412 ENTOMOLOGY 



There are only too many illustrations of the tremendous spread of an 

 insect in the absence of its accustomed natural enemies. One of these 

 examples is that of the gipsy moth, artificially introduced into Massa- 

 chusetts from Europe; another is the fluted scale, transported from 

 Australia to California. Some conception of the vast restricting influ- 

 ence of one species upon another may be gained from the fact that the 

 fluted scale was practically exterminated in California as the result of 

 the importation from Australia of one of its natural enemies, a lady-bird 

 beetle known as Nomus cardinalis. The plant lice, though of un- 

 paralleled fecundity, are ordinarily held in check by a host of enemies 



(P- 379)- 



An astonishingly large number of parasites may develop in the body 

 of a single individual ; thus over 3 ,000 specimens of a hymenopterous 

 parasite (Copidosoma truncatellum) were reared by Giard from a single 

 Plusia caterpillar. 



Parasites themselves are frequently parasitized, this phenomenon of 

 hyperparasitism being of considerable economic importance. A bene- 

 ficial primary parasite may be overpowered by a secondary parasite, 

 evidently to the indirect disadvantage of man, while the influence of a 

 tertiary parasite would be beneficial again. Now parasites of the third 

 order occur and probably of the fourth order, as appears from Howard's 

 studies, which we have already summarized. Moreover, parasites of all 

 degrees are attacked by predaceous insects, birds, bacteria, fungi, etc. 

 The control of one insect by another becomes, then, a subject of extreme 

 intricacy. 



Insects render an important, though commonly unnoticed, service 

 to man in checking the growth of weeds. Indeed, insects exercise a vast 

 influence upon vegetation in general. A conspicuous alteration in the 

 vegetation has followed the invasions of the Rocky Mountain locust, 

 as Riley has said; many plants before unnoticed have grown in profu- 

 sion and many common kinds have attained an unusual luxuriance. 



As agents in the cross pollination of flowers, insects are eminently 

 important. Darwin and his followers have proved beyond question 

 that as a rule cross pollination is indispensable to the continued vitality 

 of flowering plants; that repeated close pollination impairs their vigor 

 to the point of extermination. Without the visits of bees and other 

 insects our fruit trees would yield little or nothing, and the fruit grower 

 owes these helpers a debt which is too often overlooked. 



As scavengers, insects are of inestimable benefit, consuming as they 

 do in incalculable quantity all kinds of dead and decaying animal and 



