Physical geography. By Mary Somerville ...

400 PHYSICAL GEOGRAPHY. CHAP. XXVIII.
insect passes seventeen years in the earth, near the roots of fruit and
other trees, and lives a few weeks in the air, only long enough to provide for the continuation of the species by depositing its eggs, beneath
the tender bark of the smaller branches or twigs of plants. Very soon
after the animal emerges from the egg, it plunges into the earth to
assume its state of torpidity for seventeen years.
This animal appeared in and around Philadelphia, in May 1851;
but in other localities at different dates; in Ohio it was present in
1846, and will appear there again in 1863, and not before, and it
will recur at Philadelphia in 1868.]
Insects do not attain their perfect state till the plants they are to
feed upon are ready for them. Hence in cold and temperate climates their appearance is simultaneous with vegetation; and as the
rainy and dry seasons within the tropics correspond to our winter
and summer, insects appear there after the rains and vanish in the
heat; the rains, if too violent, destroy them; and in countries
where that occurs there are two periods in the year in which they
are most abundant-one before and one after the rains. It is also
observed in Europe that insects decrease in the heat of summer and
become more numerous in autumn: the heat is thought to throw
some into a state of torpor, but the greater number perish.
It is not known that any insect depends entirely upon only one
species of plant for its existence, or whether it may not have recourse to congeners should its habitual plant perish. When particular species of plants of the same family occur in places widely
apart, insects of the same genus will be found on them, so that the
existence of the plant may often be inferred from that of the insect,
and in several instances the converse.
When a plant is taken from one country to another in which it
has no congeners, it is not attacked by the insects of the country:
thus our cabbages and carrots in Cayenne are not injured by the insects of that country, and the tulip-tree and other magnolias are not
molested by our insects; but if a plant has congeners in its new
country, the insect inhabitants will soon find their way to the
stranger.
The common fly is one of the most universal of insects, yet it
was unknown in some of the South Sea islands till it was carried
there from Europe by ships, where it has now become a plague.
Mosquitoes and culices [gnats] are spread over the world more
generally than any other tribe: they are the torment of men and
animals from the poles to the equator, by night and by day; the
species are numerous and their location partial. In the arctic regions the Culex pipiens, which passes two-thirds of its existence in
water, swarms during the summer in myriads: the lake Myvatr, in
Iceland, has its name from the legions of these tormentors that
cover its surface. They are less numerous in central Europe, though