Summary

1. 1. The heads of resting flies will twitch to the side if the haltere is deflected rapidly forwards. Head movements are always away from the stimulated haltere and do not occur if the haltere is deflected up, down or backwards.

2. 2. The anatomy and action of the neck muscles is described.

3. 3. Cobalt fills of the whole haltere nerve show that the sensory axons project to the neuropiles of the ipsi- and contralateral pro- and mesothoracic neuropiles, to the ipsilateral metathoracic neuropiles and to the cerebral ganglion.

4. 4. Cobalt fills of the nerves to the neck muscles and from the prosternal organs show that the central projections of these nerves end in the ipsilateral prothoracic neuropile.

5. 5. Recordings from the motoneurones to the neck muscles show that they are phasically activated by forward deflexion of the halteres after a latency of 2.5–3 ms. Spikes in the motoneurones follow the vibration of the haltere, one to one, up to 200 Hz.

6. 6. Recordings from the ipsilateral mesothoracic wing nerve (N. alae) show two large units which respond after a 2.5–3 ms latency to forward deflexion of the halteres.

7. 7. Behavioural observations of walking flies show that the presence or absence of halteres has a small but nevertheless significant effect on the animals' ability to detect angular accelerations during walking or to orient with respect to gravity.