Small black flies attracted to greater butterfly orchids
No interactions present.
it's certainly not the St Marks/hawthorn fly that I am familiar with the long dangly legs.These flies are probably 4-5mm max length.
New Flora of the Isle of Man
The plant is a butterfly orchid, flowers are around 1.5cm top to bottom or 1cm side to side which would make the flies only 2-3mm at most.
It's greater butterfly orchid. I was interested to know if the insects were attarcted to a species-specific pheremone, or were an important pollinator of greater butterfly orchids. I belive that moths can play a significant role in the pollination of butterfly orchids.
The fly I normally think of as the St Mark's Fly is Bibio marci, and that is one of the largest in the family Biobionidae. But there are 20 species in the family, most of which are much smaller than B. marci, and also including the species in genus Dilophus, which seems the most likely for what is shown in the photo, as Matt says.
Their larvae live in the topsoil, feeding on decaying vegetable matter. Apparently densities of up to 37,000 larvae per square metre of soil have been recorded.
Bibio marci is known as the St Mark's Fly because it is supposed to emerge and fly at around the time of St Mark's day, 25 April. This does still seem to work, despite global warming, at least in southern England - I've only ever twice recorded them before 25 April (and then only a couple of days before), but the flight period carries on through most of May as well. Other species in the family have different flight periods, so the name St Mark's fly is not really applicable to all of them.
Entomologist and biological recorder
Supposedly there also know as march flies and lovebugs
Just a thought ... I agree that they look like bibionids and Diplophus are the smaller genus but these look even smaller than most Dilophus spp. to me - but it's hard to know without knowing how big the orchid flowers were :D I have seen large groups scatopsid flies (fairly closely related to bibionids) and they look a bit like this too, so it might be a possibility but I wouldn't like to stick my neck out and ID them as such ;)
Chris, now you've suggested Scatopsidae I think you could be right, I hadn't considered them before. I don't know scatopsids well enough to be certain that's what these are, but I'm no longer confident that they are bibionids! Thanks for pointing out this alternative.
Lat/Lng: 54.3, -4.5
OS grid ref: SC4096