There is no regular and definite course of the circulation in the Insects. The propulsive organ of the circulation is a long contractile cavity, situated in the back and termed the " dorsal vessel" (fig. 178, h). This is composed of a number of sacs ■, annulosa: insecta. 339 (ordinarily eight), opening into one another by valvular apertures, which allow of a current in one direction only - viz., towards the head. The blood is collected from the irregular venous sinuses which are formed by the lacunae and interstices between the tissues, and enters the dorsal vessel from behind, and by lateral valvular openings; it is then driven forwards, and is expelled at the anterior extremity of the body. The blood of the Insecta is corpusculated, and usually colourless. Whilst the general belief is that there is no regular system of blood-vessels (arteries and veins), and that the blood simply circulates through the interstices of the tissues, some observers affirm the partial existence of true vessels, and others maintain that the blood circulates in the spaces between the tracheae and their enveloping sheaths, which thus become converted into blood-vessels.
Fig. 178. - Ideal transverse section of an Insect, h Dorsal vessel; i Intestine; n Ventral Nerve-cord ; t t Stigmata, leading into the branched tracheal tubes ;w w Wings; a Coxa of one leg; b Trochanter; c Femur; d Tibia; e Tarsus. (After Packard.)
Respiration is effected by means of "tracheae," or branched tubes, which commence at the surface of the body by lateral apertures, called "stigmata," or "spiracles," and ramify through every part of the animal. In structure the tracheae are membranous, but their walls are strengthened by a chitinous filament, which is rolled up into a continuous spiral coil. In the aquatic larvae of many insects, and in one or two adult Insects (in Pteronarcys, one of the Orthoptera, and in one of the Phas-midae, of the same order) branches of the tracheae are sent to variously-shaped outgrowths which are termed "tracheal gills," and in which the blood is oxygenated. In all, however, with the exceptions above mentioned, these temporary internal or external appendages fall off when maturity is attained. The wings, also, whilst acting as locomotive organs, doubtless subserve respiration, the nervures being hollow tubes filled with blood and enclosing tracheae.
Entomologists have generally recognised the following kinds of breathing-organs in Insects:
1. The true tracheae, in the form of branched tubes, the walls of which are strengthened by a chitinous fibre.
2. Tracheal vesicles, or dilated receptacles directly connected with the proper tracheae, but having membranous walls not supported by a horny spiral fibre.
3. The modified tracheae of some adult Hemiptera and various aquatic larvae, in which the lips of the stigmata are prolonged into shorter or longer external tubes, by which the air is conveyed to the interior.
The nervous system in Insects, though often concentrated into special masses, consists essentially of a chain of ganglia, placed ventrally, and united together by a series of double cords or commissures. The cephalic or "prae-oesophageal" ganglia are of large size, and distribute filaments to the eyes and antennae. The post-oesophageal ganglia are united to the preceding by cords which form a collar round the gullet, and they supply the nerves to the mouth, whilst the next three ganglia furnish the nerves to the legs and wings. In larvae, thirteen pairs of ganglia may often be recognised. In the adults, however, of the higher groups of Insects (such as the Coleoptera, Hymenoptera, Diptera, and Lepidoptera), the thoracic ganglia coalesce into a single mass.
The organs of sense are the eyes and antennae. The eyes in Insects are usually "compound," and are composed of a number of hexagonal lenses, united together, and each supplied with a separate nervous filament. Besides these, simple eyes - " ocelli," or " stemmata," - are often present, or, in rare cases, may be the sole organs of vision. In structure these resemble the single elements of the compound eyes. In a few cases the eyes are placed at the extremities of stalks or peduncles, but in no case are these peduncles movably articulated to the head, as is the case in the Podophthalmous Crustaceans.
The antennae are movable, jointed filaments, attached usually close to the eyes, and varying much in shape in different Insects. They doubtless discharge the functions of tactile organs, but are probably the organ of other more recondite senses in addition.
The sexes in Insects are in different individuals, and most are oviparous. The ovum undergoes partial segmentation ; and the embryo has its future ventral surface turned outwards, and its dorsal surface inwards. Generally speaking, the young insect is very different in external characters from the adult, and it requires to pass through a series of changes, which constitute the "metamorphosis," before attaining maturity. In some Insects, however, there appears to be no metamorphosis, and in some the changes which take place are not so striking or so complete as in others. By the absence of metamorphosis, or by the degree of its completeness when present, Insects are divided into sections, called respectively Ameta-bola, Hemimetabola, and Holometabola, which, though not, perhaps, of a very high scientific value, are nevertheless very convenient in practice.