Each of these
larvae remains in the pupa state about five or six days, so that each
parent fly may be increased ten thousand-fold in a fortnight. Supposing
they went on increasing at this rate during only three months of summer,
there would result one hundred millions of millions of millions for each
fly at the commencement of summer,--a number greater probably than
exists at any one time in the whole world. And this is only one species,
while there are thousands of other species increasing also at an
enormous rate; so that, if they were unchecked, the whole atmosphere
would be dense with flies, and all animal food and much of animal life
would be destroyed by them. To prevent this tremendous increase there
must be incessant war against these insects, by insectivorous birds and
reptiles as well as by other insects, in the larva as well as in the
perfect state, by the action of the elements in the form of rain, hail,
or drought, and by other unknown causes; yet we see nothing of this
ever-present war, though by its means alone, perhaps, we are saved from
famine and pestilence.
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