Unlike the head and thorax, the abdomen is so loosely knitted together that it can undergo much expansion and contraction. This is facilitated by an imbricated arrangement of the plates, and by their being connected by means of membranes admitting of much movement (Fig. 47, m, p. [88]). In order to understand the structure of the abdomen it should be studied in its most distended state; it is then seen that there is a dorsal and a ventral hard plate to each ring, and there is also usually a stigma; there may be foldings or plications near the line of junction of the dorsal and ventral plates, but these margins are not really distinct pieces. The pleura, in fact, remain membranous in the abdominal region, contrasting strongly with the condition of these parts in the thorax. The proportions of the plates vary greatly; sometimes the ventral are very large in proportion to the dorsal, as is usually the case in Coleoptera, while in the Orthoptera the reverse condition prevails.
Cerci or other appendages frequently exist at the extremity of the abdomen (Fig. 47, n, p. [88]); the former are sometimes like antennae, while in other cases they may be short compressed processes consisting of very few joints. The females of many Insects possess saws or piercing instruments concealed within the apical part of the abdomen; in other cases an elongate exserted organ, called ovipositor, used for placing the eggs in suitable positions, is present. Such organs consist, it is thought, either of modified appendages, called gonapophyses, or of dorsal, ventral, or pleural plates. The males frequently bear within the extremity of the body a more or less complicated apparatus called the genital armour. The term gonapophysis is at present a vague one, including stings, some ovipositors, portions of male copulatory apparatus, or other structures, of which the origin is more or less obscure.
The caterpillar, or larva, of the Lepidoptera and some other Insects, bears a greater number of legs than the three pairs we have mentioned as being the normal number in Insects, but the posterior feet are in this case very different from the anterior, and are called false legs or prolegs. These prolegs, which are placed on the hind body, bear a series of hooks in Lepidopterous larvae, but the analogous structures of Sawfly larvae are destitute of such hooks.
Placed along the sides of the body, usually quite visible in the larva, but more or less concealed in the perfect Insect, are little apertures for the admittance of air to the respiratory system. They are called spiracles or stigmata. There is extreme variety in their structure and size; the largest and most remarkable are found on the prothorax of Coleoptera, especially in the groups Copridae and Cerambycidae.
The exact position of the stigmata varies greatly, as does also their number. In the Order Aptera there may be none, while the maximum number of eleven pairs is said by Grassi[[26]] to be attained in Japyx solifugus: in no other Insect have more than ten pairs been recorded, and this number is comparatively rare. Both position and number frequently differ in the early and later stages of the same Insect. The structure of the stigmata is quite as inconstant as the other points we have mentioned are.
Fig. 61.—Membranous space between pro- and meso-thoraces of a beetle Euchroma, showing stigma (st); a, hind margin of pronotum; b, front leg; c, front margin of mesonotum; d, base of elytra; e, mesosternum.
The admission of air to the tracheal system and its confinement there, as well as the exclusion of foreign bodies, have to be provided for. The control of the air within the system is, according to Landois[[27]] and Krancher,[[28]] usually accomplished by means of an occluding apparatus placed on the tracheal trunk a little inside of the stigma, and in such case this latter orifice serves chiefly as a means for preventing the intrusion of foreign bodies. The occluding apparatus consists of muscular and mechanical parts, which differ much in their details in different Insects. Lowne supposes that the air is maintained in the tracheal system in a compressed condition, and if this be so, this apparatus must be of great importance in the Insect economy. Miall and Denny[[29]] state that in the anterior stigmata of the cockroach the valves act as the occluding agents, muscles being attached directly to the inner face of the valves, and in some other Insects the spiracular valves appear to act partially by muscular agency, but there are many stigmata having valves destitute of muscles. According to Lowne[[30]] there exist valves in the blowfly at the entrance to the trachea proper, and he gives the following as the arrangement of parts for the admission of air:—there is a spiracle leading into a chamber, the atrium, which is limited inwardly by the occluding apparatus; and beyond this there is a second chamber, the vestibule, separated from the tracheae proper by a valvular arrangement. He considers that the vestibule acts as a pump to force the air into the tracheae.
Fig. 62.—Diagrammatic Insect to explain terms of position. A, apex; B, base: 1, tibia; 2, last abdominal segment; 3, ideal centre.