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.
Fig. 292.—A, fore tibia of a European grasshopper (Meconema), containing the ear: Ty, tympanum or outer membrane; Tr 1, Tr 2, tracheæ. B, diagrammatic cross-section through the tibia and ear of the same; Ty, tympanum; Ct, cuticula; CM, hypodermis: A, the auditory organ connecting with the tympanum; B, supra-tympanal auditory organ; GZ, the ganglion-cell belonging to them; Hst, the auditory rod connecting with the ganglion-cells.—After Graber, from Judeich and Nitsche.
On examining the apparatus within the leg under the drum, it is seen to consist of the trachea, the auditory vesicles and rods, ganglion cells, and acoustic nerve. The trachea is greatly modified (Fig. 292, Tr 1). On passing into the tibia the trachea enlarges and divides into two branches, which reunite lower down. The spiracles supplying the air to this enlarged trachea are considerably enlarged, while in the dumb species it is of the normal size. The enlarged trachea passes close to the tympanum, which thus has air on both sides of it: the open air on the outer, the air of the trachea on its inner surface. In fact, as Lubbock states, “the trachea acts like the Eustachian tube in our own ear; it maintains an equilibrium of pressure on each side of the tympanum, and enables it freely to transmit the atmospheric vibrations.”
Fig. 293.—The auditory apparatus in the tibia of a grasshopper, showing the tympanal nerve-endings in situ: EBI, terminal vesicles of Siebold’s organ; SN, nerve of the organ of Siebold; Gr, group of vesicles of same; SO, nerve-endings of the same; vT, front tympanum; vTr, front branch of the trachea; hT, hinder tympanum; hTr, hinder branch of the trachea; Sp, space between the tracheæ; go, supra tympanal ganglion; rN, connecting nerve-fibrils between the ganglion cells and the terminal vesicles; R, upper, n-S, lower, root of the transparent covering membrane. (Other lettering not explained by author.)—After Graber.
Fig. 294.—Auditory rod of Gryllus viridissimus: fd, auditory rod; ko, terminal piece.—After Graber, from Lubbock.