If fly larvae are put on a table by a window they immediately begin to travel away from the light. Turn the table round, and the maggots change direction so as to move directly away from the light again. They travel by a ‘tacking’ process—the head end is extended to right and left alternately, and each time the rear end is drawn after it. The animal moves on a straight course if this process is symmetrical; should a movement of the head bring it into a region of higher illumination or other undesirable condition the head is withdrawn, and a second step made to the opposite side, turning the animal round.

Copyright © 1961 The Company of Biologists Ltd.
1961
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