as a factor involved in the 

 natural control of insect pests, but it is only within com- 

 paratively recent times that their full significance has been 

 realized. With the recent introduction of several injurious 

 insects into our own country a minute comparison of the 

 controlling factors here and in the country of origin has 

 brought clearly to light the prime importance of insect para- 

 sites and attempts have been made to colonize the parasitic 

 enemies of several pests in the United States. Similar ex- 

 periments have been undertaken by foreign entomologists 

 also, and one of the most extensive yet well under way is that 

 aimed at the control of the gipsy moth which is discussed on 

 a later page. The same condition appears to prevail gener- 

 ally in regard to the diseases of insects caused by fungi (Fig. 

 26), bacteria and other microorganisms, but this matter has 

 been carefully investigated in only a few cases and it would 

 seem inadvisable to make very definite statements at the 

 present time. 



