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 {111}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. [Illustration: 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 {112}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. [Illustration: FIG. 62.—Diagrammatic Insect to explain terms of position. A, apex; B, base: 1, tibia; 2, last abdominal segment; 3, ideal centre.] SYSTEMATIC ORIENTATION.