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1 Department of Zoology, University of Cambridge, Cambridge, Cambs, United Kingdom
* To whom correspondence should be addressed. E-mail: mb135{at}hermes.cam.ac.uk.
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 lasting a few hundred ms in which a burst of spikes in the trochanteral levator motor neurons moves the hind legs into their fully cocked position, thus engaging a mechanical lock between a coxa and a femur. Second, a cocked phase lasting a few seconds, in which a trochanteral depressor motor neuron spikes continuously at a frequency gradually rising to 50Hz, but the hind legs remain stationary. Levator motor spikes are sporadic. Third, the jump movement lasting less than 1ms, 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 4.9s long cocked phase all 174 spikes were synchronous and in another 27s period of continuous spiking all but one of 1176 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.
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