Insects of Britain and Ireland: the genera of Syrphidae (hoverflies)
Adult flies. The flies resembling a potter bee, or Nomada-like, or sphecid wasp-like (compare X. pedissequum with Gorytes, Crabro, Ectemnius and Nomada, X. citrofacciatum with Nomada and Ectemnius spp.); black with yellow markings; small to medium sized; 8–13 mm long. Wings 6.5–10.25 mm long.
The head about the same width as the thorax. The face ground-coloured at least part yellow; not flat or retreating between antennae and mouth. The frons smooth. The eyes bare. Antennae relatively short, drooping; with their bases well separated; yellowish (greenish yellow). The third antennal segment ovoid or orbicular. The antennal bristle dorsal; simple (pubescent).
The humeri bare. The thorax pubescent without stiff bristles interspersed; patterned (with yellow); with longitudinal stripes. The thoracic striping lateral only; exhibiting sharply defined, yellow lateral lines. The scutellum at least partly yellow. Wings patterned to plain (pale grey, tawny towards the base and brown along the fore border, the tinting less conspicuous in X. citrofasciatum); with a conspicuous dark stigma, or without a conspicuously dark stigma; without black flecks along the hind edges; 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 before 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 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 anterior anepisternum bare.
The abdomen much wider than the thorax; ovate (sometimes rather angled at the widest part in X. citrofasciatum). The lateral margins of the tergites exhibiting beaded rims. The abdomen with 5 segments apparent, or 6 segments apparent. The male abdomen with 5 visible segments. The abdomen contrastingly patterned. The tergite patterning involving 2 to 4. The colour-patterned tergites marked with yellow (the lateral yellow marks on 2 triagular, those on 3 and 4 straight). The tergite bands medianly interrupted; narrow. The spiracles of the third abdominal segment borne in the middle of each side.
Larvae and pupae. The larvae posteriorly blunt and tail-less; scarcely flattened (oval in section); patterned; dorsally smooth, without projections; mouth with a triangular sclerite on either side; anal segments without lappets. The larvae predatory (those of X. citrofasciatum being found in ants' nests, feeding on ant-attended root aphids).
Classification. Subfamily Syrphinae; tribe Syrphini.
British representation. 2 species in Britain.
Illustrations. • X. pedissequum, male, (with Syrphus, Scaeva and Helophilus). • X. citrofasciatum: 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’.