Bog hoverfly (Eristalis cryptarum)

GenusEristalis (1)
SizeLength: 9 - 11mm (2)

Classified as Vulnerable in Great Britain (3).

The bog hoverfly is a bee mimic; it is very similar in general appearance to a bee (1). Like all members of the Eristalis genus, this species does not have a pattern of spots on the eyes (4). It can be distinguished from other Eristalis species by the orange tibia(5). Male and female hoverflies are relatively easy to tell apart; in males the eyes meet at the top of the head, whereas in females they are separated (4).

Once found throughout the counties of southwest England reaching as far east as the New Forest, the bog hoverfly has suffered a catastrophic decrease in range, and was thought to be extinct until it was rediscovered in 1993. It is now known from only a number of sites on Dartmoor in Devon (6). Outside of Britain, this hoverfly has a broad distribution in northern and temperate areas of Europe (7).

As the common name suggests, this species is found in boggy habitats. It occurs in valley mires, on heathland and moorland (7) and rhôs pastures (8) (rhôs is a Welsh word that means wet, sometimes heathy pasture) (9). The bog hoverfly is associated with mossy narrow water channels (also called runnels), caused by the emergence of ground water, where there are plenty of flowering plants as an adult food source (6). It seems to prefer areas of open, low vegetation, furthermore, patches of scrub are important refuges during bad weather (6).

Very little is known of the ecology of this species but recent research, funded by English Nature, has led to a number of new observations. Like most hoverflies, adults fly only on warm sunny days (6), they have been seen feeding on the flowers of bogbean, marsh marigold and cuckooflower (8). They fly low, rarely reaching heights of over 1 metre above the ground (6). Peak times of adult abundance occur in May, mid-July and mid-September; this has led some people to believe that there are three broods each year, each with a different time of adult emergence, however the evidence is insufficient to demonstrate whether this is the case (6). Males seem to hold 'mating territories' which they defend against other hoverflies and even bumblebees and other insects by chasing them away (6).

The larvae of this species have never been identified, but females have been seen ovipositing on fresh cow dung and on sphagnum moss near dung (2).

Whilst the cause of the precipitous decline of this species is not known (7), current threats may include the invasion of the habitat by shrubs, particularly willows, caused by a decrease in grazing and by a lack of scrub control. This results in too much shade for the bog hoverfly to survive (8).

The bog hoverfly is a UK Biodiversity Action Plan (UK BAP) priority species and a Species Action Plan has been written to co-ordinate conservation action required. This plan aims to maintain the current populations, enhance their size and restore the species to five new sites in the former range before 2010 (7). This hoverfly is also included in English Nature's Species Recovery Programme, which has funded research and survey work. Before the bog hoverfly can be conserved successfully, much more must be discovered about its life cycle and the elusive larval stage (6).

For more on this species see:
The Dartmoor Species Action Plan for the bog hoverfly. Dartmoor Biodiversity Steering Group:

Information authenticated by Jon Webb, English Nature:

  1. National Biodiversity Network Species Dictionary (August 2002):
  2. Jon Webb (2002) Pers comm.
  3. Shirt, D. B. (Ed) (1987) British Red Data Books: 2. Insects. Nature Conservancy Council.
  4. Gilbert, F. S. (1986) Hoverflies. Naturalist's handbook. Cambridge University Press, Cambridge.
  5. Eristalis key. (August 2002):
  6. Perrett, J. (2000) Eristalis cryptarum: a study of the phenology and extent of the Devon population, 1999-2000. English Nature Research Report number 414. English Nature, Peterborough.
  7. UK BAP (August 2002):
  8. Dartmoor Species Action Plan for bog hoverfly. Dartmoor Biodiversity Steering Group (August 2002):
  9. Devon Biodiversity Action Plan. Rhôs pasture. (September 2002):