Mottled bee-fly (Thyridanthrax fenestratus)

GenusThyridanthrax (1)
SizeLength: 7 - 9 mm (2)

Classified as Rare in Great Britain (3).

As the common name suggests, the mottled bee-fly is somewhat similar in appearance to a bee (4). Like all flies, it has a single pair of wings; the hind wings have been modified into drumstick-like balancing appendages known as 'halteres' (5). The large wings are mottled with black blotches (1), and this mottling continues on the thorax and abdomen, which are dark brown to blackish in colour with beige patches (4).

In Great Britain, this species is currently restricted to Dorset, West Sussex, Surrey, Berkshire and the New Forest and the Weald in Hampshire. It has been lost from many sites and has declined at many more. Elsewhere this bee-fly has a broad distribution in north-central Europe, however its status is unclear (3).

Inhabits open heathland preferably in a matrix of heather (Calluna vulgaris) of varying age classes, sparse vegetation and bare sand in unexposed conditions (2). The sand must not be too disturbed so as to make it loose (2).

There is strong evidence to suggest that the mottled bee-fly is a parasitoid of the sand wasp (Ammophila pubescens) or of the caterpillars gathered by the wasp for its larvae(3). This means that the bee-fly larvae develop inside either the larvae of the sand wasp or inside the caterpillars that the wasp collects; the host dies as a result of the development of the bee-fly. This is a 'thermophilous' (heat-loving) species, and occurs only in warm areas. Adults feed on nectar, so a source of flowers is essential (3).

The mottled bee-fly is threatened by unsuitable heathland management practices such as scrub and tree encroachment causing excessive shading, too little disturbance causing loss of bare sand, heathland fires and damage by the recreational use of heaths, such as horse riding, and by military operations (3).

This bee-fly is a UK Biodiversity Action Plan (UK BAP) priority species, and a Species Action Plan, outlining conservation targets has been published. This plan aims to make sure that viable populations are in place at all known sites by 2010 (3). All current populations occur on National Nature Reserves (NNRs) or Sites of Special Scientific Interest (SSSIs), and a few occur within proposed Special Areas of Conservation (SACs). Work to conserve this species could also benefit the sand lizard (Lacerta agilis), the heath tiger beetle (Cicindella sylvatica) and the bee-fly (Bombylius minor) (3).

Information authenticated by Jon Webb, English Nature:

  1. National Biodiversity Network Species Dictionary (August 2002)
  2. Jon Webb (2002) Pers. Comm.
  3. UKBAP (August 2002):
  4. From observation of images.
  5. Allaby, M. (1991) The concise Oxford dictionary of Zoology. Oxford University Press, Oxford.