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 

 Entomolog}^ of the United 

 States Department of Agricul- 

 ture, in the fight against the 

 gypsy moth. At least ten or 

 twelve species must be col- 

 lected, imported, and colo- 

 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 

 will avail to limit the ravages 

 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 aquatic 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 H>Tnenoptera. 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. 



Fig. 53. — Eggs of C/ir^sopa sp. Predaceous 

 on plant lice. Enlarged. Original. 



