A male longhorn moth, Nemophora degeerella (Lepidoptera: Adelidae). The males of this species have antennae which are about 6 times longer than their bodies. They use these antennae for display when they swarm in woodland clearances:
The females have much shorter antennae (see here).
Photos taken in Whiteknights Park, Reading University grounds, Reading, UK, on 2011-05-19.