Loire Valley Nature: Drone Flies

To identify flies to family level use a simple wing venation key such as A Key to the Families of British Diptera, an AIDGAP by D. Unwin, published by the FSC and available as a free download (scroll down to the titles list) or a more detailed key which includes many physical characters such as The European Families of the Diptera by P. Oosterbroek. The expert dipterists on the DipteraInfo forum are also very happy to identify, often to species level, from good photographs.

Drone Flies Eristalis spp are widespread and common members of the Hover Fly family Syrphidae. The Eristalini can be identified by the deep loop in the R4+5 wing vein (see here). They are superb Honey Bee Apis mellifera mimics, and are frequently mistaken for bees. All the species are very variable, especially in terms of the area of light / brighter markings on the abdomen. As a general rule, they are darker when they have been pupae in cold weather and lighter or brighter when they have pupated in warm weather.

The larvae are known as rat-tailed maggots and live in organically rich aquatic habitats such as ponds and ditches. Adults are found in any place that has an abundance of flowers. There are about a dozen species that occur in this area.

E. interruptus: an abundant late summer species here. Syn E. nemorum. This species can be identified by its dark front tarsi (feet), very small square pterostigma (a dark mark near the middle of the leading edge of the wing) and black facial stripe. It is very similar in size and appearance to the Dwarf Drone Fly E. arbustorum, which does not have a facial stripe and has a bigger pterostigma. E. interruptus engages in 'mating stacks', where one or more males fly above a female, guarding her (whereas E. arbustorum does not do this). The orange marking on abdominal tergite 2 does not reach the hind edge and the black goes all the way from one side to the other (in E. arbustorum the orange does reach the hind edge and the black does not go all the way across).

This species is found in open habitats such as meadows and wasteland. Larvae have been found in farmyard drains and similar places. The adults are active from April to September, with highest numbers in July and August.

Male, on Wild Carrot Daucus carrota, in our orchard, August. Note the dark front feet, the dark stripe down the centre of the face and the barely visible square pterostigma.

Male, on Wild Carrot Daucus carrota, in our orchard, August. Note the way the black on abdominal tergite 2 extends the whole way across, under the orange markings.

Tapered Drone Fly E. pertinax: Large for a drone fly. Both sexes have long tapering abdomens, especially the males. The front and mid feet are pale (all other species have dark feet). The hind tibia are pale at the knees and dark by the feet. Very like E. tenax, which is more broad and cylindrical, with dark feet and tibia. There is a variable orange triangle either side of tergite 2 (second abdominal segment).
Spring specimens are noticeably hairier and darker, due to the colder temperatures they have experienced when pupae (this is a common phenomena amongst flies).

Common and widely distributed, often one of the most abundant hover flies. Larvae can be found in farmyard drains and other organically rich very wet places. Adults begin emerging early in the year, in March, and can be seen until November (numbers peak in May and August). Males hold small aerial territories in woodland rides and can be seen hovering a few metres above ground defending their patch.