NEUROMUSCULAR BASIS OF COURTSHIP IN GRASSHOPPER GOMPHOCERIPPUS RUFUS (L)

The males of many grasshoppers perform courtship songs which are suitable for physiological study because of their stereotypy and ease of elicitation. The courtship of Gomphocerippus rufus (Acrididae) includes not only stridulation with the hindlegs but also movements of the head and its appendages. It consists of a single behavioural sequence repeated continually over some 15 seconds (at 32°C). The sequence is divided into three subsequences. One way to obtain insight into the nervous control of such behaviour is by analysis of the motor output. Muscle potentials are recorded with 30 μ steel wire electrodes. Apart from these light leads, the animal is free to move. Up to 15 electrodes can be implanted into the different thoracic muscles without noticeably influencing behaviour. Song is recorded simultaneously with the muscle activity. As the innervation of the muscles is simple, and as there is a fixed relationship between the action potentials of nerve und muscle, the nervous activity can be deduced from the muscle recordings. The recorded potentials can be related to the activity of single motor units. The motor units of a muscle are always activated in a particular order. Some units are continuously active during song, while others are activated only when the song is above a certain intensity. When both types are active, the former leads the latter by 0.2-2 msec. The activity of whole muscles within a functional group follows a similar pattern. The sound produced by the up or the downstroke of the leg occurs at a fixed interval after the muscle potential. Sound production can therefore be described quantitatively in terms of the activity of a known group of musles. The interval between up and downstrokes during song is fixed, but their repetition rate, and thus the phase relation between up and downstroke, varies. The interplay of whole muscle groups on different sides of the body and in different segments were also investigated. Simultaneous recordings from the muscles of the two hindlegs show that their co-ordination alters between different subsequences. In the first subsequence the legs move synchronously, while in the second and third subsequences the legs either alternate or show a fixed delay between strokes. The song of subsequence 3 is divided into short chirps. During the first two leg cycles of a chirp the legs move metadromically with a delay of 5-6 msec, but for the rest of the chirp they move antidromically. During the initial metadromic period either leg can lead, but the lead may change both during and between chirps. Regardless of any change of lead that may have occurred, there is always a fixed interval of 36-38 msec between the last stroke of a chirp and the first stroke of the same leg in the next chirp. Simultaneous recordings from leg and neck muscles show that head shaking and stridulation are coupled; each sideways movement of the head is followed by a chirp, with a phase lag of 120°, not a constant delay. The intervals between successive leg movements and between successive chirps are limited to certain preferred values. Thus chirps occur at intervals of 400 msec at the start of subsequence 1, and at 100 msec intervals at the end of this sequence, but the transition is abrupt and without intermediate frequencies. Between the individual up or downstrokes of subsequence 2 and 3 there are 4 such preferred intervals which are related to each other in the ratios 2:3:4:6. Some of these intervals occur not only in song but also in the flight of the grasshopper. © 1968 Springer-Verlag.