Sub-millisecond precision and variability in a cricket calling song pattern

Nightly calling songs of male Gryllus bimaculatus start with bouts of activity which gradually increase in intensity until a stable singing pattern is generated. The temporal pattern of the calling song consists of loud chirps with 3–6 sound pulses and subsequent silent inter-chirp intervals. The chirp rate is close to 3 Hz; the pulse rate between 26 and 36 Hz. Pulse periods occur with millisecond or sub-millisecond jitter driven by rapid membrane potential depolarisations of pattern generating neurons, making them one of the most precise insect motor patterns. In contrast inter-chirp intervals occur with a considerable variability and jitter, due to an underlying slow ramp depolarisation driving the pattern generating neurons and centrally initiating a new chirp period. Comparing a European and a Japanese line, reveals clear differences in the timing of the pulse pattern. Japanese males generate shorter pulse periods and pulse durations. This is linked to their significantly higher calling song frequency as compared to the European line, indicating signs of allopatric speciation. Males singing in earshot do not adjust their chirp or pulse pattern in respect to the song of their neighbour. Their individualistic behaviour does not provide support for possible chorusing or alternate singing strategies in populations.