Hoverflies: Insect Mimics
As their name suggests, hoverflies are expert at hovering and can often be seen poised overor high up in a woodland glade. They are remarkable for their mimicry of bees and wasps – a mimicry that goes much further than mere similarity of colouring.
Hoverflies are among the commonest and most familiar of the many flies you can see in garden and countryside. There are more than 200 species in Britain. Like all true flies (Diptera), they have only one pair of wings, the hind pair being greatly modified into small knobbed balancing organs (halteres).
Many hoverflies are banded with yellow on a blackish or metallic background – colouring that gives them a superficial resemblance to wasps or bees. Some look extremely realistic, adopting even more of the appearance of wasps and bees. For instance, the wings of some have a dark front margin, suggesting the rolled wings of the common wasp: others have longer antennae than is usual in flies, again in imitation of bees and wasps; and most make a realistically bee-like buzzing sound if trapped.
Although such mimicry is widespread in insects, it seems to be particularly well developed in hoverflies. In their association with flowers – where they can easily be seen by birds and other predators – such a resemblance may give them a selective advantage. Predators will avoid the harmless hoverflies in much the same way that they avoid the stinging insects. The larvae of hoverflies, which do not have such mimicking colouring, are preyed on by birds and parasitised by ichneumon and other parasitic wasps.
A life-cycle centred on aphids
A large group of hoverflies produce larvae that feed on aphids and are important in keeping down the numbers of these destructive insects. Some of this group are common garden insects, most of them blackish with yellowish bands or paired crescents (lunules) on their bodies. One of the best known is Syrphus ribesii; it is one of the larger species, at 12mm (1/2in) long and with a wingspan of 25mm (1 in). Its abdomen is dark, with three orange-yellow bands and two crescents at the base. You can frequently see it sitting on flower heads or hovering near flowers in sunny weather from April to November.
S. ribesii and its many allies, feeds at flowers; the females need to include pollen as well as nectar in their diet if their eggs are to develop. Males wait on projecting parts of plants for females to pass, then fly in pursuit. Mating usually takes place on vegetation. Females lay their eggs singly or in groups of two or three on plants infested with aphids. They are attracted to the aphids by odour and hover over the aphid colonies for a short time before landing to deposit their eggs. Each female may lay 100 eggs a day, with a total of perhaps as many as a thousand. The eggs, less than 1 mm long, are white and hatch in three to four days.
The minute white larvae immediately start to feed,their hook-like mouthparts into the bodies of the unfortunate aphids and sucking out their soft body contents. There are three larval stages, with moults in between -in all a 10-day growth period in which a single larva may consume over 800 aphids. The aphids make no attempt to escape, although the ants that attend the aphids to collect their honeydew often try to protect them. The Syrphus larva responds to this by exuding a protective slime which deters the ants.
When full-grown, the larva attaches itself by its hind end to a leaf or twig, and, as with all more advanced flies, turns into a pupa within its last larval skin, which becomes a hard, pear-shaped, brownish case. After a further 10 days or so, the adult fly emerges by splitting the upper side of the case or puparium.
Several generations of S. ribesii follow one another in this way, until autumn. At this time of year fully grown larvae seek out hiding places in thewhere they can pass the winter. They do not become pupae until March or April.
Overwintering and migration
Other species, for example Scaevapyrastri, a common blue-black hoverfly with creamy lunules on its abdomen, have successive generations throughout the year, and hibernate as adult flies in old trees, out-houses and similar sheltered places. In this case, however, only mated females hibernate; the males, their purpose accomplished, die. The females feed up in the autumn and fill their crops with nectar, increasing their body weight by as much as 50%. They are the first on the scene when aphid activity begins in the spring.
Another species, Episyrphus balteatus, a narrower insect with double bands on its abdomen, also overwinters as an adult fly. It is one of the species that regularly migrates, presumably to seek out fresh breeding sites. It is frequently found in huge numbers in coastal districts, suggesting that it has come from the Continent. It has even been found 25km (15-½ miles) out to sea. Sometimes this species appears in unusually large numbers in gardens, but within a day or two they all disappear.
Nest invading hoverflies
It is not only the aphid-feeding species that have a marked wasp or bee-like appearance. Five fairly large species, belonging to the genus Volucella, are similarly adorned. They lay their eggs in the nests of bumblebees and social wasps.
Volucella bombylans is a large hairy fly, 14mm (1/2in) long, with a wingspan of 3cm (1-1/4in). It occurs in two forms, one with hairy yellow bands like a yellow-banded bumblebee, the other black with a red tail like the bumblebee Bombus lapidarius. The larvae of this species hatch from eggs laid in bumblebee nests where they act as scavengers, feeding on dead bees, dead larvae and any other rubbish.
The other species of Volucella are less hairy and scavenge in the nests of social wasps. The adults enter wasps’ nests unmolested and lay their eggs on the paper envelopes. The commonest wasp nest-dwelling species is V.pellucens, which has a conspicuous whitish base to its abdomen. The young larvae that hatch from its eggs mostly fall to the midden below the nest. Here dead wasps and larvae, and any other rubbish that has accumulated, form their staple diet. Some manage to penetrate into the nest, squeezing into the paper cells with the wasp larvae, on whose secretions and excretions they feed. Late in the season the Volucella larvae penetrate the nest more fully and eat living wasp larvae and pupae. In the autumn, the Volucella larvae become full grown, reaching a length of about 20mm (3/4in); they are broadly oval in shape and covered with spines. They become dormant and hibernate as larvae, either in the remains of the nest, or in soil nearby. They change to pupae in the following May and June and emerge as flies in July and August.
Other hoverflies, of the genus Microdon, show quite a close resemblance to the honeybee. They occur particularly in old woodlands and in the heathlands of Surrey. Sussex and the New Forest. The two commonest species are Microdon eggeri and M. mutobilis; both are about 9mm (1/3in) long.
Microdon hoverflies are on the wing in June, flying low over the ground and around rotten tree stumps containing ants’ nests, particularly those of the common black garden ant, Lasius niger. The flies investigate holes in the stumps and eventually choose those suitable for laying eggs. The eggs hatch after 12 days and the larvae move actively into the ants’ nests, where they become sluglike in shape and movement. The larvae seem to be present in fairly large numbers in the chambers where the ants are most numerous.
The larvae are ignored by the ants. They appear to feed mostly on the pellets of dis-carded food that the ants eject from a special pouch in their mouths. When fully grown the larvae travel to the upper and drier parts of the nest, where they remain until the spring. They change to pupae in April or May.