Are hover flies active day and night? We see hover flies during the day but they are not observed at night. Is this because they are not active at night? Or are they active but less likely to be observed in the dark? The absence of evidence is not evidence of absence.
Two scientists* followed the activity of hover flies in an activity monitor that detects movement. The hover flies were on a cycle of 12 h light followed by 12 h of darkness. On the first night of monitoring, the dark phase of the circadian cycle was totally dark. On the second night, the dark phase was interrupted by a 5 h period of light in the middle of the dark phase. On the first day, all of the male hover flies tested flew when the light was on and did not fly when the light was off. On the second night, a few male hover flies flew when the light was on, but substantially fewer than the previous two days. No flies were active in the dark.
The conclusion: There is a circadian rhythm that leads to greater activity during the day than the night. However, the repression of flight activity can be overcome by strong light. Also, hover flies fly only in the light. The darkness completely suppresses flight activity.
*Malin Thyselius · Karin Nordström. Hoverfly locomotor activity is resilient to external influence and intrinsic factors. J Comp Physiol A (2016) 202:45–54.