Phototactic choice between two lights of various intensity in flies.

Under the conditions of double choice in a Y-maze, proportion of flies gathering in its arm depended on its relative illumination, approximately so that (1) at, or below the lower critical relative intensity limit, it assumed a constant minimum value; (2) at, or above the species-specific upper critical limit, the proportion of choice assumed a constant maximum value; whereas (3) between these illumination limits it roughly depended linearly on the logarithm of relative intensity of the light. (However, variations of behavioural responsiveness of flies towards white lights with different intensities, underlying their choice in the maze, suggest that the dependence of proportion of choice on relative light intensity has presumably not a zigzag but a sigmoid-shape course). The slope of the graph of linear proportionality of choice to the logarithm of light intensity differed in the housefly and the blowflies, similarly as did the extreme values of the proportion of choice beyond the critical intensity limits. The proportion of choice depended on a type of the maze (being different in a Y- and a T-maze), but not on its size, and it did not depend either on adaptation of flies to light or darkness prior to the experiment, or on the age of flies.