Water-filled holes

Small water-filled holes (usually along streams) may be sometimes literally packed with various Dytiscidae and Hydrophilidae, althouth they appear entirely sterile at the first sight. The (usually hidden) beetles can be collected by vigorous straining and/or removal of detritus (which can also be left to dry up on the ground cloth). Water-filled tree holes are often inhabited by larvae of Scirtidae; some small Hydrophilidae have been described from water-filled leaf bases.