Larvae of Chaoborus punctipennis Say (Diptera: Culicidae) were maintained under constant conditions in two 152‐ × 10‐cm columns containing lake water and sediments. Visual counts of the larvae in the columns were made hourly with a flashlight equipped with a red filter and darkened reflector. Activity was observed during dark periods; however, no activity was observed during light periods. A diurnal rhythm of activity persisted up to 10 days in constant darkness. The phase of the rhythm was reset by entrainment under differing light regimes, as well as by temperatures of 5C or less. These critical low temperatures also abolished activity. The offset of light was demonstrated to be the Zeitgeber for this rhythm; therefore it is necessary that under natural conditions these larvae receive some offset stimulus from the setting sun.

A factorial experiment was designed to elucidate the environmental factors controlling vertical migration of Chaoborus punctipennis Say larvae. Variables were oxygen concentration, temperature, and presence or absence of food (plankton ) . Replicate experiments were conducted and hourly counts were made. Under low oxygen conditions (f = 0.57 ppm), 35.5% of the planktonic larvae were found in the stratum immediately above the sediments and 30.0% in the surface stratum 2 hr after sunset. At high oxygen concentrations (X = 6.81 ppm), an average of 88.2% of the larvae remained in the bottom stratum and only 1.1% migrated to the surface stratum 2 hr after sunset. Temperature change and prcsencc or absence of food modified behavior; however, varying these factors under high oxygen concentrations failed to induce vertical migratory behavior.

When Chaoborus larvae (Diptera:Culicidae) were kept under constant light conditions within sediment cores their distribution exhibited a 24‐hr periodicity. There was a gradual upward movement of larvae in the sediments; the greatest percentage (x̄ = 90%) was consistently found in the 0–1‐cm stratum at solar sunset. The data indicate the larvae “test” the light conditions at the mud‐water interface at solar sunset. When the light intensity was below some critical threshold value, the larvae emerged and became planktonic. When the light intensity was above this value, the larvae reentered the sediments. The depth to which they burrowed was a function of the light intensity to which they were exposed. A method of resetting the endogenous rhythm in nature is proposed.

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