Small China-mark Cataclysta lemnata
Wingspan 18-24 mm.
The males of this species are slightly smaller and whiter than the females, but both sexes have a distinctive row of blue-centred black dots on the hindwing.
The adults are on the wing between June and August, when the females in particular come to light. It is fairly common around still or slow-moving water throughout Britain and Ireland.
The larvae are semi-aquatic, feeding on duckweed (Lemna spp.). They build floating cases made from fragments of the foodplant.