Definition of "cleg"

Sir Christopher Pack did cleave like a clegg, and was very angry he could not be heard ad infinitum.

1657, Thomas Burton, Diary, section I

Now that was in summer, the time of fleas and glegs and golochs in the fields, when stirks would start up from a drowsy cud-chewing to a wild a feckless racing, the glegs biting through hair and hide to the skin below the tail-rump.

1932, Lewis Grassic Gibbon, Sunset Song (A Scots Quair), Polygon, published 2006, page 39

The clegs continue to swarm all around. I wonder how many there are. […] Remaining seated on the block, I seize clegs out of the surrounding air at random, and with scissors cut out a tiny triangle from the rear edge of each one's right wing before releasing it.

1998, V. K. Riabitsev, One Season in the Taiga‎, page 138

Cattle were grazing languidly on the lush grass and flicking their tails to keep away the clegs that constantly plagued them and, having recently suffered a nasty bite from one, I was wary of them myself.

2007, John T. Wright, An Evacuee's Story: A North Yorkshire Family in Wartime‎, page 361

Whilst the swarms which surround you are annoying, they do not bite. It is the midges, clegs and ticks you should be on the lookout for.

2011, Denis Brook, Phil Hinchliffe, North to the Cape: A Trek from Fort William to Cape Wrath, page 49