Fig. 2. Initial movements of flies at the onset of walking phonotaxis are not stereotyped. Flies lunge forward with both prothoracic legs in response to a sound source directly ahead (A). In response to a lateral sound source, flies may initiate phonotaxis by stepping with either the ipsilateral (B) or contralateral (C) prothoracic legs, or with the contralateral mesothoracic leg (D).