[Illustration: FIG. 172.—Middle of body of _Pachytylus nigrofasciatus_, to show tympanum, _e_. (After Brunner.)] [Illustration: FIG. 173.—Mecostethus grossus: A, Insect with wings expanded; B, profile of head and prothorax. (After Brunner.)] If the dorsal aspect of the first segment of the hind body of an Acridian Insect be carefully examined there may be seen in the majority of species an organ which has somewhat the appearance of an ear (Fig. 172), and which there is great reason for believing to be really an organ of that nature. It is situate a little over the articulation of the hind leg, very close to the spot where the sound is, as above described, produced. There are three forms of these Acridian ears as described by Brunner:[217] (1) a membrane surrounded by a rim; (2) the membrane somewhat depressed, a portion of the segment projecting a little over it; (3) the depression very strongly marked, and the sides projecting over it so much that all that is seen externally is a sort of broad slit with a cavity beneath it. This last is the condition in which the ear exists in the genera _Mecostethus_ (Fig. 173) and _Stenobothrus_, which are among our few native grasshoppers. On minute examination this ear proves to consist of a tympanum supplied internally with nerve and ganglion in addition to {286}muscles, and tracheal apparatus of a complex nature; it is no doubt delicately sensitive to some forms of vibration. Unlike the stridulating organ, these ears exist in both sexes; they are found in a great majority of the species of Acridiidae. The forms in which the ears are absent are usually at the same time wingless and destitute of organs of stridulation; but, on the other hand, there are species—some of them wingless—that are, so far as is known, incapable of stridulation and yet possess these ears.