Flies are members of the order Diptera, an immense group of insects that includes deer flies, horse flies, house flies, crane flies, gnats and mosquitoes. Members of Diptera are different from other insects because they only have one pair of wings as an adult instead of two. Black flies are a member of a subgroup of flies called the higher flies, which also includes horseflies, house flies, fruit flies and cluster flies.
The large black flies that you see in your home when the weather is cool are called cluster flies. They are larger than ordinary house flies. They fly sluggishly and are nearly dormant when they are in your house. They do not feed on animal manure or garbage. They seek shelter when it starts to get cold and that's why you see them in your house. Cluster flies are black or dark grey. They have distinct yellow, golden and tawny hairs on the back of the head and at the top of the thorax, but these colours darken with age. The wings overlap and have a scissor-like appearance. Cluster flies gather in closets, attics and empty rooms, seeking shelter and warmth. They don't move about much when it's cold but will re-emerge when it gets. They don't bite.
The cluster fly is a type of blow fly, which can be shiny black or grey, metallic green or blue. The blow fly is large and has a bloated appearance. They are sometimes referred to as bottle flies because they be shiny blue and green rather resembling the colour of glass bottles as well as shiny black or bronze.
Flesh flies are black or grey and get their name from their behaviour of seeking scraps of meat or carrion on which they can lay their eggs. This fly has three dark stripes on its thorax and a checkerboard pattern on its belly. The flesh fly is bigger than the common house fly.
Large Black Flies Versus Humans
When you see a large black fly population this means that the nearby streams are healthy and clean because these flies do not tolerate pollution. But the large black flies are annoying because they swarm around humans. Female flies bite; most of the bites are aimed at animals and birds and not humans, but people can have a reaction and blood loss from the bite of a black fly or from inhaling the fly accidentally. When you are bitten by a black fly, you are going to itch, swell and bleed. The fly will target your head or areas where your clothing fits tightly. Black flies like to crawl up your sleeves and get under your socks and trousers.