The findings are reported in Current Biology by Roland Strauss, of the University of Wrzburg, and Simon Pick, of the University of Ulm, both in Germany.
In their studies, the researchers presented freely walking flies with a chasm in their path. The flies had shortened wings and could not fly over the gap, and they were thus forced to decide whether to attempt to cross the gap "by foot." The researchers found that the flies first visually measure the gap width and then only cross the gap if it is a traversable width. If the chasm is short, flies engage in an extraordinary crossing behavior that the researchers showed involve three motor programs. High-speed video analyses revealed that flies can flexibly combine, and iteratively improve, these three behavioral adaptations in order to traverse gaps much broader than their own body size. The decisive phase for the climbing success of a fly is the formation of a "bridge" with the hind and middle legs still holding on to the starting side and the front legs just about reaching the edge of the target side. Forward shifting of the body into the gap, primarily by little hind-leg steps, contributes the most to gap-crossing. Auxiliary actions of the middle legs keep the body close to a horizontal position and help in reaching the other side. In the last step, the front legs stretch as much as possible to grasp the other side.
In some ways, the motor programs employed by the flies show high similarity to vertebrate motor programs--for example, when a person is tryi
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