VALUE OF PARASITES 49 
ture is found unfavorable to an essential parasite; or it is discovered 
that in its new home the parasite itself is attacked by some enemy 
from which it was free in its native land. Rarely, however, the 
experiment is successful; and the few instances that do work 
out satisfactorily abundantly repay for the labor and cost of all. 
The most extensive attempt at the introduction of parasitic and 
predaceous enemies from abroad for the control of a menacing pest is 
that now in progress, under 
the auspices of the Bureau of 
Entomology of the United 
States Department of Agricul- 
ture, in the fight against the 
gvpsy moth. At least ten or 
twelve species must be col- 
lected, imported, and colo- 
» 
» sn Bat anal 
nized successfully, in order to 
make this work a complete 
success ; but there is excellent 
prospect of ultimate accom- 
plishment; and meanwhile, 
no other known measures 
Fig. 53.— Eggs of Chrysopa sp. Predaceous 
will avail to limit the ravages on plant lice. Enlarged. Original. 
of the pest. 
Among our valuable families of predaceous insects are the ground 
beetles, tiger beetles, and lady-bird beetles in the order Coleoptera ; 
the assassin bugs and many aquatie forms in the order Hemiptera; the 
dragon flies constituting the order Odonata; and several families in 
the two-winged flies or Diptera. 
Two orders furnish the majority of our parasitic species, the Diptera 
and the Hymenoptera. The number of parasites within these orders 
is enormous. The principal families include the Tachina flies and the 
Syrphus flies in the Diptera, and the Ichneumons, Braconids, and 
Chalcids in the Hymenoptera. 
