The Locustidae resemble the Acridiidae in the possession of specialised ears and sound-producing organs; neither of these is, however, situate in the same part of the body as in Acridiidae. The ears of Locustidae are placed on the front legs, below the knee; a tympanum (Fig. 193, A), or a crack giving entrance to a cavity in which the tympanum is placed (Fig. 193, B), being seen on each side of each of the anterior pair of limbs. In this family, as in the Acridiidae, three kinds of ear are recognised according to the condition of the tympanum, which is either exposed (Fig. 193, A) or closed by an overgrowth of the integument (Fig. 193, B), or in a condition to a certain extent different from either of these. The existence of ears placed on the legs is a curious fact, but it is beyond doubt in the Locustidae, and there is good reason for believing that analogous organs exist in this situation in other Insects that have special means of sound-production, such as the ants and the Termites. [Illustration: FIG. 193.—Ears of Locustidae: A, portion of front leg of _Odontura serricauda_, adult; _p_, prominence of integument; _r_, rim of ear; _T_, tympanum; _b_, thickened area thereof; _Fu_, remains of groove in which the structure was developed. B, portion of front leg of _Thamnotrizon apterus_; _i_, inner margin; _a_, slit-like external aperture of ear; _di_, overlapping cover of the ear. (After Graber.)] The structure of these organs in the Locustidae has been investigated by Graber,[247] and their acoustic functions placed beyond doubt, though to what special kind of sounds they may be sensitive is not ascertained, this point being surrounded by even greater difficulties than those we have discussed in the case of the Acridiidae. In the Locustidae there is a special structure of a remarkable nature in connexion with the ears. In Acridiidae {317}a stigma is placed close to the ear, and supplies the internal structures of the organ with air. There are no stigmata on the legs of Insects, consequently admission of air to the acoustic apparatus in Locustidae is effected by means of a gaping orifice at the back of the prothorax, just over the base of the front leg (Fig. 101); this communicates with its fellow of the other side, and from them there extend processes along the femora into the tibiae, where they undergo dilatation, so as to form vesicular cavities, one of which is in proximity to each drum of the ear. These leg-tracheae are not connected with the ordinary tracheal system; the prothoracic stigma exists in close proximity to the acoustic orifice we have described, but is much smaller than it. It is not yet clear why the acoustic apparatus should require a supply of air apart from that which could be afforded by the ordinary tracheal system. This special arrangement—to which there is hardly a parallel in Insect anatomy—has still to be accounted for; we do not know whether the necessity for it may be