Hoverfly biology

Adults

The adult stage is what we normally see and think of as 'the hoverfly. They are relatively short-lived and survive for a few days to a few weeks. One mark-release-recapture exercise revealed that the large, black-and-white Great Pied Hoverfly Volucella pellucens can live for at least 35 days, although the average adult life-span was 12 days. Great Pied Hoverfly Volucella pellucens Recent mark-recapture studies of Hammerschmidtia ferruginea found the maximum adult lifespan to be 55 days. The primary function of the adult is to mate, disperse and lay eggs. Energy and protein are required to form the eggs. In the main, adult hoverflies obtain these by visiting flowers to obtain energy-giving nectar and protein-rich pollen. Most hoverflies lack specialised mouthparts, so prefer to visit flowers in which the nectar and pollen are exposed and easy to reach. White umbellifers, such as Hogweed, and members of the daisy family, such as thistles and knapweeds, are favourites.

One instantly recognisable genus of British hoverflies, Rhingia, is exceptional however, in having its mouthparts extended into a long snout, or rostrum. The rostrum has a groove on the underside into which the rather elongated proboscis fits. Consequently, it can visit flowers with a moderately deep tube, like Red Campion and Bluebell, to take advantage of a food source that is out of the reach of other hoverflies.

The closely related genera Xylota, Brachypalpoides and Chalcosyrphus tend not to visit flowers at all. Instead, they feed on honeydew, the sugary secretion of aphids, and pollen grains stuck to the leaf surface. They run over the surface of leaves, rapidly turning backwards and forwards in a very characteristic manner that makes them look like an ichneumon or spider-hunting wasp.

A number of smaller hoverflies, notably members of the tribes Bacchini (Melanostoma, Platycheirus), visit the flowers of wind pollinated plants such as plantains, grasses and sedges to obtain pollen. These plants provide a particularly rich food source through the production of great quantities of pollen.

Males of many hoverflies show some degree of territorial behaviour. They guard sunny spots by either hovering in a shaft of sunshine or sitting on a sunny leaf, from whence they dart out at other passing insects either to chase off a rival or to mate with a female. The same individual will return repeatedly to the same spot. Mark-release-recapture studies have also found that the same male of Volucella pellucens will return to hover in the same position along woodland rides on several successive days, indicating a strong and sophisticated level of territoriality.

The very noticeable high-pitched whine often heard in British woodlands on sunny Summer days is generated by thousands of male Syrphus species involved in territorial behaviour. In order to be ready to move at a moment's notice, they keep their wing muscles 'ticking over'. This causes the thorax to vibrate and this vibration is transmitted to the leaf surface, which acts like a sounding board.

Courtship is usually very brief. If the male encounters a female during his darting territorial flights, he grapples with her and, if accepted, they couple, either in flight or after falling onto vegetation. Mating usually lasts for a few minutes.

The small, honey-bee-like hoverfly Eristalis nemorum exhibits a more conspicuous courtship-related behaviour than most other hoverflies. A female is often seen feeding on a flower with a male hovering a few inches above her. It is not clear whether this is courtship behaviour or 'mate-guarding'. If a male has mated with a female, it is not unusual, across a range of insect groups, for him to then stand guard over her to stop other males mating with her before she has a chance to lay the eggs he has fertilised. This may be the case with E. nemorum but occasionally two or more males may be involved, jockeying for position over the female; this behaviour could therefore be a form of courtship. An interesting project for someone would be to investigate what is really going on!

Occasionally you may encounter a hoverfly with a distended abdomen with distinct pale whitish bands emerging between the segments. Such flies are usually dead and are firmly fixed to a plant; the genera Melanostoma and Platycheirus are particularly afflicted.

These have succumbed to a fungus that grows inside the insect and emerges between the body plates in order to release its spores. Mass occurrences of dead hoverflies attached to the undersides of umbellifer flowers or to grass stems are most frequently encountered in late Summer and early Autumn. Fungi that attack insects are referred to as 'entomophagous' and are one of the more noticeable causes of death amongst hoverflies.

Eggs

The ovipositor of female hoverflies consists of soft and flexible telescoped sections which, due to a complex arrangement of muscles and sensilla, has incredible sensitivity and manoeuvrability. It enables the fly to deposit eggs, singly or in small batches, very carefully and accurately, in a suitable place for the larvae to find food. Species whose larvae feed on aphids often lay eggs in or near aphid colonies. Those species with plant-feeding larvae usually lay eggs on particular host plants. Identifying the plant on which a female is egg-laying can, therefore, provide a useful clue to the identification of species within some genera (e.g. Cheilosa, Merodon, Eumerus).

Hoverfly eggs are usually somewhat elongate-oval in shape with one end wider than the other and often slightly curved. They are typically cream-coloured or pale yellow with a sculptured surface of fine pits and ridges or net-like patterns. These patterns can be distinctive and, potentially, used to identify a species, but very high magnification is required for them to be fully appreciated. When seen through a Scanning Electron Microscope the patterns are often quite beautiful. Larger species generally lay larger eggs; those of Volucella are around 2 mm in length, those of Syritta only 0-5 mm.

In the few cases where it has been observed, eggs...