Insects of Britain and Ireland: the genera of Syrphidae (hoverflies)
Formerly Xylota p.p.
Adult flies. The flies alternatively hivebee-like, or resembling Colletes (C. eunotis), or sphecid wasp-like (C. nemorum, cf. Crabro scutellatus and Ectemnius continuus); blackish with orange to grey abdominal marks in C. nemorum, or dark brownish and unmarked in C. eunotis; small to medium sized; 7.9–10.6 mm long. Wings 6.5–10.5 mm long.
The head wider than the thorax (mostly), or about the same width as the thorax. The face entirely dark in ground colour; flat or concave in the region between the antennae and the mouth; without a central knob; impressed. The eyes depicted as bare; not rimmed along their facial borders for much of the height of the face. Antennae relatively short, drooping; seated on a tubercle; black. The third antennal segment ovoid or orbicular. The antennal bristle dorsal; about as long as the third segment to much longer than the third segment; simple.
The humeri hairy, and readily visible behind the head. The thorax inconspicuously patterned to plain; with longitudinal stripes, or without longitudinal stripes. The scutellum flat, with a well defined edge; black (sometimes very reddish-hairy). Wings plain; divergent in repose, or incumbent and almost parallel 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 4.4–5.7 (in British species); broadly oval, or oblong, or linear (often, more or less). The male abdomen with 4 visible segments. The abdomen not conspicuously furry; contrastingly patterned (sometimes colour-tailed only). The tergite patterning involving 2, or 2 and 3, or 2 to 4. The colour-patterned tergites marked with whitish, or yellow to tawny, or silvery or greyish (in C. eunotus). The tergite bands medianly interrupted (to form well defined, subquadrate marks); narrow to wide. The spiracles of the third abdominal segment borne at or near the anterior corner of each side.
Larvae and pupae. The larvae tailed (shortly so). The anal segment tapered gradually to the spiracular process. The larvae slightly flattened; plain; with hooks on the thorax (two lateral to each anterior spiracle, and another small one on a separate base below these); mouth without triangular sclerites; anal segments with lappets (the anal segment with 3 pairs, these evenly sized). The larvae probably saprophagous; in sap runs, in rot-holes, and in decaying sap under loose bark (in deciduous and coniferous trees).
General comments. The thorax long-haired, and the hind femur enlarged. C. nemorum is very weakly distinguished from Xylota.
Classification. Subfamily Milesiinae; tribe Xylotini.
British representation. 2 species in Britain.
Illustrations. • Chalcosyrphus nemorum (Four-spotted Hover-fly: B. Ent. 425). • Chalcosyrphus nemorum: B. Ent. 425, legend+text. • Chalcosyrphus nemorum: B. Ent. 425, text cont.. • C. nemorum: Verrall.
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’.