Neuroethology of sound production in gomphocerine grasshoppers (orthoptera: acrididae)

Summary

• 1. Sound production of grasshoppers is based upon central motor patterns which are hierarchically organized at various functional levels (Fig. 2). These involve coordination (i) of single motor units belonging to the same muscles, (ii) of muscles relating to one hindleg, and (iii) of laterally homologous muscles moving the left and the right hindleg, respectively.

• 2. The motor organization at the first two levels has been investigated comparatively in several species. The neuromuscular activity underlying stridulation was recorded in freely moving animals using flexible wire electrodes (diameter 20–30 μm) which allowed the functional identification of individual motor units (Fig. 3). Up to 30 electrodes were chronically implanted into the metathoracic muscles without restricting the behaviour.

• 3. The motor patterns are based upon certain principles some of which are generally valid for all species: (i) The individual units are characterized by discrete types of activity (Fig. 4). (ii) Motor units and muscles are always recruited in a fixed order (Figs. 1, 3, 10).

• 4. The co-ordination of single units does not follow a general scheme but it reflects features of the specific song patterns (Fig. 5). The muscles producing the movements of one hindleg are also specifically co-ordinated: (i) In rapidly stridulating species the sets of antagonistic muscles are coupled in an unequivocal (unimodal) manner from which the pattern of leg movements can easily be deduced (Figs. 11, 12, 13). (ii) On the other hand, in slowly stridulating species the antagonistic muscles are coupled in two ways, since several of them are recruited both before the beginning and at the end of the leg strokes (Figs. 7–9). Maximum of motor activity is usually found at the end of the upward movements where the activities of both the elevator and depressor muscles partly overlap (Fig. 7b).