When tested in Hirsch-Hadler mazes, Drosophila pseudoobscura descended from natural populations is phototactically and geotactically neutral on the average. Decidedly positive and negative populations have been obtained by artificial selection. The behavior is independent of temperature (15-27°) and age (2-17 days). The flies perceive light as weak as 0.2 lux; from 7 to 3000 lux the behavior difference between positive and negative strains increases little, and perhaps decreases at 8000 lux. A remarkable transformation occurs with red light; the population that is negatively phototactic with shorter wave lengths or with white light becomes photopositive.
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