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Agree it is Rhagio scolopaceus, the Downlooker Snipefly. The most similar species is Rhagio strigosus, but that is much rarer and confined to the Chilterns and Surrey as far as we know. (R. strigosus is slightly smaller, often has more yellow markings, and the antennae are a different shape - but you'd need a really good close-up photo from side-on to show that.)
Entomologist and biological recorder
Actually I was thinking of Panorpa scorpionfly which is wrong, it is a snipefly as Martin points out. Should have recognised this as we often have several of these flies sitting on us as we use the surveying equipment in meadows during the summer. The flies hitch a free ride and presumably keep a look out for their prey as I think they are predators on other insects.
Mike, could you get a specimen next time this happens? I had a tentative theory that the rare species (R. strigosus) seemed to be much more prone to sitting on people than the common one (R. scolopaceus) - no doubt your observations will disprove the theory but it would be interesting to find out!
will circulate your comment to survey team as main surveys have now finished for 2009 so might not get much back till we start the surveys next May-July.
Lat/Lng: 51.4, -2.5
OS grid ref: ST6570