Insects of Britain and Ireland: the genera of Syrphidae (hoverflies)

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L. Watson and M. J. Dallwitz

Chrysotoxum Meigen

Adult flies. The flies variously vespid wasp-like (cf. C. faciolatum and Vespula queens), or Nomada-like, or megachilid bee-like, or sphecid wasp-like (C. bicinctum and C. festivum, cf. Argogorytes and Ectemnius respectively), or not suggestive of mimicry; black and yellow; medium sized; 11.3–13.6 mm long. Wings (7–)8–12(–13) mm long.

The head about the same width as the thorax. The face ground-coloured at least part yellow; flat or concave in the region between the antennae and the mouth; without a central knob. The eyes hairy, or bare. Antennae relatively long, porrect; with their bases approximated; black and yellowish. The third antennal segment elongate- ovoid or orbicular to linear. The antennal bristle dorsal; about as long as the third segment (from near the apex); simple (bare or pubescent).

The humeri bare. The thorax patterned; with longitudinal stripes. The thoracic striping lateral only (yellow, interrupted). The scutellum at least partly yellow, or at least partly yellow and black. Wings plain (slightly longer than the body, colourless or greyish, usually tinted tawny or ferruginous along the fore border); without a conspicuously dark stigma; without black flecks along the hind edges; 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 before the middle of the adjoining discal cell. Vein R4+5 with a conspicuous curve projecting into the cell R5 to without a conspicuous curve projecting into the cell R5 (that of C. festivum exhibiting a gentle curve); 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 slightly stepped to strongly stepped. The upper outer marginal cross-vein conspicuously bent near the base the base; not re-entrant. The alula distinct. The thoracic squamae without long hairs dorsally.

The abdomen wider than the thorax; broad, oval, or oblong, or obovate (convex). The lateral margins of the tergites exhibiting beaded rims. The male abdomen with 5 visible segments. The abdomen contrastingly patterned (and sometimes yellow-tailed). The tergite patterning involving 2 to 4. The colour-patterned tergites marked with yellow. The dorsum of tergite 2 exhibiting a wineglass-shaped black area, or without a wineglass-shaped black area. The tergite bands medianly interrupted, or partially interrupted (then usually complexly so); narrow (C. festivum, C. vernale), or wide (then with characteristic figure: see illustrations). The spiracles of the third abdominal segment borne in the middle of each side.

Larvae and pupae. The larvae poorly known, probably posteriorly blunt and tail-less; probably scarcely flattened; probably pale to dark brown; dorsally smooth, without projections; mouth with a triangular sclerite on either side (?); anal segments without lappets. The larvae predatory (aphidivorous? - associated with ants’ nests).

Classification. Subfamily Syrphinae; tribe Syrphini.

British representation. 8 species in Britain.

Illustrations. • Chrysotoxum octomaculatum Curtis: B. Ent. 653. • Chrysotoxum octomaculatum Curtis: B. Ent. 653, legend+text. • Chrysotoxum octomaculatum Curtis: B. Ent. 653, text cont.. • C. arcuatum, C. bicinctum, C. elegans, C. festivum, C. octomaculatum: Verrall. • C. cautum: Verrall. • C. festivum, with Volucella. • 18 genera (from Walker). • C. cautum, 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.’.