Australian native Nomia bees, all males, settling down to roost on an arched flower stem of Gaura plant. The females live in solitary nests or in groups of nests, while the males sleep in the open like this. They like thin blades of grass or similar strands and cluster like this on the end of the blade of grass, probably for protection against predators.

This Gaura plant in my garden was covered in Nomia bees each evening, for about three weeks.