Small, black, olive, brown, rust, red, flies that you find everywhere there is water, especially cold, nutrient rich water (tailwaters, spring creeks). I believe a mosquito is a type of chironomid, or midge. Those little squirmy's you see in stagnant old tire water are mosquito larva!!
Midges come in 3 forms, or stages (mayflies only have 2):
1) Larva - there is little differentiation between head, thorax and abdomen - like a worm. A blood-worm is a good representation.
2) Pupa - Distinct head, thorax and abdomen - the zebra midge is a representation. We fish these way more than larva- larva hang out in the mud, or hidden elsewhere.
3) Adults- look like mosquitos, just w/out the blood sucking apparatus, and usually a bit smaller 'round here. We often fish emerger patterns like a brooks sprout, sipper midge for this 'un. A griffith's gnat can represent a clump of midges, maybe a single adult midge.
I carry lots of midge pupa in my fly boxes, mostly black and olive ones, because they're more common. Around here they're about 18-24, for the most part. There are exactly 97 billion kinds, colors, sizes of midges, but the aforementioned cover about 96 billion of 'em. Pump a fish tummy next time your out - there will be midges if it's a trout and it's not in a hatchery. I give you the Billy Mays guarantee that stomach pumps are effective in determining midge choice.
Hope this helps, Mr. Raider