INSECTS IN RELATION TO MAN 327 



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

 is that of the gypsy moth, artificially introduced into Massachusetts 

 from Europe; another is the fluted scale, transported from Australia to 

 California. Some conception of the vast restricting influence of one 

 species upon another may be gained from the fact that the fluted scale 

 has practically been exterminated in California as the result of the im- 

 portation from Australia of one of its natural enemies, a lady-bird beetle 

 known as Novius cardinalis. The plant lice, though of unparalleled 

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

 described. 



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 

 vegetable matter. This function of insects is most noticeable in the 



