Black flies are not usually regarded as important trout fare, but scientific studies of some rivers have shown them to make up the majority of the trout's diet. Such places are few and far between, but anglers should be aware of the possibility and keep a lookout for high concentrations of the larvae.
The adults are nasty, annoying, biting flies.
Larva & pupa biology
Environmental tolerance: Requires clean, cool, pure water
Black fly larvae live together in colonies, sticking to rocks in riffles and filtering plankton from the waters. Several of them are shown in my underwater photos. These colonies may be found in very very fast water.
This is my favorite underwater picture so far. It shows a bunch of Simuliidae (black fly) larvae clinging to a rock and swinging in the fast current. There are also at least four visible mayfly nymphs, probably in the family Baetidae.
The large caddisfly case (really less than 1/2 inch) is a Brachycentridae larva. The other cases are actually the protective sheaths of black fly (Simuliidae) pupae. The two antler-like pieces sticking out of each one are not legs, but antennal sheaths.
Although many anglers have been driven from the stream by these nasty little flies, the fish love them. I remember a crazy day on a tiny Pike County brook trout creek. The black flies were legion, but so were the brookies. As long as I could stand it (I was prepared with a headnet, but it was only a partial defense) the little trout hammered a Griffith's Gnat on almost every cast.
Even if you can't tolerate the adult flies, an imitation of the larvae is very good. In fact, Don Holbrook (the author of Midge Magic) recently told me that his imitation of these larvae was the single most reliable and productive pattern of all of his "midge" imitations. He complained (only partly in jest) that the widespread spraying for black flies could ruin his fishing!
I've been looking around the web to find out what they are and I saw a site that said they were drain flies? It makes sense to call them that but what are their true name? It can't be drain flies can it? Anyway they are taking over my tank. They start out like 1cm long white inch worms that have a black tip on the face and tail, they then form a brownish-black shell, like a cocoon and then they turn into tiny flies the size of a gnat or fruit fly and they don't die under water they are water proof. I hate to say it if you guys are bug lovers but they have to move out, possibly to the swampy stream out back. Does anyone know their name and if they are harmful to goldfish at an epidemic rate? They breed like crazy and I can't keep up.
Just wanted to acknowledge your assistance and say thanks for your help in identifying a bug I happened to seine out of the Upper Delaware this week - on a drizzly afternoon, there were tiny BWO's coming off in small numbers; but also present (in greater numbers) was a size 24/26 down-winged light-olive insect, with black mottling, that I had not seen before. They rode the current for quite a while, and the trout were quietly sipping them. After describing the fly (as best I could), you were able to provide photos, along with the suggestion that these were "blackflies" (Simulidae). I had no idea that these flies were present in the Delaware system, but will certainly carry a few imitations from now on (just in case).