Physical geography. By Mary Somerville ...

CHAP. XXVIII. DISTRIBUTION OF INSECTS. 399
each particular species, must have been originally created in the
places they now inhabit.
Mountain-chains are a complete barrier to insects, even more so
than rivers; not only lofty mountains like the Andes divide the
kinds, but they are even different on the two sides of the Col de
Tende in the Alps. Each soil has kinds peculiar to itself, whether
dry or moist, cultivated or wild, meadow or forest. Stagnant water
and marshes are generally full of them; some live in water, some
run on its surface, and every water-plant affords food and shelter to
many different kinds. The east wind seems to have considerable
efect in bringing the insect or in developing the eggs of certain
species; for example, the aphis, known as the blight in our country,
lodges in myriads on plants, and shrivels up their leaves after a continued east wind. They are almost as destructive as the locust, and
sometimes darken the air by their numbers. Caterpillars are also
very destructive; the caterpillar of the Y moth would soon ruin the
vegetation of a country were it not a prey to some other. Insects
sometimes multiply suddenly to an enormous extent, and decrease
as rapidly and as unaccountably.
Temperature, by its influence on vegetation, has an indirect effect
on the insects that are to feed upon plants, and extremes of heat
and cold have more influence on their locality than the mean annual
temperature. Thus in the polar regions the mosquito tribes are
more numerous and more annoying than in temperate countries, because they pass their early stages of existence in water, which shelters them, and the short but hot summer is genial to their brief
span of life.
In some instances height produces the same effect in the distribution of insect life as difference of latitude. The parnassius Apollo,
a butterfly native in the plains of Sweden, is also found in the Alps,
the Pyrenees, and a closely allied species in the Himalaya. The
parnassius smyntheus, true to the habitat of the genus, has recently
been found on the Rocky Mountains of North America. Some insects require several years to arrive at their full development; they
lie buried in the ground in the form of grubs: the cockchafer takes
3 years to reach its perfect state, and some American species requite
a much longer time. [For example, the Cicadac septcncEecem, or
seventeen-year locust, whose appearance about Philadelphia was first
recorded in May 1715; and since that date, "punctually in the
same month, every seventeenth year, now certainly for nearly one
hundred and fifty years, has this extraordinary insect been known
to make its visit. No causes have affected it during that period,
not even so far as relates to the month in which it appears."' This
John Cassin-in the "Proceedings of the Academy of Natural Sciences
of Philadelphia," for September 1851.