The mechanism of visual pattern fixation in the walking fly,Drosophila melanogaster

Summary

In freely walking flies,Drosophila melanogaster, stationary stripes induce fixation processes. If two vertical black stripes (width 6°) are presented against an illuminated white background and if the two stripes are separated by an angular distance ofα _ν < 60°, the flies prefer the direction of the bisector of the angle between the two objects, i.e. the angular distribution of the flies' positions is characterized by one maximum pointing at the center between the two stripes. If two stripes are presented in angular distancesα _ν > 75°, the angular distributions of the flies' positions always show two maxima directed towards the two stripes (Fig. 3a). This result is also true for 6°-wide vertical stripes which are illuminated from behind in an otherwise dark drum (Fig. 4a). In experiments with a two- and three-stripesenvironment, the variability of the flies' reactions to the stripes has been measured (Figs. 3–6). From these results, a functiontr (μ) can be deduced which describes the strength of the turning tendencies induced in the flies by a stripe as a function of the walking directionμ of the fly (turning tendency function). This function is characterized (1) by a sharp increase oftr (μ) with increasing angular distance from the direction of the object to the maximum response atμ=25–30°, (2) by a sharp decrease oftr (μ) from the maximum toμ=60°, and (3) by a very slow approach oftr (μ) to the labile zero position of the turning tendency function atμ=±180° (Fig. 7). Additive superposition of two or three of these turning tendency functions, which are phase-shifted for different anglesα _ν, allows one sufficiently to describe the walking directions of the flies. This is possible because the stable zero positions of the summation turning tendency functionstr _s (μ) coincide with the maxima of all the frequency distributionsf (β) of the flies' positionsβ (Fig. 8).