Effect of Dim Light on Locomotor Activity Rhythm in the Onion fly, Delia antiqua

The phase of locomotor activity of the onion fly, Delia antiqua, in L (400 lux) Ldim (1.0 lux) cycles delayed as compared with that in LD cycles. The free-running period (τ) in constant dim light (LdimLdim) was longer than that in constant darkness (DD), suggesting that the phase delay of locomotor activity in LLdim cycle was caused by the increase in τ. At LdimD 12:12 in which the light intensity of the photophase was 1.0 lux, the locomotor activity free-ran with the period shorter than 24 hr until about week 2 after eclosion but thereafter entrained to LdimD in spite of τ might become longer than 24 hr. This suggests that the flies may become more sensitive to light intensity with age.