This is a particularly good year for the St Mark’s fly (Bibio marci). They have sometimes been in such numbers that avoiding clouds of them has been quite difficult. The flies you normally see are males, black and about 12mm long with big eyes and dangly legs, bobbing up and down at about head height. The male’s eyes are split into two, one half to keep an eye out for the later emerging females and the other for checking how far they are from the ground. The females are slightly bigger but with smaller legs and eyes.
The fly’s name comes from the normal first appearance of the males on St Mark’s Day (25 April), but this year because of the cold spring the swarms came weeks later. In places the flies are still numerous. They are an important fruit pollinator and also known as the hawthorn fly because they often drink nectar from these flowers.
When the females emerge they survive about six days, during which time they must find a mate and lay their eggs in the soil. When the larvae hatch they spend the autumn and winter in the ground living on rotting vegetation. The wonder is how, each spring, both sexes manage to synchronise their short and energetic adult lives.
