What’s that fly with long dangly legs?

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At this time of year, for a couple of weeks, there are lots of large, black flies flying around with their legs dangling down. On the coast here in Cornwall they’re everywhere, getting blown into the long grasses and paired up as shown left.

Insight into St Mark’s Fly (Bibio marci): the image on the left shows the differences between the male and female flies. The male on the left has a large head with large eyes and clear wings whilst the female on the right is slightly larger and has a small head and smoky wings.

The females lay their eggs in the soil and die soon after. The larvae grow all through the autumn and winter, emerging as adult flies around St Mark’s Day, 25 April, the following spring.

Adults only live for about two weeks so the St Mark’s Fly spends most of its life in the larval stage.

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