Hoverflies are true flies and belong to the order Diptera, family Syrphidae. There are more than 6000 species of fly in Britain of which over 280 are hoverflies. They range in size depending on the species from a few millimetres to 2 cm. The characteristic feature of true flies is that they have only two wings (one pair) compared to most other insects such as bees and wasps which have four wings (two pairs).

Hoverflies get their name from their ability to hover in mid-air. Many hoverflies mimic wasps, honey-bees or bumblebees with stripes, bands and markings of black and yellow. However, hoverflies do not bite or sting. Flies including hoverflies are an essential part of a healthy garden. Adult hoverflies are pollinators and the larvae of most species are predators or help recycle organic matter (detritivores).

Larvae

Approaching half of British hoverflies are aphid predators as larvae and are as important as ladybirds in reducing populations of aphids. They can also prey on other sap-sucking garden insects such as leafhoppers, whiteflies and scale insects. Tolerating the presence of aphids and other prey on some plants will provide a food source for hoverflies and can help promote a natural balance in the garden.

Some species rely on dead wood so creating a log pile can support them as well as a wide range of animals in the garden. Drone flies and some other hoverflies like stagnant water rich in organic material; creating a hoverfly lagoon will support these animals.

Adults

Most adult hoverflies rely on nectar for their energetic flights and have mouthparts that dab like a sponge. This means they prefer to visit flowers with an open bloom with easily accessible nectar and pollen.

Aphid-feeding

About 40% of hoverfly species found in Britain feed on aphids as larvae. They are very effective predators and contribute to the natural balance in a garden. These include many of the common species such as the marmalade hoverfly (Episyrphus balteatus), a species that migrates in huge numbers from Europe in the summer. Predatory larvae are mobile and rely on chemical senses and touch to locate prey. Once located they pierce the aphids with their mouth hooks and suck out the body contents.

Larvae in wasp and bee nests

The larvae of some hoverflies are scavengers and live as commensals (one organism obtains food from another without damaging or benefiting it) in the nests of bumblebees and wasps, feeding on dropped food, dead workers and in some cases wasp grubs. Hoverflies with this feeding habit include the species in the genus Volucella which are bumblebee or wasp mimics as adults and at 2 cm in length includes the largest British hoverfly, the spectacular hornet mimic Volucella zonaria that has become more widespread in recent years. In early autumn at the same time as wasp and bumblebee nests die out the hoverfly larvae leave. They drop down to pupate into whatever is underneath the nest, usually soil or leaf debris; if the wasp nest is in a loft they can fall into the rooms below, where the presence of the 2 cm long often spikey maggots can cause alarm.

Plant-feeding larvae

A few hoverfly larvae are herbivorous feeding on plants. Larvae of the large bulb fly (Merodon equestris)  feed on the inside of bulbous plants in the Amaryllidaceae family, especially daffodils. Larvae of the small bulb flies (Eumerus funerallis and E. strigatus) can also be found in bulbs but are usually only a secondary problem following other bulb damage. The larva of one species of hoverfly mines the leaves of Sempervivum. First found in the UK in 2008 the Sempervivum leaf miner Cheilosia caerulescens remains an uncommon insect largely restricted to South East England.

Ant-feeding larvae

Microdon larvae are highly specialised predators which feed on the eggs and brood of ants. The larvae body shape is adapted to survive the bites and stings of host ants. They are hemispherical and look very similar to woodlice.

Larvae feeding on rotting wood

A number of hoverfly larvae live in rotting wood such as the genera Criorhina and Xylota. Adult hoverflies can be seen crawling into crevices between large roots looking for egg-laying sites. Wet, decaying stumps are favoured. Some of these species have become very rare as their dead wood habitat has become uncommon.

Larvae in stagnant water

For accurate identification, a good quality photo, a binocular microscope, sometimes a pinned specimen and some expertise is needed.

Websites that can be helpful with identification:

British Naturalists' Association
NatureSpot has an info page on many species
Nature Guide UK
Hoverfly recording scheme
Steven Falk's comprehensive collection of hoverfly images on Flickr