=The ears or tympanal and chordotonal sense-organs of Orthoptera and other insects.=—The ears or tympana of locusts (Acrydiidæ) are situated one on each side, on the basal joint of the abdomen, just behind the first abdominal spiracle. That this is a true ear was first suggested by J. Müller, and his opinion was confirmed by Siebold, Leydig, Hensen, Graber, Schmidt, Lubbock, etc.[49] [Illustration: FIG. 290.—Ear of a locust (_Caloptenus italicus_), seen from the inner side: _T_, tympanum; _TR_, its border; _o_, _u_, two horn-like processes; _bi_, pear-shaped vesicle; _n_, auditory nerve; _ga_, terminal ganglion; _st_, stigma; _m_, opening, and _m′_, closing, muscle of the same; _M_, tensor muscle of the tympanum membrane.—After Graber. ] The apparatus consists of a tense membrane, the _tympanum_, surrounded by a horny ring (Fig. 290). “On the internal surface of this membrane are two horn-like processes (_o_, _u_), to which is attached an extremely delicate vesicle (_bi_) filled with a transparent fluid, and representing a membranous labyrinth. This vesicle is in connection with an auditory nerve (_n_) which arises from the third thoracic ganglion, forms a ganglion (_ga_) upon the tympanum, and terminates in the immediate neighborhood of the labyrinth by a collection of cuneiform, staff-like bodies, with very finely pointed extremities (primitive nerve-fibres?), which are surrounded by loosely aggregated ganglionic globules” (Siebold’s Anatomy of the Invertebrates). [Illustration: FIG. 291.—Fore tibia of _Locusta viridissima_. _td_, cover of the drum; _tr_, fissure between the drum and its cover.—After Graber, from Lang. ] In the green grasshoppers, katydids, and their allies, the ears are situated on the fore tibiæ, where these organs can be found after a careful search (Figs. 291, 292). The presence of the structure is indicated by the oval disc, the drum, which is a thin tense membrane covering the auditory apparatus of nerves, ganglion cells, and auditory rods beneath. The tympana, or drums, are not present in all Locustidæ and Gryllidæ, and, as Lubbock states, it is an additional reason for regarding them as auditory organs, that in those species which possess no stridulating organs the tympana are also wanting. In many of the Locustidæ the tympana are covered or protected by a fold of the skin projecting over them. These covered ones are, Graber thinks, derived from the open ones. [Illustration: