Eristalis: Distinguishing males of E rupium and E jugorum

Some time ago I was working on a key to European Eristalis (still working on it every now and then!). It seems a good way of learning new criteria for distinguishing the species, and the keys I know, excellent as they are, are not primed for identifying from photographs (the first couplet is often 'aristae plumose or bare'!) - maybe it will get posted here at some point!

Anyway, I was aware of the black posterior fringe of hairs on the front femora of E jugorum males (a handy way to distinguish from horticola males from the key of van Veen. And while working on the difficult 'yellow-hind-metatarsus group' I became aware that E rupium and E obcura males also have a posterior fringe of black hairs on the front femur. It suddenly dawned on me that a male rupium with a dark hind metatarsus could be very hard to distinguish from a male jugorum if there wasn't a good angle on the face.

Soon after, flo-dycob queried an observation I had identified as a male jugorum, and I knew I'd run into precisely this problem.

Well it's taken a while, but here are some features I think can be used to distinguish male rupium and jugorum. Individually some may be more or less reliable, but I think it's a good package. Comments and corrections most welcome!

jugorum rupium
Face produced forwards and down Face produced down only.
Hind metatarsus black, never yellow, swollen Hind metatarsus black to yellow, not swollen.
Femur 1 with dense posterior fringe of black hairs Femur 1 with sparse posterior fringe of black hairs
Femur 2 with posterior fringe of pale hairs Femur 2 with posterior fringe of black hairs at least apically
Stigma short Stigma elongate
T2 spots occupy almost whole lateral margin when fully formed, inner angle just subrectangular T2 spots end well before the hind margin laterally even when fully formed, inner angle distinctly acute.
T3 pale haired T3 posteriorly extensively black haired.
Frons pale haired (dark ground colour shows through) Frons with black hair.
Subcosta yellowish at base Subcosta brown at base
Scutellum distinctly brighter than scutum Scutellum and scutum not contrasting in colour

I would just add that to me the general appearance of these flies is that male jugorum are rather bright, bushy and tidy flies, whereas male rupium often (but not always) seem rather dull and scrawny.

Lähettänyt matthewvosper matthewvosper, 14. lokakuuta 2021 22:18

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