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Gloucestershire Naturalists' Society |
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Gloucestershire Naturalists' Society |
November Floods
For information about the current floods along the Severn and their impact on local wildlife see the News page.
Volucella zonaria and Volucella inanis: spectacular new additions to Gloucestershire’s hoverfly fauna.
Volucella zonaria and Volucella inanis are nationally notable hoverflies that were unknown in Gloucestershire before 2001. They have since gradually become established here, and are being seen frequently at this time of year.
Volucella zonaria (about 19 mm. in length ) is easily the largest of British hoverflies. Volucella inanis is smaller, but at about 16 mm. is still among the half dozen or so largest British species. Both are good mimics of social wasps, in particular the Hornet (Vespa crabro) – indeed Volucella zonaria is sometimes known as the Hornet Hoverfly. Many hoverflies are good mimics of bees and wasps, their mimicry giving them protection from predators such as birds. However these two Volucella species are associated with wasps in a more direct way: their females lay their eggs in the nests of hornets and other social wasps, and their larvae develop there as scavengers.
Volucella inanis has probably been a native species for centuries, but Volucella zonaria was only found in Britain as an occasional vagrant until the 1940s. Before the current millennium both species were virtually confined to south-eastern and southern England, with the exception of a population of Volucella zonaria in the Bristol area (including sites in Vice County 34 West Gloucestershire, but well south of Gloucestershire’s present day borders). Since then both species have been expanding their range westwards and northwards. The first record for Gloucestershire (as currently defined) of Volucella zonaria was in 2001, but it was only in 2004 that Volucella inanis began to be seen on a regular basis in the county and only in 2007 that Volucella zonaria was seen in numbers. The spread of these two hoverfly species is suspected to be a result of climate change.
Apart from its larger size, the female of Volucella zonaria is easily distinguishable from Volucella inanis by the fact that its thoracic dorsum is predominantly, or sometimes entirely, chestnut coloured; that of Volucella inanis is mainly black with some chestnut at the sides and a small chestnut onion-dome shaped mark at the rear. This distinction makes the female Volucella zonaria look especially hornet-like in its colouring. Difficulty can arise however in separating the males of the two species, as the thoracic dorsum of male Volucella zonaria is black (like both sexes of inanis) rather than chestnut. (As with most hoverflies, the males have the eyes contiguous, while those of the females are separated)
The two species can usually be differentiated by looking at tergite 2 (the front-most fully visible segment of the upper surface of the abdomen). In Volucella inanis the yellow markings occupy the greater part of the segment, are almost rectangular in shape, and are always as bright as the yellow areas on the other segments of the abdomen. In the case of Volucella zonaria the equivalent markings are restricted to a smaller area of the front half of tergite 2, their rear edge is curved and they are often darker than the yellow areas of the other tergites, sometimes being almost chestnut-coloured like the thorax of the female.
The most reliable way of distinguishing the two species is to look at sternite 2, the segment of the underside of the abdomen immediately below tergite 2. This can be done in the field by looking at the insect from the side. In Volucella zonaria sternite 2 is always black while in Volucella inanis it is always yellow.
While many scarce hoverflies can only be found in such habitats as ancient woodland or primary wetland, Volucella zonaria and Volucella inanis favour gardens and urban parks. Although they feed on nectar and pollen from a wide range of flowers, Volucella inanis is often at bramble and Volucella zonaria at Buddleia, while both species seem to have a special affinity with the Hebe cultivar “Great Orme”. The peak of their flight season is August.
Recent news suggests that Volucella zonaria, itself a relative newcomer to Britain, may be in danger of eventually losing its status as the country’s largest hoverfly. There have been two reports this year of sightings in the south-west of Milesia crabroniformis, a continental hoverfly, which, as its name implies, is another hornet mimic, and which is even larger, at nearly 25 mm., than Volucella zonaria.
David Iliff
David's pictures below show different views of the 2 species in the following order:
Volucella zonaria male - top left and female - top right
Volucella inanis male - middle left and female - middle right
Volucella zonaria side view - bottom left and Volucella inanis side view - bottom right.
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GNS Grants
The Society is pleased to announce that small grants are available to help finance projects concerned with biological recording, education, public awareness and environmental or nature conservation within the county.
The terms and conditions for payment of grants are available here.
A copy of the grant application form is here 
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Gloucestershire Naturalists' Society is the natural history recording organisation for Gloucestershire, UK, recording all of the wildlife in the county. The society is membership based and organises lectures and field meetings.
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