Sericomyia silentis (Harris)   
   (= S. borealis (Fallén))   
(a hoverfly)

Kingdom: Animalia
Phylum: Arthropoda
Subphylum: Hexapoda
Class: Insecta
Order: Diptera – true flies

British distribution: Widespread, especially in the north and west.
World distribution: Throughout much of Europe.

Sericomyia silentis
Sericomyia silentis
Sericomyia silentis
Sericomyia silentis visiting Centaurea nigra (rayed variant), Wigtownshire, July 2005.
(Flower colour rendered incorrectly, see note below.)

With a body length generally of about 16mm, Sericomyia silentis is one of our largest hoverflies (family Syrphidae).

Hoverflies are classic Batesian mimics – harmless but closely resembling bees or wasps (order Hymenoptera). S. silentis is a magnificent but rather fearsome-looking insect that could easily be taken to be a large social wasp (Vespa species). As shown below, it hovers, but it also has a rather 'busy' manner as it moves from flower to flower, again like a bee or wasp.

It is a species primarily of peatland areas and its distribution in Britain matches the distribution of its habitat, i.e. it is most common in the north and west and is much more local and scarce in central and south-east England.

The larva is of the "rat-tailed maggot" type, so called because it has a long, rear, tail-like, extendable breathing-tube, enabling it to live submerged in deoxygenated aquatic sediments. Larvae of S. silentis evidently occur in the rotting vegetation and semi-liquified peat of moorland ditches and flooded peat-cuttings; Verrall (1901) quotes correspondance recording the discovery of larvae that proved to be this species, found where vegetated peat turves had been thrown back into a pool in the cut area.

Sericomyia silentis hovering
S. silentis hovering.
Sericomyia silentis hovering, enlarged view

Its size, broad abdomen, and its pattern of yellow, narrowly wedge-shaped abdominal bars, which usually do not quite meet in the centre of the abdomen, are distinctive, though it should be noted that there are quite a number of smaller hoverflies with rather similar patterning. S. silentis is most likely to be confused with the one other British and European species of the same genus, S. lappona, which is scarcer but has much the same distribution in Britain. S. lappona is a little smaller and the abdominal bars are parallel-sided and typically a paler yellow.

A note on the flower shown in the photographs
The plant is Hardheads or Black Knapweed, Centaurea nigra, a very common plant; indeed data show it to be one of the most constant species in Wigtownshire botanical recording. However, although bract characters show it to belong to the subspecies nigra, it has rayed flowers usually seen only in subsp. nemoralis (as formerly interpreted, now C. debeauxii), a plant primarily of southern England. The unexpected sight of this population on a peaty roadside was the original reason for the photography.
Unfortunately, a problem with at least some digital cameras, the Nikon D100 included, is that certain rich purple and purplish-blue flower colours are not faithfully recorded. The flower colour in these photographs should be a much richer red-purple than is shown.

The excellent and definitive account of British hoverflies is:
•   Stubbs, A.E., & Falk, S.J. (2002). British hoverflies. An illustrated identification guide, ed. 2, British Entomological and Natural History Society, Reading.
Another valuable source of reference has been:
•   Verrall, G.H. (1901). British flies, vol. 8: Platypezidae, Pipunculidae and Syrphidae of Great Britain, reprint, 1969, E.W.Classey, Hampton.

© A.J. Silverside
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