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Hello,

this 8mm fly - an Eustalomyia species I suppose - is from beginning of June from near the river Mosel. Can perhaps be said, which species it is? Eustalomyia histrio? Eustalomyia festiva?

Thanks,
Michael

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Not vittipes which has yellow tibiae, or hilaris which has median vitta on scutum widened posteriorly. That leaves festiva or histrio; need to see either length of arista or hind tibia av setae number to decide.

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Hello Michael,

thank you for your answer. Unfortunately neither the arista nor the seta in the hind tibia are visible on the other photos I have.

Michael

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Hi,

can I please ask what length of arista and how many tibia av setae indicate the difference between festiva or histrio, so I can check a specimen from today?

thanks,

Jeremy

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Here is my key. Sorry I can't space it out correctly

EUSTALOMYIA
Males and females
1 Legs and palpus partly yellow vittipes (Zetterstedt)
– Legs and palpus completely black 2
2 Arista long haired, the length of the hairs on upper and lower side are each equal to width of postpedicel. Median vitta of scutum widened behind into a rounded spot
hilaris (Fallén)
– Arista short haired or pubescent. Median vitta of scutum equally wide throughout 3
3 The length of hairing on arista in total equal to width of postpedicel. Hind tibia with 7 av-setae festiva (Zetterstedt)
– Hairing of arista significantly shorter, the arista is only pubescent. Hind tibia with 12 av-setae histrio (Zetterstedt)

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many thanks Michael. It looks like I have both hilaris as well as festiva/histrio, hilaris looks easier as the vitta widens to a rounded spot. Do you happen to know, are they all Ectemnius parasites? They were either investigating last year's holes or loitering nearby

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I think I have found them on rotten tree trunks with Crabro holes. Here are a few references
Eustalomyia festiva Zett, Hennig, 1966-76, reared from pupa of Agrotis (Lepidoptera),
Eustalomyia sp, Hennig, 1966-76, in nests of wasps (in dead wood) of subfamily Crabroninae; feeding on food store of paralysed insects (kleptoparasites).,
Eustalomyia sp, Smith, 1971,
Eustalomyia sp, O'Toole, 1978, host records,

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I've applied the key to my two species and here are the results:

festiva + hind tibia with 7 bristles. The two small ones close together at the top are hairs. The 2nd and 4th bristles from the top are very short but under the microscope are clearly bristles not hairs. There's a useful image of histrio in the gallery showing the extra bristles. E. festiva large, 10mm. Male?

hilaris + feathery arista. Fly 8mm. Female?

I hope these check out ok and add to the observation overall. Are males bigger than females or is festiva simply bigger than hilaris? E. hilaris seen investigating holes used last year by large Crabronid wasps including Ectemnius.

(now any chance of a key for Hydrophoria?) ;) (I expect that's actually quite lengthy)

many thanks,

Jeremy

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7 av bristles

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hilaris

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double plumose

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Size in Eustalomyia (and other commensals) is of little value as the larvae are dependent on how much food they can get, and if in short supply they will pupate without waiting and then a small adult will emerge.

Subhylemyia longula is a parasite on snails, and the adults vary in size enormously. Small ones have a wing length of only 3 mm, while eggs laid on a large snail can produce adults with wing length of 6mm.