I have apparently caused something of a controversy and confused people in equal numbers by my references to being bitten by clegs in past dig diaries. For those of you that don't come from areas with a strong Viking/Norse background, a cleg is a horse fly, and the little beggars can and do bite you even through quite thick clothing.
The spelling varies, growing up in Cumbria as I did we always called them clegs and I had in my head that spelling, but honestly can't remember ever seeing it written down. You don't need to know how to spell it to learn to swat them pretty quick, difficult to get a good pic when you're busy swatting, although I will try & get a post-swat picture next time I'm bitten, but here's a Wikipedia image meantime:
Regarding the official Orkney spelling I've just consulted my copy of Hugh Marwick's The Orkney Norn, which is the standard reference, and he spells it klegg, from the Old Norse kleggi, in Danish klaeg and in Norwegian klegg. (I don't think that Old Norse has a 'c' anyway, so the klegg spelling makes perfect sense) Jamieson's Scottish Dictionary (published in 1879) has the spelling cleg, with one 'g' as does Chambers's Scots Dictionary published in 1911. I don't have a Cumbrian dictionary but a quick Google found one online and they use the spelling clegg with two 'g's.
However I've just consulted my good friend Patricia Long (an Orcadian born and bred for many generations, unlike myself who's just a ferrylooper) and she has always spelt it 'cleg'. Incidentally if you want a good read about all things Orcadian I can heartily recommend Patricia's blog, and especially the article about the discovery of Skara Brae, which her research has shown is credited to the wrong William Watt - not the then Laird, but in fact an illegitimate son of the same name.
Anyway the main thing you need to know about kleggs/clegs/cleggs is to swat them immediately whether on yourself or anyone else (although it is considered polite to warn the person you're swatting before whacking them to avoid any misunderstandings).