In general, the word "midge" is used to describe any of a number of tiny, fragile, dipteran (two-winged) flies related to the mosquito. In this article I will concern myself only with the biting midges (Ceratopogonidae family), as I have some little personal experience of them. (The non-biting midges belong to the Chironomidae family.)
Midges lay their eggs by the hundred in moist vegetation or small pools, as the larvae are aquatic, and form an important source of food for larger aquatic insects and fish.
Biting and feeding
There are some 800 species of biting midge, who are equipped with specialised mouthparts enabling them to pierce skin and suck blood. Their victims are varied, ranging from dragonflies through birds and small mammals to humans. Due to their diminutive size, they are ill-equipped to feast on larger animals, and in general, humans (with their relatively thin skins) are at the top end of their victim-size range. Some midges are highly specialised, others may attack a number of species.
Biting midges are extremely small, with wingspans of only about 2mm. They form dense clouds comprising hundreds of individuals. Hunting by smell, a cloud will swarm around the victim, and settle to feed. Each bite is tiny, and due to the small size, initially goes undetected.
The feeding process is similar to their mosquito relations. Biting midges are equipped with two long, finely-toothed mandibles and a maxillae. They land on their chosen victim and locate a suitable biting site, then cut their way into the skin in a series of sawing movements. a small pool of blood then forms, and the midge feeds, sometimes for several minutes. Whilst the initial bite may be undetected, once the little blighter has finished and gone, the healing process causes itching and swelling.
Incidentally, only the females bite as they need protein to produce their eggs. The males feed on vegetation, and have slightly different mouthparts.
Midges are a worldwide scourge, but some areas seem more infested than others. The 'midgies' of my experience were in the Western Highlands of Scotland, but the Americas and Australia certainly have their share of these biting beasties. These have their own tiny draculae, are known by many names - sand flies being one example.
Reactions to bites
Easy - everyone bitten by midges curses them loud and long.
Recent research has also confirmed something which victims have known for some time, namely that some people are attacked more frequently than others. As they hunt by smell, they are attracted to people with different sweat chemistries. The University of Edinburgh has carried out research at its Centre for Tropical Veterinary Medicine, and discovered that some people attract more insects than others. Using the species Culicoides impunctatas Goetghebuer, they found wide variances in the attraction to certain types of chemicals. This will hopefully lead to more effective repellents.
Personal experience also suggests that people react differently to the bites themselves. On our recent trip to Scotland, Dale was affected more than I was (though admittedly, she had more bites), each bite swelling to a raised red lump, with accompanying irritation. She also attracted more attention from them than I did - I reckon it's bad blood, she claims sweet blood ;-).
Various insect repellents are used to reduce attacks. Deet seemed to work for me, and a friend of mine swears by smearing tobacco juice on exposed areas. Brewer's yeast was also recommended by some locals - apparently if you eat it, the scent puts the little buggers off. Sadly, nothing except a full-body anti-midge mesh suit will be 100% effective!