Packard.] INSECTS OF THE GARDEN. 
15 
crust in its place. Now there are two conditions to be ful¬ 
filled in this act of parasitism; for the insect preyed upon 
must maintain its hold upon life, feeble as it may be, long 
enough to enable the enemy lurking within, to build up its 
tissues and add to its own strength by daily depleting from 
Fig. 4. 
Ichneumon (Macrocentrus). 
the stores of vitalized food about it; and on the other hand 
the parasite must carefully avoid touching the vital parts of 
its host. It must content itself with feeding upon the fatty 
portions alone of the body. 
That family of the Hymenoptera of which the ichneu¬ 
mon-fly (Fig. 4) is a type, and many species of true flies 
15 
