(Hübner, )Though winged males occur in Europe, only the parthenogenetic wingless female form of Dahlica triquetrella has been recorded in Britain; with confirmed records from Kent, Westmorland and Essex, but it may have been overlooked. In mainland Europe, its preferred habitat is on algae covered bark of trees on the edges of woods with a southern aspect, but it is also found on algae or lichen on rocks, stones, posts and house walls, and in meadows on moss. The diet of algae and lichen has to be supplemented by dead insects for development to be successful.
The adult lives a very short time, so the cased larva or pupa are more likely to be found. The silk case, triangular in cross section and tapered at both ends, is covered in sand, frass and bits of dead insect. The larva overwinters fully fed in a rock crevice or crack in tree bark, sometimes near the ground. The larva is active as the snow melts in Europe, the date depending on altitude (up to 2200m), February in Britain, at temperatures of 1 or 2 degrees Celsius. After selecting a suitable site and sewing the front of the case shut, the larva pupates in the case. In March or April the pupa extrudes from the case for the wingless female to emerge and lay her eggs in the vacated case. It appears that parasitised larvae are the first to move to pupation positions. At one Swiss site, 99% of early larvae were affected; two weeks later, none were parasitised.
Positive identification depends on dissection of the genitalia, or examination of the headplate of the pupal exuviae.