Is it a hornet? No, it's Britain's biggest fly

At first glance, Britain may not stand out as the home of impressively large insects - the largest beetles are tropical, as are the stick insects, large water bugs and largest butterflies. We do have some large dragonflies such as the Emperor Dragonfly Anax imperator, and a male Stag Beetle Lucanus cervus is always striking, but Britain is not home to the 'island gigantism' seen in, for example, the Giant Wetas of New Zealand, particularly the Little Barrier Island Giant Weta or Wetapunga Deinacrida heteracantha, the world's largest orthopteran (grasshoppers & crickets). The world's largest fly is Gauromydas heros, a member of the family Mydidae found in Brazil. It can be up to 60 mm long (excluding appendages) and is a wasp mimic, but relatively little is known about it with few specimens being collected as the adult lifespan appears short.


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.

This is of course the Hornet Robberfly Asilus crabroniformis, Britain's largest species of fly, seen here resting on a dry stick in damp grassland. Although it can't rival G. heros, large specimens are about half its size with the body length varying from 18-28 mm (this individual was at the upper end of the range).


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)

Although it has no specific legal protection, it was listed as Notable (scarce) by Falk (1992) who noted key threats as:

• 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).

Since then, Stubbs & Drake (2001) have also noted a new threat in the form of avermectins used as a veterinary treatment to remove parasites from the guts of livestock. In some cases, this makes dung harmful to the dung beetles which appear to be important in the fly's life cycle (see below). Further, WBP (2008) suggest a possible effect of climate change as adult activity appears to be temperature-regulated (i.e. reliant on high ambient air temperatures), though it is unclear whether this would simply increase levels of adult activity and if so, whether this would be beneficial or harmful.


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.

These are simple enough in concept, but of course not so straightforward to implement. Asilus-friendly habitat management should be quite easy to achieve (at least in principle, assuming the time and resources to control scrub etc), though beyond the bounds of the nature reserve it is less clear how the loss of cattle dung can be reversed. However, education of land-owners is important and simple measures can ensure a continuous supply of horse dung, although again it will require a considerable change in behaviour to reduce the use of avermectins which are popular broad-spectrum treatments. In the end, this is a charismatic species which, with a little effort and creativity, could be more widely known and popular as 'Britain's biggest fly'.




References

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]