The connexions between stretch receptors of the wings and motoneurones innervating flight muscles have been studied anatomically and physiologically. 2. Filling with cobaltous chloride shows that the single neurone of a forewing stretch receptor has a complex pattern of branches within the mesothoracic ganglion and branches which extend into the pro- and meta-thoracic ganglia. The single neurone of a hindwing stretch receptor has extensive branches in the metathoracic ganglion and branches in themesothoracic ganglion. The branches of both receptors are confined to the ipsilateral halves of the ganglia. 3. A stretch receptor gives information about the velocity and extent of elevation of a wing. 4. Each spike of a forewing stretch receptor casuses an EPSP in ipsilateral mesothoracic depressor motoneurones and an IPSP in elevators. The connexions are thought to be monosynaptic for the following reasons. The EPSPs in the first basalar (depressor) motoneurone follow each spike of the stretch receptor at a frequency of 125 Hz and with a constant latency of about 1 msec. In a Ringer solution containing 20 mM-Mg2+ the amplitude EPSP declines gradually. The IPSP'S upon elevators have similar properties but occur with a latency of 4-6 msec. 5. The connexions therefore comprise a monosynaptic negative feed-back loop; elevation of the wing excites the stretch receptor which then inhibits the elevator motoneurones and excites the depressors. 6. A hindwing stretch receptor synapses upon metathoracic flight motoneurones in the same way, causing EPSPs in depressor and IPSPs in elevator motoneurones. 7. No connexions of either fore- or hindwing stretch receptors have been found with contralateral flight motoneurones. 8. Interganglionic connexions are made by both receptors. For example, both fore- and hindwing stretch receptors cause EPSPs upon the meso- and metathoracic first basalar motoneurones. 9. Stimulation of the axon of a stretch receptor with groups of three stimuli repeated every 50-100 msec thus simulating the pattern which it shows during flight, causes subthreshold waves of depolarization in depressor motoneurones. When summed with an unpatterned input, the stretch receptor is able to influence the production of spikes in motoneurones on each cycle. During flight, it is expected that the stretch receptor will influence the time at which a motoneurone will spike and hence have an effect on the amplitude of the upstroke and upon the phase relationship between spikes of motoneurones.
