Although this and the following chapter may not be interesting to all my
readers, I think it is only right to add some remarks on the structure and
classification of insects, so that any one who wishes to follow up the
subject may gather a few general ideas which may induce them to take up
some technical and scientific work in which they will get fuller and more
exact data on the difficulties which are involved in such simple questions
as "What is an insect?" "How are the different orders of insects
distinguished from each other?" "What is a species?" etc.
To realize the characters of an insect in its perfect or "imago" state, we
may for the moment forget what often seems to be its most important
features, and which are frequently its most extensive parts, viz. its limbs
or {125} appendages; by limbs are meant its wings, legs, horns or antennae,
jaws or mandibles, etc.: strip these all off, and we have a limbless trunk,
which many would not recognize as belonging to an insect at all; still this
limbless trunk possesses characters which assert its insect nature, as it
may be known from other limbless trunks by being divided into three parts
by two great transverse divisions; in most insects these are extremely well
marked, and in all they have a very real existence. The parts thus divided
off are known by the names of head, thorax, and abdomen. Anybody knows how
easy it is to break off the head or body of a dried insect. Now the head or
body breaks off at one of these divisions, and it is this partitioning of
the body into three sections which makes one of the strongest characters in
the definition of an insect. The three parts, thus divided off, each
possesses special functions in the life of the creature. In the head are
contained the principal organs of sense and brain; in the thorax, the
organs of locomotion; and in the body those of digestion, reproduction,
etc.
This division into three parts does not however {126} always hold good in
the early stages of the insect's life, and we must remember that the
creature commences life on leaving the egg, and not merely on its emergence
from the chrysalis, so that we have to reckon with caterpillars, grubs and
all sorts of curious immature forms in our conceptions of an insect.
These early stages do not as a rule interest the public much, but it is
well to bear in mind that the "perfect insect" stage is reached by some
insects along apparently a very different road from that travelled by
others. Some leave the egg as caterpillars or grubs, and after various
changes of skin become apparently lifeless chrysalids, from which they
emerge as perfect insects. Others leave the egg as diminutive likenesses of
their parents, and run or hop about much as they do, attaining the perfect
insect stage simply by a series of changes of skin, without any definite
quiescent or chrysalis condition.
The observation, therefore, which one often hears that insects never grow,
has to be taken with caution; all insects grow in their early stages, but
it is an obvious truth that insects do not {127} grow after they attain the
imago or "perfect insect" condition. A small fly will never become a large
fly, nor a small beetle a large beetle. This is only because we do not
recognize their caterpillars or grubs as flies and beetles; but a
grasshopper we know grows, because its early stages are of the same general
form as the perfect insect, and we see the little ones hopping about in
some places, and if we visit the same place later on we notice that they
have grown, but as soon as they cast their last skin and obtain the free
use of their wings, growth ceases, as it does in a fly or a beetle or in
any other insect.
It must not be supposed that the limbs of insects are of no value in their
identification. We only removed them in order to emphasize the great
importance of the character derived from the regional constrictions of the
body, which is considered to be certainly one of the most, if not the most,
important of any. Besides this character every perfect insect should have
six legs, four wings, and various appendages on the head, such as antennae,
mandibles, maxillae, labium, etc.; some of these may be so modified as
hardly to {128} be recognizable, but they are hardly ever absent
altogether; for instance, the two fore wings of a beetle are modified into
what are called wing cases, and fold over its back, protecting the two hind
wings, which are more or less membranous, as are those of a bee. They have
not the functions of locomotive organs, and are used in flight as poisers.
Again in the case of a fly, the hind wings seem to be absent, but they are
considered to be represented by two little projecting organs which look
like large headed pins or nails, but which are quite useless for locomotive
purposes.
The organs of the mouth are especially liable to modification, and on these
the older authors used to frame their classification. Insects were divided
by them, primarily, into two great divisions, viz. those which had a biting
and those which had a sucking mouth; treated in this way, the following
orders fall into the division with biting mouths:--
_Coleoptera_, or beetles; _Hymenoptera_, or bees, wasps, ants, etc.;
_Orthoptera_ and _Neuroptera_, which include the grasshoppers, earwigs,
cockroaches, dragonflies, May flies, etc. {129}