[Illustration: FIG. 63.—Diagram of arrangement of some of the internal organs of an Insect: _a_, mouth; _b_, mandible; _c_, pharynx; _d_, oesophagus; _e_, salivary glands (usually extending further backwards); _f_, eye; _g_, supra-oesophageal ganglion; _h_, sub-oesophageal ganglion; _i_, tentorium; _j_, aorta; _k_{1}_, _k_{2}_, _k_{3}_, entothorax; _l_{1}-l_{8}_, ventral nervous chain; _m_, crop; _n_, proventriculus; _o_, stomach; _p_, Malpighian tubes; _q_, small intestine; _r_, large intestine; _s_, heart; _t_, pericardial septum; _u_, ovary composed of four egg-tubes; _v_, oviduct; _w_, spermatheca (or an accessory gland); _x_, retractile ovipositor; _y_, cercus; _z_, labrum.] {115}Many of the anatomical structures have positions in the body that are fairly constant throughout the class. Parts of the respiratory and muscular systems and the fat-body occur in most of the districts of the body. The heart is placed just below the dorsal surface; the alimentary canal extends along the middle from the head to the end of the body. The chief parts of the nervous system are below the alimentary canal, except that the brain is placed above the beginning of the canal in the head. The reproductive system extends in the abdomen obliquely from above downwards, commencing anteriorly at the upper part and terminating posteriorly at the lower part of the body cavity. In Fig. 63 we show the arrangement of some of the chief organs of the body, with the exception of the muscular and respiratory systems, and the fat-body. It is scarcely necessary to point out that the figure is merely diagrammatic, and does not show the shapes and sizes of the organs as they will be found in any one Insect. MUSCLES. The muscular system of Insects is very extensive, Lyonnet[31] having found, it is said, nearly 4000 muscles in the caterpillar of the goat-moth; a large part of this number are segmental repetitions, nevertheless the muscular system is really complex, as may be seen by referring to the study of the flight of dragon-flies by von Lendenfeld.[32] The minute structure of the muscles does not differ essentially from what obtains in Vertebrate animals. The muscles are aggregations of minute fibrils which are transversely striated, though in variable degree. Those in the thorax are yellow or pale brown, but in other parts the colour is more nearly white. The muscles of flight are described as being penetrated by numerous tracheae, while those found elsewhere are merely surrounded by these aerating tubules. The force brought into play by the contractions of Insect muscles is very great, and has been repeatedly stated to be much superior to that of Vertebrate animals; very little reliance can, however, be {116}placed on the assumptions and calculations that are supposed to prove this, and it is not supported by Camerano's recent researches.[33]