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superabundance of this one element. ‘he fact that several of these 
species are ordinarily dependent in part upon vegetable food is not 
to be placed to their discredit, but, on the contrary, rather increases 
their efficiency as checks upon insect oscillations. The numbers of 
any species strictly dependent upon insects for food must, of course, 
rise and fall with the numbers of the species upon which it preys, 
or indeed a little after them. There consequently can never be any 
surplus of such species maintained for the suppression of arising 
outbreak among the injurious insects. If, on the other hand, our 
carnivorous beetles can sustain themselves during a deficiency of 
insect food by resorting to vegetation, a large surplus may be. held 
ready for instant attack upon any injurious insect which commences 
to appear in unusual numbers. 
This argument applies with special force to the Coccinellida, 
which have been shown to feed so largely upon the omnipresent 
and everywhere abundant moulds and blights of vegetation.* 
We are thus brought to see the points of evident superiority of 
the insectivorous beetles over the parasitic Hymenoptera. The 
latter must share in all the ups and downs of the host species, and 
can only be of service in finally putting a period to uprisings already 
well under way. In fact, there is considerable reason to suspect 
that these strictly dependent parasites often cause the oscillations 
which they afterwards have the credit of suppressing; and it is a 
very significant fact, in this connection, that the most irregular and 
destructive, insects are, as a rule, the worst ridden by parasites. 
When the army-worm, for example, commences to throng the fields 
in hordes, an extraordinary opportunity is afforded its parasitic 
enemies to multiply, and this increase in their numbers necessarily 
proceeds at a geometrical ratio, until it is arrested by a resulting seri- 
ous diminution in the numbers of the worms themselves. The parasites 
must thus necessarily far outstrip their hosts for a time, and, as a 
consequence, eventually reduce them to insignificance. But with 
the disappearance of the latter the parasites must suffer in turn; 
and so an unending alternation goes on, needing no other explana- 
tion in many cases than the superabundance of parasites. 
With respect to the families treated in this paper, however, we 
have not a particle of evidence upon which to rest such a charge; 
but everything indicates that their services to agriculture are ren- 
dered at no more expense than the trivial injuries to vegetation for 
which a few of them are responsible. 
* The discovery of this fact opens the way for some interesting and promising experi- 
ment. If any class of predaceous insects can be bred artificially to advantage, it is 
probably the Coccinellide, since the above kinds of food could be furnished.them in 
unlimited quantities, at trivial expense. It remains to be seen, however, whether they 
could reproduce without animal food, 
