INSECTS IN RELATION TO MAN 39 
Sat 
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 Calli- 
fornia as the result of the importation from Australia of one 
of its natural enemies, a lady-bird beetle known as Novis 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 
