Diel activity of a boreal water beetle (Dytiscus alaskanus: Coleoptera; Dytiscidae) in the laboratory and field

SUMMARY. 1. The diel activity of Dytiscus alaskanus (Coleoptera: Dytiscidae) was studied by trapping in the field and by direct observation of behaviour (mounting, chasing, grappling, swimming and surfacing) in the laboratory. 2. In the field in the number of beetles trapped during the dark hours was approximately fifty‐fold greater than trapped during daylight hours. 3. In the laboratory, there was a marked increase in mounting, chasing and grappling when the ambient light level fell below 0.41mm −2 . Swimming showed only a slight diel pattern and surfacing none at all.