Morphology of the sense organs in the proximal parts of the tibiae ofGryllus campestrisL. andGryllus bimaculatus deGeer (Insecta, Ensifera)
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- Eibl, E. Zoomorphologie (1978) 89: 185. doi:10.1007/BF00993947
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The tibial surfaces directed outward when the leg is in the normal position have a considerably greater density of innervated hairs and bristles than the surfaces on the inner side of the leg. On the forelegs, hairs and bristles surround the tympana (Fig. 4). This arrangement of the sensilla is thought to be associated with the burrowing habit of the crickets.
The special cuticular differentiation (tympana, chitinous ridges) and the particular arrangement of the tracheal system in the vicinity of the auditory receptors (apposition of two tracheal tubes which are in direct communication with one another, Figs. 4, 6, and 7) seem to improve the ability of detection of airborne sound and seem to be important in the frequency analysis carried out by the ears. There are no such differentiated regions in the tibiae of the middle and hind legs.
On all three pairs of legs each tibia bears a group of 14–15 campaniform sensilla (Fig. 5); the groups have very similar spatial arrangements. Their nearness to the subgenual organ, and the fact that they are incorporated into the tibial cuticle immediately proximal to the crease, indicate that they measure the loading of the tibia.
The subgenual organs in all three pairs of legs consist of ca. 25 sense cells each; these, with their accessory structures, are spread out in a fanlike arrangement, and their dendritic processes bend to run in the hemolymph canal (Figs. 8-13).
Only in the anterior tibia is there a complete tympanal organ, with a main process consisting of a row of 30–35 sense cells parallel to the long axis of the leg (Fig. 9). Structures homologous to the caudal and frontal processes of the tympanal organ of the anterior tibia, with 25–30 sense cells, are also to be found in the middle and hind tibia (Figs. 11–13). Therefore it is presumably the main process that is chiefly responsible for detection of airborne sound by the forelegs. The sensory apparatus in the middle and hind legs that corresponds to the caudal and frontal branches of the tympanal organ may well mediate the residual sensitivity to airborne sound which remains after ablation of the tympanal organs (cf. Nocke, 1972; E. Schuhmacher, 1975).
The paths followed by the sensory axons from the tibial organs described are almost identical in the three thoracic segments. The subgenual nerve (SGN, Fig. 9) and the tympanal nerve (TN, Fig. 9) each also contain axons of the campaniform sensilla; the tympanal nerve includes, in addition, axons of the sensory cells in the subgenual organ.