Structure of Insects.
Although insects present a great variety of forms, they nevertheless
agree in general features; thus by studying the structure of some
generalised species, which will give a broad idea of the main
characteristics, one is enabled to recognise different structural
modifications assumed by various species. For this purpose a weta,
grasshopper, or cockroach may be taken as a type.
Just as in the case of the crayfish, so the body of an insect is
completely covered and protected by a continuous “shell,” very solid
in some insects, more or less pliable in others, but even in the most
delicate forms tending to become rigid and brittle after death. This
shell acts as a skeleton and as a very effective armour-plating,
protecting and supporting the soft body within. Unlike the shell of
the crayfish, which is mainly calcareous, that of insects consists of
a horny substance called _chitin_, secreted by the underlying skin,
and constitutes what is known as a _cuticle_. It is due to this horny
cuticle or shell that the form and colour of most insects are preserved
after death, though the enclosed body tissues decay unless preserved in
some suitable medium.
The cuticle, though forming a complete covering, does not enclose the
body in an inflexible shell; flexibility is allowed by the cuticle
being formed of a segmented series of strongly-chitinised sections
alternating with skin-like, feebly-chitinised, and very elastic
sections; this arrangement gives freedom of movement to the enclosed
body, as is readily seen in the movements of a caterpillar.
There are three distinctly separated divisions of the insect
body--head, thorax, and abdomen--each consisting of a varying number
of segments (Fig. 3). The head segments are so closely fused as to
be practically untraceable, the cuticle forming a rigid capsule; the
thorax, to which the head is attached, carries the wings (when present)
and the legs, and consists of three segments; posterior to the thorax
is the abdomen, comprised of several segments, which show the typical
segmentation of insects better than any other part of the body.
The head capsule is more or less freely movable on the thorax, and
bears certain sensory organs, together with the mouth appendages.
The sensory organs are the eyes and the feelers, or antennæ. On each
side is a compound eye of varying size, according to the insect; each
eye consists of a variable number (from a comparative few to several
thousand) of microscopic, hexagonal lenses, each of which records a
separate image. Between the compound eyes, on top of the head, are
three simple eyes in some insects, but in others one or all of these
may be absent. Between the compound eyes on the front aspect of the
head is a pair of feelers, or antennæ; they consist of a variable
number of joints, are freely movable and highly sensory, thread-like
or hair-like, short, or longer even than the whole body, and may be
bare or clothed to a varying degree with hair or bristles. On the
antennæ are the organs of touch, smell, and sometimes hearing.
[Illustration: FIG. 3.]
When the head of a weta, grasshopper, or cockroach is removed from
the body and boiled for a few minutes in a 10 per cent. solution of
caustic potash, and then washed in water in order to remove the muscles
and other tissues, a large opening will be seen on the posterior
surface where the head was attached to the thorax; also, if the mouth
appendages are pulled apart, they will be seen to surround another
opening on the lower aspect of the head capsule, marking the position
of the mouth. The digestive canal passes from the mouth through the
posterior opening into the thorax.
The mouth appendages are as follows (Fig. 3):--Suspended from the fore
aspect of the mouth opening is a more or less conspicuous movable
flap, which forms the upper lip, while from the posterior aspect of
the same opening is another suspended appendage forming the lower lip;
this latter appendage is really a complicated one, and bears a pair
of short, jointed appendages--the palps--which are sensory organs,
while on its inner surface--_i.e._, within the mouth--is a swollen
area or tongue, an organ very greatly modified in certain insects.
Between the upper and lower lips, and suspended from both sides of the
mouth opening, is a pair of true jaws immediately behind the upper
lip, followed by a pair of accessory jaws immediately before the lower
lip; these jaws do not move up and down, but have a side-wise action,
closing and opening like scissor blades. While the true jaws are each
of one piece, the accessory jaws consist of several parts, and each
bears in addition a jointed palp, as in the case of the lower lip. The
upper and lower lips serve to hold the food in the mouth, the true
jaws nibble or tear off portions of the food and masticate it (if the
term can be used), while the accessory jaws, aided by the lower lip,
manipulate the food during the process of feeding.
The comparatively simple arrangement of mouth parts found in the weta,
grasshopper, and cockroach, as described above, is characteristic of
all insects that gnaw or chew their food--_e.g._, earwigs, beetles
and their larvæ or grubs, the caterpillars of moths, and so on. There
is, however, a vast number of insects that has developed more or less
complex variations of this generalised pattern, according to the manner
of feeding.
The mouth parts of the worker honey-bee, for example, have the jaws
adapted for eating pollen and moulding wax for the comb; the accessory
jaws, however, are lengthened, though their palps are reduced to
mere vestiges in contrast with the elongated palps of the lower lip;
the most remarkable modification is that of the greatly elongated
tongue, with its spoon-like tip adapted for reaching nectar of flowers
having deep-seated nectaries. For the same purpose, the mouth parts
are modified in a moth (Fig. 3) to form a long proboscis, which lies
curled up in a spiral beneath the head when not in use; in this case
the proboscis is the modified accessory jaws, the remaining mouth
parts, with the exception of the well-developed palps of the lower lip,
being greatly reduced. In a blood-sucking insect, such as the female
mosquito, all the mouth parts are well developed, but are very delicate
and greatly lengthened and suited for piercing the skin. The greatest
modification is found in the blow-fly proboscis, which is a soft,
sucking tube, with no outward resemblance to the generalised plan,
except for the palps of the accessory jaws. The mouth parts of insects
(_e.g._, aphids) which feed on the nutrient sap of plants, just in
the same way as mosquitoes do on blood, are modified for puncturing
the tissues of plants; in such insects the upper lip is short, and
both pairs of palps are atrophied, but the jaws and accessory jaws are
greatly lengthened in the form of bristle-like stylets, which lie in
a groove along the equally lengthened lower lip (Fig. 3). The manner
in which insects feed is of great importance in controlling them with
insecticides, and the two types to bear in mind are those that chew
their food and those that suck the sap of plants, reached by puncturing
the tissues.
As already stated, the thorax consists of the three segments
immediately behind the head, and carries the organs of locomotion; its
three segments are distinct, and may be referred to, respectively, as
the fore, middle, and hind thorax. The cuticle of each thoracic segment
consists of a number of chitinised plates connected by membranous
areas; these plates are arranged in three series--the back or dorsal;
the lower or ventral, forming the sternum; and the lateral, or
side-pieces, connecting the dorsal and ventral ones.
At the lower surface of each thoracic segment is attached a pair of
legs, the members of each pair being separated by the sternum of the
segment to which they belong. The presence of three pairs of legs is a
character by which insects can be distinguished from all other animals;
indeed, on account of this feature, insects are sometimes called the
hexapods, or six-legged animals. Each leg is covered by a continuation
of the body cuticle, and is five-jointed; the first two joints at the
attachment to the body are small; the next two are long, and form
the greater part of the limb; while the fifth, or foot, consists of
a varying number of small joints, the terminal one bearing a pair of
claws.
In the typical winged insects there are two pairs of wings: one pair
attached to the middle thorax, and the other to the hind thorax; owing
to the development of muscles controlling flight, the middle and hind
thorax of winged insects are usually better developed than the fore
thorax; this is especially noticeable in the thorax of two-winged flies
(daddy-long-legs and blow-flies), where the hind wings are reduced to
vestiges, the power of flight being thus confined to the middle thorax,
which forms by far the greater portion of the whole thorax.
Each wing, arising from the junction of the dorsal and lateral thoracic
plates, is a bag-like extension of the cuticle, flattened leaf-like, so
as to form a double flexible membrane. The wing membrane is supported
by several ribs or veins, which may be very numerous (grasshopper) or
few (aphid), while the fore edge, where it cuts the air in flight, is
bordered by a stouter vein, ensuring rigidity. The fore and hind wings
of some insects work independently, but in agreement of movement, while
in others the fore and hind wings of each side are coupled along their
adjoining margins, giving greater rigidity during flight.
The abdomen of insects consists of a varying number of visible
segments; each segment is covered by an upper and lower chitinous plate
connected by membrane, there being no side plates as are found in
the thorax. There are no organs of locomotion (except in a very few
cases), the only appendages being those connected with reproduction;
the latter are well developed in the female weta, where the egg-laying
apparatus, or ovipositor, projects blade-like from the apex of the
abdomen. In very many insects, however, the external reproductive
organs are not readily seen without special study.
All insects, from the largest to the most minute, contain internally
a well-formed heart and a digestive, reproductive, respiratory, and
nervous system (Fig. 3), while the spaces surrounding these organs
are, for the most part, packed with a complex system of muscles. The
heart is a delicate tube lying along the middle of the back or dorsal
surface of the body, immediately under the skin, and extends almost
from one end of the insect to the other; in an almost similar position,
close to the lower or ventral surface of the body, the nervous system
is situated, and consists of a chain of nerve centres, or ganglia,
connected by a double nerve cord, the most anterior of these ganglia
being in the head and forming the brain, the following three lying in
the thorax, one to each segment, while the remainder are confined to
the abdomen, one ganglion to each segment, as in the thorax. In many
insects the number of nerve centres is reduced, owing to the fusion of
two or more. The reproductive organs are located in the abdomen.
The digestive system consists of a tube (Fig. 3), with its appendages,
opening at the mouth and at the posterior end of the body; this
alimentary canal may be straight and simple, or convoluted and complex,
according to the insect and the nature of its food. Respiration in
insects is carried on by means of a system of air tubes (Fig. 3), which
branch and re-branch to form an intricate system of delicate tubular
airways, carrying the atmosphere to all tissues of the body; the main
air tubes open at the surface by a series of breathing pores normally
arranged along each side of the body, except on the head; these pores
are best seen on a caterpillar or on the abdomen of adult insects.