What's That Bug? does not endorse extermination

A scary looking fly, what is it??
August 20, 2009
I was camping in the uk last week and noticed a few of these strange looking flies, I thought nothing more of it until I have noticed very nasty bites on my ankles and my head, I was wondering if you could identify it for me, and tell me if i need to worry!
Karl Chapman
Derbyshire, UK

St. Mark's Fly

Heather Fly

Hi Karl,
We hesitate to tell you not to worry, but you need not worry about this insect.  We believe this is a March Fly in the family Bibionidae.  We are going to leave actual species identification to a Dipterist, but we found several UK species with similar looking photos.  There is a Fever Fly, Dilophus femoratus, that is shown in close-up on the Bio-Images Virtual Field Guide (UK) page that looks close.  A closer match would be Biblio johannes, called a St. Mark’s Fly, on the same Bio-Images website.  Though the color doesn’t match, the spine at the joint of the foreleg matches another St. Mark’s Fly, Biblio marci, also pictured on the Bio-Images website. The Nature Observer’s Scrapbook page has this to say about St. Mark’s Flies:  “St Mark’s fly owes it’s common name to its annual habit of appearing around St Mark’s day, 25th April.
It seems odd to me that an insect as substantial as this should be deemed to be a ‘midge’. This is the largest of the 18 strong Bibionidae family of black day flying midges. The females are about 13mm in length and the males about 10mm.
It is slow and cumbersome in flight with its legs dangling clumsily – and that is while it is on it’s own. When they are mating, it is not unusual to find them in even more unwieldly flight, still coupled together with the larger female dragging the hapless male to the next resting place.
The differences between male and female can clearly be seen in the upper image. The female is significantly larger but has a much smaller head with smaller eyes set on either side of the head. The male on the other hand has large eyes touching each other.
The single, strong, forward pointing spine on the outside of the tibia of the front legs (highlighted in the lower image) is an identifying feature of the Bibio family, helping to distinguish it from the similar Dilophus family – to which the fever-fly (see below) belongs.
The conformation of the wings is such that when folded, one wing completely overlays the other.
It breeds underground and the larvae feed largely on decaying vegetation but are also blamed for damage to crop roots.
One ‘oddity’ of Bibio species is that the larval structure appears to be more primitive than the adult fly conformation would lead one to expect, indicating some evolutionary aberration in their development.”  Based on the head, this fly is a female since males have much larger eyes.

St. Mark's Fly

Heather Fly

Clarification from Karl
Hi Karl:
There are apparently 13 species of ‘March Flies’ in the genus Bibio listed for Great Britain. From what I can tell the closest match to yours is B. pomonae; the Heather Fly. It was the only one I could find with red colouring on the legs that doesn’t extend beyond the femur. The peak of the flying season in GB is July-August so the timing would be right. The species is very widely distributed in Europe, Iceland to Russia and down to the Mediterranean. According to some references it prefers higher elevations, hence the common name; others give hedgerows as the preferred habitat. The larvae feed on roots and the adults eat nectar, so I suppose your bites remain a mystery. For reference you could check out http://www.diptera.info/photogallery.php?photo_id=865 or http://www.commanster.eu/commanster/Insects/Flies/SuFlies/Bibio.pomonae.html. Nice photos bye the way. Regards. K

What's That Bug? does not endorse extermination

6 Responses to St. Mark's Fly from U.K.: Heather Fly

  1. kkroeker says:

    Hi Karl:

    There are apparently 13 species of ‘March Flies’ in the genus Bibio listed for Great Britain. From what I can tell the closest match to yours is B. pomonae; the Heather Fly. It was the only one I could find with red colouring on the legs that doesn’t extend beyond the femur. The peak of the flying season in GB is July-August so the timing would be right. The species is very widely distributed in Europe, Iceland to Russia and down to the Mediterranean. According to some references it prefers higher elevations, hence the common name; others give hedgerows as the preferred habitat. The larvae feed on roots and the adults eat nectar, so I suppose your bites remain a mystery. For reference you could check out http://www.diptera.info/photogallery.php?photo_id=865 or http://www.commanster.eu/commanster/Insects/Flies/SuFlies/Bibio.pomonae.html. Nice photos bye the way. Regards. K

  2. Dill Fox says:

    Biblio marci
    Is the St. Marks Fly
    I hope you dont mind the correction

    • bugman says:

      We welcome corrections at What’s That Bug? but we are hesitant to accept corrections that are not accompanied by either professional qualifications (are you a dipterist?) or by reputable links that clearly demonstrate the identification. We acknowledge that we are amateurs with no professional qualifications, and we attempt to the best of our ability to provide accurate information. Your “correction” substantiates both our original research and Karl’s comments that the March Fly in the images belongs in the genus Biblio and we understand that exact species identifications often require inspection of specimens by experts.

  3. Sylvia says:

    I have lots of these in my garden I find they fly around for a few weeks then just disappear after having laid their eggs on my Solomon seal plant which gets eaten after it has flowered and then it dies and all the little caterpillars just disappear until the end of March hence the name st marks

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