The chain of GANGLIA lying on the ventral side of the abdomen consists primitively of pairs of ganglia corresponding with the twelve segments, which are connected by longitudinal and transverse commissures. But many changes in the ganglia may be seen in insects caused by partial or entire amalgamation of single ganglia, so that in a few cases only one abdominal ganglion is present. In conclusion, a definite INTESTINAL NERVOUS SYSTEM is always present.
Of the organs of sense the FACETTED EYES, situated at the sides of the head, deserve special mention, as do also the ORGANS OF TOUCH and SMELL, situated on the antennæ, and the ORGANS OF HEARING and taste, or finer sensations, situated at the mouth and in the buccal cavity.
The sounds emitted by insects are, as a rule, produced by the friction or beating of certain chitinous parts, but sounds are also produced in breathing (flies).
The ORGANS OF RESPIRATION, the so-called tracheæ, are highly developed; there are openings (stigmata) at the sides of the body which draw in air by means of the active participation of the muscles of the body. The number of stigmata varies between two and ten pairs; the tracheæ themselves branch off from the trunks in the most varied manner, and carry air to the internal organs.
The colourless BLOOD circulates between the tissues and organs, and is kept circulating by the contraction of a chambered dorsal vessel provided with ostia, and which terminates in a short aorta opening at the anterior end.
Insects are SEXUALLY DISTINCT; their sexual glands are in pairs and have a tubular structure, but the testicular tubules are united together by a capsule into an oval testicle; exceptionally, also, the excretory canals are double, as also the sexual orifices; usually the paired canals unite into a single oviduct or spermatic duct which terminates at the posterior end of the body after receiving the products of various glands.
As to the HISTORY OF THE DEVELOPMENT of insects, all that is necessary to mention here is that the young hatched from eggs only exceptionally (as in Apterygota) resemble the adult parent (insecta ametabola); as a rule they differ from them not only in the shape of the body, but also more or less by their manner of life, and only attain the form of the parent through METAMORPHOSIS. This is a gradual process (insecta hemimetabola) in the Rhynchota and Orthoptera, or a sudden one with a stage of inanition (insecta metabola) in the other orders. This stage of rest or inanition, the PUPA, concludes the larval life (caterpillar, maggot, etc.); during the pupal stage no nourishment at all is taken, but the internal organs undergo changes; in some forms the rest is not absolute, as voluntary local movements may take place (pupæ of gnats).
The insects are divided into numerous orders according to the form of the mouth parts, the structure of the wings, as well as the manner of the development; with the exception of the lowest group (Apterygota), which is most nearly related to the ancestors of the insects, and which has no wings and undergoes no metamorphosis, all the remaining orders, which are termed Pterygota, have wings on the thorax, though there are, of course, a few species and families of this group which have lost their wings.
The Pterygota include—
(1) Orthoptera.—Biting mouth parts, anterior wings leathery, posterior wings thin, folded longitudinally; metamorphosis incomplete (grasshoppers, crickets, cockroaches).