CLEG / HORSEFLY : Haematopota pluvialis
Also known as the common cleg or notch-horned cleg.
The horsefly bite is usually red, surrounded by a raised area of skin known as a weal, and is tender and painful. The problem is that they look so much like so many other flies that we may panic unnessarily.
If you are passing through an area with livestock such as cows or horses, you may well come across such flies. In Scotland they are usually referred to as clegs. While they live in grassland, but feed as miniature vampires. If they cannot find an animal to feed on, they may turn to you. You may not notice the bite until afterwards when it starts to itch.
Mountaineering Scotland notes : Concentrated in the northern highlands, these relatively large insects have a vicious bite. You will only feel the bite as the fly disengages to fly off, by which time it is too late. They do not swarm in large numbers and thankfully are relatively uncommon. They are most active in the summer months from June to September.
While not as common here, they are nevertheless found here.
The majority of people react to the bite with a large red weal, which is exceptionally itchy. The itch can continue for many days afterwards, and up to two weeks. You should not scratch the site of the bite. Antihistamine creams applied to the bite area help take the pain away and stop the itching.
The weal may rise like a blister.
Wildlife Trusts tells us that there are 30 species of horse-fly in the UK. Of these the Notch-horned cleg-fly is a horse fly dark grey in colour, with grey-brown mottled wings and intricately striped, iridescent eyes.; this is one of the most frequently encountered species and also one of the smaller ones. Some of us have felt the painful bite of the Notch-horned cleg-fly (a 'horse-fly') while out walking in grasslands or woods, although it prefers to feed on the blood of cows and horses.
As with so many flies of different types, being certain that what is buzzing around you can be difficult. Compare this for instance to the siphona range of flies index.asp?pageid=732654 or even the hosue fly.
The Notch-horned cleg-fly is a common species of horse fly that lives in long grassland and damp woodland habitats. The females have sharp, biting mouthparts and usually feed on the blood of large mammals, such as cows and horses. The males lack these mouthparts, so feed on nectar. Females wait in shady areas for their prey to pass-by, locating it by sight with their large, compound eyes. The eggs are laid on stones and plants, or in mud, close to water. When the larvae hatch, they fall on to the damp earth where they predate other invertebrates.
Influential Points website (see link below) has perhaps the best information on identifying them. Haematopota pluvialis is a small to medium-sized tabanid with a body length of 8-10 mm. The wings are pale mottled brown and the abdomen has a rather faint series of lateral pale spots on a dark background. The first antennal segment is rather bulbous, is partly shining black and has a strong sub-apical notch. The presence of the sub-apical notch is the best way to distinguish Haematopota pluvialis from Haematopota crassicornis, which has no notch. This website has further information on feeding and breeding as well as detailed photographs.
It also points out that while all clegs feed on the blood of animals such as cattle and horses, they also feed on nectar.
MOUNTAINEERING SCOTLAND : https://www.mountaineering.scot/safety-and-skills/health-and-hygiene/insects