Impulse Patterns in the Flight Motor Neurones of Bombus Californicus and Oncopeltus Fasciatus*

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Abstract
1. Each of the motor neurones innervating the indirect flight muscles of Bombus and Oncopeltus fire regularly during flight, not in patterned bursts. The several motor neurones innervating one muscle fire at about the same rate and have a weak tendency to fire synchronously, but all possible relative timings occur in each flight. Neurones innervating different muscles have no preferred relative timing, and may have different frequencies. 2. The motor patterns of Bombus and Oncopeltus are very similar. These patterns could be generated by a model including common excitatory input to each of the neurones innervating one muscle and by weak electrotonic short-latency synaptic coupling between the motor neurones. Different sets of neurones would receive different excitatory input, and antagonistic sets seem to be coupled by a weak inhibitory mechanism. 3. Recruitment of motor units was observed in Bombus during low-frequency activity. 4. Synergistic units in Bombus fire in near-perfect synchrony during periods when the bee is warming up but not flying. When the bee begins to fly, there is a sudden shift to the output pattern characteristic of flight. 5. The patterns of impulses in these insects have characteristics shared by both dipteran patterns and by orthopteran and lepidopteran patterns. The flight systems of Orthoptera, Lepidoptera, Hemiptera, Hymenoptera and Diptera also have many anatomical similarities. The anatomical similarities and the several common characteristics of the motor output patterns in these orders support the hypothesis that there was one primordial flight mechanism common to the origin of these orders, from which the different mechanisms which exist today have evolved.