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.

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.