Dancing fly

These tiny flies that seem to be laying eggs in the water-filled shell and decaying remains of a garden snail (one at 11 o'clock on the shell rim, the other at 5 o'clock just inside the shell) are Sepsis fulgens. They are about 5mm. long and are said to be ant mimics - their black shiny bodies certainly give that appearance, although I've never come across any evidence for the functional significance of this similarity.

They've emerged in large numbers in our garden during the recent spell of hot, humid weather. In this image you can see the two white halteres just behind the wings, which show that these are true flies (dipterans) and not winged ants, which are hymenopterans with two pairs of wings.

Sepsis has very distinctive behaviour, scuttling over leaves, dancing and waving its wings at other flies.

The similarity with an ant is particularly striking in this side view.