THE FOUR-WINGED PARASITES 



179 



A considerable proportion of these tiny parasites belong 

 to the genus Aphidius. The fly generally deposits her egg 

 within the abdo- 

 men of the aphid. 

 The egg shortly 

 hatches into a 

 tiny footless 

 maggot that lives 

 within the body 

 of its host, ab- 

 sorbing the ma- 

 terials of growth 

 from it, and often 

 being visible 

 while the host in- 

 sect apparently 

 is still alive. Fi- 



Aphis with parasite 

 larva'Inside 



PARASITE 

 of 



GRAIN. APHIS 



MAGNJFIED 



Aphis Killed by 

 parasite 



nally the parasite larva becomes fully developed, absorb- 

 ing practically all the contents of the aphid's body, kill- 

 ing it, and leaving only the inflated skin of the latter to serve 

 as a protection to the parasite. Within this brown inflated 

 skin, the larval parasite changes to a pupa and very soon 

 afterward changes again to a tiny fly, resembling the one 

 that laid the egg. This fly gnaws out a circular piece in 

 the skin of the dead host, making a hole through which 

 it emerges to continue the good work of aphis destruction. 

 Very often these little parasites are important means of 

 checking outbreaks of destructive aphides. The parasite 

 of the grain aphis, illustrated herewith, is a notable example 

 of this. Under certain conditions which are favorable to 

 the development of the plant louse, but not favorable to the 

 development of the parasite, the former become exceedingly 

 destructive in the grain fields of the South and West. 



