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These two flies were collected in the same North Wales wood on 11 October 2015, and they both key out to Clusiodes albimana. But they differ in colour, size and the bristles around the mouth. Alan Stubbs' 1982 key says “The face is normally dark in C. albimana but a scarce yellow-faced variety as been named pallidior; the genitalia seem to be very similar and the question of the taxonomic status of this form is left undecided.” Has there been any subsequent work on this species, please? Is the dark form always larger and more bristly? Might it be caused by the species having a variable larval life, those spending longer as a larva producing larger adults, as I think is the case with some saproxylic beetles?

John Bratton

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The pale one

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The pale one. Look at bristles around mouth.

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The pale one, abdomen.

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The dark one.

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The dark one. Look at bristles around mouth.

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The dark one, abdomen.

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Both look like C. albimana to me. The rounded shape of the genital capsule is distinctive for this species. All the other species have a longer and narrower shape. I find that the colours vary a lot in Clusiodes from dark to really quite light brown in the same species.

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PS I collect loads of these most years. I must start keeping them all and investigating the size and colour association you ask about. It could make an interesting mini-study.

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I would say the smaller, paler one has only two fronto-orbitals. That leaves it in another species group. Applying the key by Lonsdale & Marshall (2007) I get to C. gentilis.

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I caught two of the little pale clusiids. One has two orbitals and if I've interpreted orbitals correctly, the other has three - see below. The Stubbs key is not definite about the number of orbitals. He said “Normally three pairs of orbital bristles - albimana.” “Normally two pairs of orbitals” leads to gentilis.
So....

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I was relying more on the shape of the outer lobe of the genitalia, and I think the dark and pale specimens both match fig. 2b, albimana.

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Here is a Clusiodes gentilis for comparison, coll. 31 May 2010, Vaynol Park, North Wales.