Whilst bathing my horses today, I noticed a very large black and yellow flying insect repeatedly landing on them before before being brushed off by the horse(s). At one point I stood back and watched it land on the rump of the horse, to which it couldn't be disturbed by the animal. It proceeded to dig into the skin, very much like a horse fly would, and appeared to suck the blood. I've noticed them hanging around my horses in the years I've been on the west coast of Wales (UK) and again previously when I lived in the Midlands. I've never heard of any other horse owners mention this kind of insect before and I've only ever seen it around my horses - always in twos or alone.
The best way to describe it is a very large, fat hornet that appears to have been on a body building course whilst injecting itself with steroids! It makes a very low BZZZZZZZZZZZZZZZ noise and has large, more defined black and yellow stripes whilst also being quite "hairy".
Any suggestions?

Thanks.

Amber,

We get quite a few questions like this over the summer months and they almost invariably turn out to be female broad-bodied chaser dragonflies Libellua depressa http://www.flickr.com/photos/suttonnature/9086964043/. Certainly, there is no other insect in the UK that is a likely candidate from what you describe.

Dragonflies take smaller (usually) aerial invertebrates and will not suck the blood of your horse, they do not have the mouthparts for it.

Unless it was Tabanus sudeticus the largest species of Horse-fly that you get in the UK (and indeed Europe I believe). They have pale yellow bands on the abdomen and their thorax can look quite fuzzy. Much bigger than many other Horse-fly species.

Worth a punt, I suppose! I've just uploaded a photo of T. sudeticus, so see if this is more or less like your critter than the chaser: http://www.flickr.com/photos/suttonnature/9342249701/