A possible sensory function for the stop—go patterns of running in phorid flies

. Running activity was filmed at high speed in two species of phorid fly, Megaselia scalaris and Puliciphora borinquenensis , which differ in weight by about one order of magnitude. Under all experimental conditions, activity consisted of brief runs regularly interrupted by short halts when the insect was completely immobile. Runs and halts were approximately equal in duration and occurred five to eight times a second. In crowded conditions the flies made longer stops and usually also shorter runs. Maximum instantaneous velocities during running were 0.2 m s ‐1 in M. scalaris and 0.09 m s ‐1 in male P. borinquenensis. Maximum relative velocities were 80 and 112 fly‐lengths s ‐1 respectively. In M. scalaris , step frequencies reached at least 57 Hz and this rate can probably be exceeded. Fast running was accompanied by small side‐to‐side oscillations of the body at the step frequency. Fast turns were normally made at the start of runs and only rarely at other times. Painting the compound eyes or removing the antennae did not disrupt the stop—go pattern of running, although flies so treated were reluctant to run. Possible visual and chemosensory functions for stop—go running are discussed.