Insects of Britain and Ireland: the genera of Syrphidae (hoverflies)
Adult flies. The flies cimbicid sawfly-like, or resembling a wood-wasp (with X. signis remarkably similar to Sirex juvencus in shape and patterning, but much smaller), or sphecid wasp-like (X. sylvarum, with reference to the posture in life, rather than morphology); black, black and red, or black marked with yellowish or greyish; small to medium sized; (9–)10–14 mm long. Wings 5.5–12 mm long.
The head wider than the thorax. The face entirely dark in ground colour; flat or concave in the region between the antennae and the mouth (but not descending far below the level of the eyes, by contrast with Brachypalpus and Criorhina); without a central knob; impressed. The eyes depicted as bare. Antennae relatively short, drooping; seated on a tubercle; black. The antennal bristle dorsal; about as long as the third segment to much longer than the third segment; simple (basally slightly pubescent).
The humeri hairy, and readily visible behind the head. The thorax pubescent without stiff bristles interspersed; patterned (with two anterior pale spots); without longitudinal stripes. The scutellum flat, with a well defined edge; black. Wings plain (pale grey, or tinged with brown between costa and disc); with a conspicuous dark stigma to without a conspicuously dark stigma; divergent in repose. Wing veins R2+3 and R4+5 not forming a closed cell. The anterior cross vein R-M in cell R5 crossing it at or beyond the middle of the adjoining discal cell. Vein R4+5 without a conspicuous curve projecting into the cell R5; without a backwardly projecting, incomplete transverse veinlet. The lower outer marginal vein slightly diverging from the posterior wing margin to more or less parallel with the posterior wing margin. The upper and lower outer marginal cross veins strongly stepped. The upper outer marginal cross-vein conspicuously bent near the base the base; not re-entrant. The alula distinct.
The abdomen about the same width as the thorax to narrower than the thorax; about 5.8–7.8; narrowly obovate, or oval, or oblong (rather parallel-sided, or even slightly narrowed in the mid-region), or linear (almost); contrastingly patterned (sometimes tawny- or yellow-tailed only). The tergite patterning involving 2 and 3 (mostly), or 4 and 5 (when tailed only). The colour-patterned tergites marked with whitish, or tawny, or orange to red, or silvery or greyish. The interrupted band on tergite 2 reduced to small paired spots, or not reduced to small spots. The tergite bands medianly interrupted, or partially interrupted and entire (X. tarda and X. segnis having tergite 2 partially interrupted proximally, and 3 entire); usually narrow, or wide (notably in X. tarda and X. florum).
Larvae and pupae. The larvae tapering anteriorly and posteriorly from the middle, or tapering posteriorly; tailed, or posteriorly blunt and tail-less (X. segnis). The anal segment when tailed, shortly tapered gradually to the spiracular process. The larvae to 12–15 mm long, scarcely flattened (sub-cylindrical); dark or light brown, plain; without thoracic hooks; mouth without triangular sclerites; anal segments with lappets. The larvae saprophagous, or coprophagous, or consuming stored produce; variously in sap runs, in rot-holes, in decaying sap under loose bark, in rotten wood, in decaying tree roots, and in decaying vegetation (as well as in silage and rotten potatoes).
General comments. The thorax short-haired, unlike Chalcosyrphus and Brachypalpus.
Classification. Subfamily Milesiinae; tribe Xylotini.
British representation. 7 species in Britain.
Illustrations. • X. abiens, X. florum, X. segnis, X. sylvarum, X. tarda: Verrall. • X. sylvarum, female (with Brachypalpoides, Criorhina and Eristalis). • 15 genera (from Walker). • X. segnis, with assorted other Syrphidae (adult forms).
To view the illustrations with detailed captions, go to the interactive key. This also offers full and partial descriptions, diagnostic descriptions, differences and similarities between taxa, lists of taxa exhibiting or lacking specified attributes, and distributions of character states within any set of taxa.
Cite this publication as: ‘Watson, L., and Dallwitz, M.J. 2003 onwards. Insects of Britain and Ireland: the genera of Syrphidae (hoverflies). Version: 28th July 2015. delta-intkey.com’.