Phototactic responses along a gradient of light intensities for the sibling species Drosophila melanogaster and Drosophila simulans.

The phototactic responses of four recently collected isofemale strains of Drosophila melanogaster and Drosophila simulans were measured in a light gradient from 590 to 10 lux. High light intensities were preferred by most flies, but a small proportion of flies preferred the lowest light intensity. Based on the strains tested, D. simulans showed greater phototaxis than D. melanogaster, and within each species variability was found. The niche breadth of D. melanogaster appears likely to be greater than that of D. simulans for phototaxis in the light gradient. These results are in general qualitative agreement with earlier results published on dispersal activities from the same populations.