Distribution of Insects.
THREE hundred thousand insects are known: some with wings, others
without; some are aquatic, others are aquatic only in the first stage of
their existence, and many are parasitical. Some land insects are
carnivorous, others feed on vegetables; some of the carnivorous tribe
live on dead, others on living animals, but they are not half so
numerous as those that live on vegetables. Some change as they are
developed; in their first stage they eat animal food, and vegetables
when they come to maturity.
Insects maintain the balance among the species of the vegetable creation
by preventing the tendency that plants have to encroach on one another.
The stronger would extirpate the weaker, and the larger would destroy
the smaller, were it not checked by insects which live on vegetables. On
the other hand, many plants would be extirpated by insects were these
not devoured by other insects and spiders.
Of the 8000 or 9000 British insects the greatest part are carnivorous,
and therefore keep the others within due bounds.
Insects increase in kinds and in numbers from the poles to the equator:
in a residence of 11 months in Melville Island, Sir Edward Parry found
only 6 species, because lichens and mosses do not afford nourishment for
the insect tribes, though it is probable that every other kind of plant
gives food and shelter to more than one species; it is even said that 40
different insects are quartered upon the common nettle.
The increase of insects from the poles to the equator does not take
place at the same time everywhere. The polar regions and New Holland
have very few specifically and individually; they are more abundant in
Northern Africa, Chile, and in the plains west of the Brazils; North
America has fewer species than Europe in the same latitude, and Asia has
few varieties of species in proportion to its size; Caffraria, the
African and Indian islands, possess nearly the same number of species;
but by far the richest of all, both in species and numbers, are central
and intertropical America. Beetles are an exception to the law of
increase towards the equator, as they are infinitely more numerous in
species in the temperate regions of the northern hemisphere than in
equatorial countries. The location of insects depends upon that of the
plants which yield their food; and, as almost each plant is peopled with
inhabitants peculiar to itself, insects are distributed over the earth
in the same manner as vegetables; the groups, consequently, are often
confined within narrow limits, and it is extraordinary that,
notwithstanding their powers of locomotion, they often remain within a
particular compass, though the plants, and all other circumstances in
their immediate vicinity, appear equally favourable for their
habitation.
The insects of eastern Asia and China are different from those in Europe
and Africa; those in the United States differ specifically from the
British, though they often approach very near; and in South America the
equinoctial districts of New Grenada and Peru have distinct groups from
those in Guiana.
Though insects are distributed in certain limited groups, yet most of
the families have representatives in all the great regions of the globe,
and some identical species are inhabitants of countries far from one
another. The Vanessa Cardui, or “Painted Lady Butterfly,” is found in
all the four quarters of the globe and in Australia; and one, which
never could have been conveyed by man, is native in southern Europe, the
coast of Barbary, and Chile. It is evident from these circumstances that
not only each group, but also 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 effect 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 corresponds with 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 perfect state; they lie buried in the
ground in the form of grubs: the cock-chafer comes to maturity in 3
years, and some American species require a much longer time.
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
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 by
ships from Europe, and it has now become a plague.
The mosquito and culex 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 in 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 one species of mosquito, the Simulium columbaschense,
which is very small, appears in such clouds in parts of Hungary,
especially the Bannat of Temeswar, that it is not possible to breathe
without swallowing many: even cattle and children have died from them.
In Lapland there is a plague of the same kind. Of all places on earth
the Orinoco and other great rivers of tropical America are the most
obnoxious to this plague. The account given by Baron Humboldt is really
fearful: at no season of the year, at no hour of the day or night, can
rest be found; whole districts in the upper Orinoco are deserted on
account of these insects. Different species follow one another with such
precision, that the time of day or night may be known accurately from
their humming noise, and from the different sensations of pain which the
different poisons produce. The only respite is the interval of a few
minutes between the departure of one gang and the arrival of their
successors, for the species do not mix. On some parts of the Orinoco the
air is one dense cloud of poisonous insects to the height of 20 feet. It
is singular that they do not infest rivers that have black water, and
each white stream is peopled with its own kinds; though ravenous for
blood, they can live without it, as they are found where no animals
exist.
In Brazil the quantity of insects is so great in the woods, that their
noise is heard in a ship at anchor some distance from the shore.
Various genera of butterflies and moths are very limited in their
habitations, others are dispersed over the world, but the species are
almost always different. Bees and wasps are equally universal, yet each
country has its own. The common honey-bee is the only European insect
directly useful to man; it was introduced into North America not many
years ago, and is now spread over the continent: it is now naturalized
in Van Diemen’s Land and New Zealand. European bees, of which there are
many species, generally have stings; the Australian bee, like a black
fly, is without a sting; and in Brazil there are 30 species of stingless
bees.
Fire-flies are mostly tropical, yet there are four species in Europe; in
South America there are three species, and so brilliant that their pale
green light is seen at the distance of 200 paces.
The silkworm comes from China, and the cochineal insect is a native of
tropical America: there are many species of it in other countries. The
Coccus Lacca is Indian, the Coccus Ilicis lives in Italy, and there is
one in Poland, but neither of these have been cultivated.
Scorpions under various forms are in all warm climates; 2 or 3 species
are peculiar to Europe, but they are small in comparison with those in
tropical countries: one in Brazil is six inches long. As in mosquitos,
the poison of the same species is more active in some situations than in
others. At Cumana the sting of the scorpion is little feared, while that
of the same species in Carthagena causes loss of speech for many days.
Ants, Formicidæ (Hymenoptera), are universally distributed, but of
different kinds. Near great rivers they build their nests above the line
of the annual inundations. The insects called white ants, belonging to a
different genus and family Termitidæ (Neuroptera), are so destructive in
South America, that Baron Humboldt says there is not a manuscript in
that country a hundred years old.
There are upwards of 1200 species of spiders and their allies known;
each country has its own, varying in size, colour, and habits, from the
huge bird-catching spider of South America, to the almost invisible
European gossamer floating in the air on its silvery thread. Many of
this ferocious family are aquatic; and spiders, with some other insects,
are said to be the first inhabitants of new islands.
The migration of insects is one of the most curious circumstances
relating to them: they sometimes appear in great flights in places where
they never were seen before, and they continue their course with
perseverance which nothing can check. This has been observed in the
migration of crawling insects: caterpillars have attempted to cross a
stream. Countries near deserts are most exposed to the invasion of
locusts, which deposit their eggs in the sand, and, when the young are
hatched by the sun’s heat, they emerge from the ground without wings;
but as soon as they attain maturity, they obey the impulse of the first
wind, and fly, under the guidance of a leader, in a mass, whose front
keeps a straight line, so dense that it forms a cloud in the air, and
the sound of their wings is like the murmur of the distant sea. They
take immense flights, crossing the Mozambique Channel from Africa to
Madagascar, which is 120 miles broad: they come from Barbary to Italy,
and a few have been seen in Scotland. Even the wandering tribes of
locusts differ in species in different deserts, following the universal
law of organized nature. Insects, not habitually migratory, sometimes
migrate in great flocks. In 1847 lady-birds or coccinellæ and the bean
aphis arrived in immense multitudes at Ramsgate and Margate from the
continent, in fine calm weather, and a mass of the Vanessa cardui flew
over a district in a column from 10 to 15 yards wide, for 2 hours
successively. Why these butterflies should simultaneously take wing in a
flock is unaccountable, for had it been for want of food they would
probably have separated in quest of it. In 1847 the cabbage butterfly
came in a mass from the coast of France to England. Dragon-flies migrate
in a similar manner. Professor Ehrenberg has discovered a new world of
creatures in the Infusoria, so minute that they are invisible to the
naked eye. He found them in fog, rain, and snow, in the ocean, in
stagnant water, in animal and vegetable juices, in volcanic ashes and
pumice, in opal, in the dusty air that sometimes falls on the ocean; and
he detected 18 species 20 feet below the surface of the ground in
peat-earth, which was full of microscopic live animals: they exist in
ice, and are not killed by boiling water. This lowest order of animal
life is much more abundant than any other, and new species are found
every day. Magnified, some of them seem to consist of a transparent
vesicle, and some have a tail: they move with great alacrity, and show
intelligence by avoiding obstacles in their course: others have
siliceous shells. Language, and even imagination, fails in the attempt
to describe the inconceivable myriads of these invisible inhabitants of
the ocean, the air, and the earth: they no doubt become the prey of
larger creatures, and perhaps bloodsucking insects may have recourse to
them when other prey is wanting.