ORIENTATION BY OPPOSED BEAMS OF LIGHT

Computations of the effective angular inclination (H) of the photoreceptive surfaces of the two sides, based upon measurements of orientation angles under the action of beams of light directly opposed or crossing at right angles, show that with larvae of Calliphora and of Lucillia H declines as the total illumination decreases (i.e., as the angle of orientation away from the more intense light increases). H is greater with the two lights opposed at 180°; this may be due to the difference in refraction. For the more sharply pointed larvae of Lucillia, H is less than half as great as in Calliphora.