Landing reaction of Musca domestica. 3. Dependence on the luminous characteristics of the stimulus.

1. The house fly shows landing reaction to an expanding visual stimulus or to a decrement of the environmental light level. The quantitative relationship between the dependence of landing reaction on expansion and on light level decrement is analyzed. 2. The different properties of the reactions at different light levels may be correlated with the two visual systems in Diptera and may be explained by the existence of an alterting mechanism that w eighs stim uli for landing reaction at low illumination levels. 3. The landing reaction resembles the optomotor reaction with respect to the influence of the contrast of the stim ulating pattern. The landing reaction appears to be invariant to the phase relations between the spatial components of the periodic patterns used in the present experim ents.