Nick Baker’s hidden Britain
The unnoticed water snipe fly
Many insects are easy to miss in the melee of summer buzzing. Our eyes and ears become bamboozled by the gaudy and gallant – butterflies, dragonflies, bees, beetles – and we overlook the more subtle species. But low-key insects, such as this month’s dipteran delight, are worth seeking out.
Atherix ibis is not easy to find. It’s one of a group known as the water snipe flies, of which Britain and Ireland have three species. They tend to go unnoticed, despite possessing the attractive family traits of a long, banded, tapering body; dark-patterned, back-swept wings; and glittering green eyes. Keep a lookout for them almost anywhere there is flowing water.
Much of what we know about Atherix ibis comes from its larvae. Charismatic by maggot standards, they are aquatic, carnivorous and venomous. Bad news for juvenile stoneflies and caddisflies, which they hunt among the gravel beds of streams and rivers. A venomous nip and their prey is quickly immobilised then sucked dry.
However, it is the egg-laying behaviour of the female flies that really makes a name for Atherix. In early summer, after they’ve mated, the females engage in a phenomenon so bizarre it takes some explaining. During a spell of still, balmy weather, they gather for one final, slightly sinister denouement to an otherwise fairly regular fly life.
Nobody really knows why the females do this, but they seek out a position above flowing water. This can be on overhanging leaves, branches, boughs, mossy rocks or even bridges. In fact, bridges are probably disproportionately represented in our records, as the flies are much more noticeable here. The final act starts with one fly. Over a period of hours or days, other mated females join the throng. How they co-ordinate this is uncertain, but it is thought that they emit some kind of odour – an aggregation pheromone.
As the flies pile in, they attach to one another by means of microscopic hooked hairs called acantae. These mesh together much like Velcro. Once a female has settled on a position, that’s it – she’s there for life (the little of it that she has left). Some of these aggregations can contain more than 1,500 individuals, but 300–600 is more usual.
The purpose of the matriarchal mass is to lay eggs, about 500 per female. These are deposited within the cluster of flies – afterwards, the females die. Interlocked in place, their bodies form a crust – a fly pie with a mat of stuck-down eggs as the filling. Sometimes, this cluster has a more three-dimensional form, hanging like some kind of grizzly twitching fruit. Here, the flies remain until the 2mm-long larvae hatch out, 9–12 days later.
The larvae wait until night has fallen, to avoid predation by visual hunters such as fish, then drop from the crusty mass of eggs and dead flies into the water below. It was once thought that they lingered to feed on the carcasses of their mothers, but studies have shown this not to be the case.
The matriarchal mass may be a way of giving the larvae some kind of protective shield – against either the elements or predators. It might serve as camouflage, or mimic the clusters of stinging insects such as bees and wasps.