in the caterpillar, inserting its long tail in the openings of the
cone till it touches the included insect, for its body is too large to
enter. Thus it fixes its minute egg upon the caterpillar, which being
hatched, destroys it."[954]

Entomologists enumerate many parallel cases where insects, appropriated
to certain plants, are kept down by other insects, and these again by
parasites expressly appointed to prey on them.[955] Few perhaps are in
the habit of duly appreciating the extent to which insects are active in
preserving the balance of species among plants, and thus regulating
indirectly the relative numbers of many of the higher orders of
terrestrial animals.

The peculiarity of their agency consists in their power of suddenly
multiplying their numbers to a degree which could only be accomplished
in a considerable lapse of time in any of the larger animals, and then
as instantaneously relapsing, without the intervention of any violent
disturbing cause, into their former insignificance.

If, for the sake of employing, on different but rare occasions, a power
of many hundred horses, we were under the necessity of feeding all these
animals at great cost in the intervals when their services were not
required, we should greatly admire the invention of a machine, such as
the steam-engine, which was capable at any moment of exerting the same
degree of strength without any consumption of food during periods of
inaction. The same kind of admiration is strongly excited when we
contemplate the powers of insect life, in the creation of which the
Author of nature has been so prodigal. A scanty number of minute
individuals, to be detected only by careful research, are ready in a few
days, weeks, or months, to give birth to myriads, which may repress any
degree of monopoly in another species, or remove nuisances, such as dead
carcases, which might taint the air. But no sooner has the destroying
commission been executed than the gigantic power becomes dormant--each
of the mighty host soon reaches the term of its transient existence, and
the season arrives when the whole species passes naturally into the egg,
and thence into the larva and pupa state. In this defenceless condition
it may be destroyed either by the elements, or by the augmentation of
some of its numerous foes which may prey upon it in the early stages of
its transformation: or it often happens that in the following year the
season proves unfavorable to the hatch