Neural Control and Coordination of Jumping in Froghopper Insects

Abstract

The thrust for jumping in froghopper insects is produced by a rapid, synchronous depression of both hind legs generated by huge, multipartite trochanteral depressor muscles in the thorax and smaller levator muscles in the coxae. A three-phase motor pattern activates these muscles in jumping. First, a levation phase lasts a few hundred milliseconds, in which a burst of spikes in the trochanteral levator motor neurons moves the hind legs into their fully cocked position and thus engages a mechanical lock between a coxa and a femur. Second, a cocked phase lasts a few seconds, in which a trochanteral depressor motor neuron spikes continuously at a frequency gradually rising to 50 Hz, although the hind legs remain stationary. Levator motor spikes are sporadic. Third, the jump movement lasts <1 ms, in which the spikes in the depressors stop abruptly and the legs rapidly depress. This pattern may vary in the speed of the initial levation and in the duration of the cocked phase. Recordings from the depressor muscles on both sides showed remarkable synchrony of their motor spikes. In one 4.9-long cocked phase all 174 spikes were synchronous and in another 27 s period of continuous spiking all but one of 1,176 spikes were synchronous. When a single hind leg moves rapidly, these depressor spikes are nevertheless independent of those of the other leg. These features of the motor pattern and the coupling between motor neurons to the two hind legs ensure powerful movements to propel rapid jumping.