connected with the respiratory system or the acoustic organ. [Illustration: FIG. 194.—Diagram of arrangement of parts of the ear as seen in transverse section of the tibia of a Locustid. A, J, V, H, outer, inner, anterior, posterior aspects of leg; _a_, _d_, thin part of integument forming anterior tympanum; _b_, _c_, thicker portion of same; _f_, _g_, posterior tympanum; _a_, _f_, and _d_, _h_, _g_, thick portions of integument; _i_, _k_, internal protuberances of same; _l_, _m_, _n_, _o_, walls of the anterior tracheal vesicle, _vTr_; _p_, _q_, _s_, _r_, walls of the posterior tracheal vesicle, _hTr_; _o″_, projection of tympanal orifice of prothorax; _tr-n_, tracheal nerve-end organ, _crista acustica_; _st_, rod; _de_, curtain-membrane; _hn_, _e_, supra-tympanal, nerve-end organ; _hn_, ganglion cells; _st′_, rods; _e_, point of integumental fixation of nerve endings. (After Graber.)] The chief features of the acoustic apparatus of the legs of Locustidae will be gathered from the accompanying diagrammatic transverse section through the tibia. In this figure the deep black parts indicate the outer wall of the tibia and its prolongations, the white spaces indicate the parts filled with air, while the dotted portions are occupied by blood or some of the body organs;[248] the {318}circular space _o″_ is not part of the actual structure, but represents the area of the external acoustic orifice of the prothorax; it is not, however, so large as it should be. Although the tibial ears of Locustidae are very perfect organs, there is great difficulty in deciding on the exact nature of their functions. They would appear to be admirably adapted to determine the precise locality from which a sound proceeds, especially in those cases—and they are the highest forms—in which the tympanum is placed in a cavity the external orifice of which is a slit (Fig. 193, B); for the legs can be moved in the freest manner in every direction, so as to bring the drum into the most direct line of the vibrations. But as to what kinds of vibrations may be perceived, and the manner in which they may be transmitted to the nerves, there is but little evidence. On reference to the diagram it will be noticed that the tympanum, the tympanal vesicles, and the nervous apparatus are not in close connexion, so that even the mode by which the impulses are transmitted is obscure.