What causes edge fixation in walking flies?

The orientation of freely walking flies (female Lucilia cuprina) to lines and stripes in a circular arena is described. The following observations were made. 1. The flies walked straight towards a dark line using the frontal eye region, but a pale line on a dark background was only weakly attractive. 2. In bright conditions flies walked in a curved line towards a black-white edge, the path being convex towards the dark side of the border. The curves indicated that the flies were heading for a point about 5-10 degrees to the dark side of the edge. 3. In dim conditions the edge of a dark region was not especially attractive and flies headed towards any point in the dark area. These observations can be accounted for by assuming that the fly walks towards the darkest region in its visual field (scototaxis). In bright conditions the edges of a dark region become more attractive than its centre. This change could be explained if lateral inhibition creates a 'Mach-band' effect, making the edges appear darker than the centre. Thus, fixation behaviour in walking Lucilia females seems to be a simple taxis.