However, Britain is not without its own sizeable flies as noted during a visit to Hatchet Pond yesterday. Although used heavily for recreation, this site is home to many uncommon plant and invertebrate species, and one of these was this striking insect seen shortly after arriving.
|The Hornet Robberfly Asilus crabroniformis, Britain's largest fly.|
Despite the large size and bright yellow abdominal section (it is a hornet mimic), it is remarkably well camouflaged when the brown wings cover this area. As it is a 'darting' hunter (capturing prey via short darting flights, usually of no more than 50cm, but sometimes up to 3m), this cryptic colouration is presumably important, while defense may also be involved as it has been seen hanging upside-down when resting at night, a position that displays only the cryptic brown colours of the underside - in this position it has the appearance of a dead curled-up leaf (Clements & Skidmore 1998). The hunting flights are launched from low-lying perches such as the stick in the photo above; prey items are large insects such as grasshoppers, larger beetles and flies, butterflies, moths, bees and wasps (WBP 2008) and take 10-30 minutes to drain using the fly's sharp piercing mouthparts (Stubbs & Drake 2001).
A. crabroniformis is associated with a range of open habitats such as heath and various types of grassland with rough shrubby vegetation, and is associated with old dung with females laying their eggs on or under the crust, particularly cow dung, though also horse and (if in mounds) rabbit. Despite this fairly broad habitat requirement, it is a local species rarely found in large numbers. The map below (courtesy of the National Biodiversity Network) suggests a fairly widespread distribution in southern Britain; however, some of these records date from before the loss of much of its habitat (especially in East Anglia) - most recent records are from the heaths of Dorset, Hampshire (as here) and Surrey, plus chalk grassland in Hampshire and Wiltshire, with scattered records elsewhere.
|Distribution map of Asilus crabroniformis in Britain (as of 08/08/11)|
- Habitat loss/degradation due to conversion to intensive agriculture or forestry.
- Loss of dung and associated beetles due to removal of appropriate grazing animals e.g. replacement of cattle by sheep in much of its historical Hampshire habitat (Stubbs & Drake 2001) and removal of dung during paddock management (WBP 2008). Sheep dung appears to be unsuitable for A. crabroniformis and the shift from cattle to sheep accelerated during the BSE outbreak of the 1990s (Clements & Skidmore 2002).
The life cycle is not fully understood, but the later-instar larvae are free-living in the soil and thought to be predatory on dung beetle larvae (geotrupines such as Minotaur Beetle Typhaeus typhoeus) which are associated with herbivore dung. The diet of early-instar larvae is unknown as eggs are laid in dry dung which has no large insect larvae, only small invertebrates such as nematodes and springtails. Territorial behaviour has been suggested, centred on dung-pats (e.g. by Stubbs & Drake 2001) but this has never been clearly demonstrated with Clements & Skidmore (2002) indicating no strong territoriality following their mark-recapture study - indeed individuals could be seen sharing parches and previous records of agression may be attempts at copulation. Clements & Skidmore (2002) found the mean adult lifespan to be 15.9 days in the field with peak emergence in late July and August following a larval lifespan thought to be 2-3 years, although this is not well documented (WBP 2008).
Regarding conservation, it seems clear that this is a scarce species which has declined over recent decades - it is therefore of key importance to understand the combination of habitat and stock-related factors which may benefit it, especially as even historical records appear to show populations as being somewhat erratic with local ones becoming extinct as others appear. However, this shifting distribution may be threatened with collapse if suitable areas of habitat become isolated, especially as research suggests dispersal rates may be limited with individuals rarely moving more than a few hundred metres, and often less (Clements & Skidmore 2002). This, a range of conservation measures need to be considered:
- Maintain rotational grazing management by suitable species to ensure a continuous dung supply.
- Prevent excessive scrub invasion of suitable habitat while ensuring a range of vegetation type/structure.
- Reconsideration of the widespread use of avermectins.
Clements, D.K. & Skidmore, P. (1998). The autecology of the Hornet Robberfly Asilus crabroniformis L. in Wales, 1997. Countryside Council for Wales Contract Science Report No. 263. Bangor.
Clements, D.K. & Skidmore, P. (2002). The autecology of the Hornet Robberfly Asilus crabroniformis L. in Wales, 1997-1999. Countryside Council for Wales Contract Science Report No. 525. Bangor.
Falk, S. (1992). A Review of the Scarce and Threatened Flies of Great Britain (Part 1). NCC, Peterborough.
Stubbs, A. & Drake, M. (2001). British Soldierflies and their Allies. BENHS, Reading.
Worcestershire Biodiversity Partnership (2008). Hornet Robberfly Asilus crabroniformis Species Action Plan. [accessed 08/08/11]