The acoustic nerve, which next to the optic is the thickest in the body, divides soon after entering the tibia into two branches, one almost immediately forming a ganglion, the supra-tympanal ganglion, the other passing down to the tympanum, where it expands into an elongated flat ganglion, the organ of Siebold (Fig. 293), and closely applied to the anterior tracheæ. At the upper part of the ganglion is a group terminating below in a single row of vesicles, the first few of which are approximately equal, but which subsequently diminish regularly in size. Each of these vesicles is connected with the nerve by a fibril (Fig. 293, _vN_), and contains an auditory rod (Fig. 294). They are said by Graber to be brightly refractive, hollow (thus differing from the retinal rods, which are solid), and terminate in a separate end-piece (_ko_). The rods were first discovered by Siebold, and, as Lubbock remarks, may be regarded as specially characteristic of the acoustic organs of insects. [Illustration: FIG. 295.—Chordotonal organ in nymph of a white ant.—After Müller, from Sharp. ] [Illustration: FIG. 296.—Right half of 8th body-segment of _Corethra plumicornis_: _g_, ganglion of ventral cord; _lm_, longitudinal muscle; _cn_, chordotonal nerve; _cl_, chordotonal ligament; _cg_, chordotonal ganglion; _cs_, rod of chordotonal organ; _cst_, terminal cord; _tb_, tactile setæ; _hn_, out-going fibres of the integumental nerves.—After Graber, from Lang. ] As will be seen in Fig. 293, at the upper part of the tibial organ of Ephippigera there is a group of cells, and below them a single row of cells gradually diminishing in size from above downwards. “One cannot but ask oneself,” says Lubbock, “whether the gradually diminishing size of the cells in the organ of Siebold may not have reference to the perception of different notes, as is the case with the series of diminishing arches in the organ of Corti of our own ears.” These organs were supposed to be restricted to the Orthoptera, but in 1877 Lubbock discovered what seems to resemble the supra-tympanal auditory organ of Orthoptera in the tibia of the yellow ant (_Lasius flavus_). Graber confirmed Lubbock’s account, and also discovered these organs in the tibia of a Perlid (_Isopteryx apicalis_), and Fritz Müller has detected them in the fore tibiæ of the nymph of _Calotermes rugosus_ (Fig. 295). To these structures Graber gave the name of chordotonal organs.