ing of the eggs or the development
of the pupae.

Thus the swarming myriads depart which may have covered the vegetation
like the aphides, or darkened the air like locusts. In almost every
season there are some species which in this manner put forth their
strength, and then, like Milton's spirits, which thronged the spacious
hall, "reduce to smallest forms their shapes immense"--

----So thick the aary crowd
Swarm'd and were straiten'd; till the signal given,
Behold a wonder! they but how who seem'd
In bigness to surpass earth's giant sons,
Now less than smallest dwarfs.

A few examples will illustrate the mode in which this force operates. It
is well known that, among the countless species of the insect creation,
some feed on animal, others on vegetable matter; and upon considering a
catalogue of eight thousand British Insects and Arachnidae, Mr. Kirby
found that these two divisions were nearly a counterpoise to each other,
the carnivorous being somewhat preponderant. There are also distinct
species, some appointed to consume living, others dead or putrid animal
and vegetable substances. One female, of _Musca carnaria_, will give
birth to twenty thousand young; and the larvae of many flesh-flies devour
so much food in twenty-four hours, and grow so quickly, as to increase
their weight two hundred-fold! In five days after being hatched they
arrive at their full growth and size, so that there was ground, says
Kirby, for the assertion of Linnaeus, that three flies of _M. vomitoria_
could devour a dead horse as quickly as a lion[956]; and another Swedish
naturalist remarks, that so great are the powers of propagation of a
single species even of the smallest insects, that each can commit, when
required, more ravages than the elephant.[957]

Next to locusts, the aphides, perhaps, exert the greatest power over the
vegetable world, and, like them, are so sometimes so numerous as to
darken the air. The multiplication of these little creatures is without
parallel, and almost every plant has its peculiar species. Reaumur has
proved that in five generations one aphis may be the progenitor of
5,904,900,000 descendants; and it is supposed that in one year there may
be twenty generations.[958] Mr. Curtis observes that, as among
caterpillars we find some that are constantly and unalterably attached
to one or more particular species of plants, and others that feed
indiscriminately on most sorts of herbage, so it is