INSECTS IN RELATION TO MAN 395 



cereals ; clothing, especially of wool, fur or feathers ; also fur- 

 niture and hundreds of other useful articles. 



As carriers of disease germs, insects are of vital importance 

 to man, as we have shown. 



Beneficial Insects. The vast benefits derived from insects 

 are too often overlooked, for the reason that they are often 

 so unobvious as compared with the injuries done by other spe- 

 cies. Insects are useful as checks upon noxious insects and 

 plants, as pollenizers of flowers, as scavengers, as sources of 

 human clothing, food, etc., and as food for birds and fishes. 



Almost every insect is subject to the attacks of other insects, 

 predaceous or parasitic to say nothing of its many other 

 enemies and but for this a single species of insect might soon 

 overrun the earth. There are only too many illustrations of 

 the tremendous spread of an insect in the absence of its accus- 

 tomed 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 influ- 

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

 that the fluted scale has practically been exterminated in Cali- 

 fornia as the result of the importation from Australia of one 

 of its natural enemies, a lady-bird beetle known as Novius car- 

 dinalis. 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 phe- 

 nomenon of hyperparasitism being of considerable economic 

 importance. A beneficial primary parasite may be overpow- 

 ered by a secondary parasite, evidently to the indirect disad- 

 vantage 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 



