Light control of vertical migration was examined with sonar recordings of the evening migration of Chaoborus punctipennis larvae. Frequently, a slow, upward drift in the afternoon preceded the rapid ascent of larvae. Movement out of the sediments was not consistently related to a fixed light intensity or rate of relative light change. Light intensities were variable at the initiation of and during the upward migration, whereas the beginning of rapid ascent was closely correlated with the timing of a relative light change of 1.7 .times. 10-3 s-1. The close relationship between the rate of relative change in light intensity and the rate of upward movement including the afternoon drift supports the stimulus-velocity hypothesis that once larvae leave the sediments the speed and direction of the migration pattern is primarily regulated by relative light changes. Second- and third-instar larvae migrated as higher light intensities than fourth instars, but did not differ in their rate of movement in relation to light change.
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