These little guys are found everywhere; all the time. If a fish isn't eating a stonefly, mayfly, terrestrial, or caddis, it is probably feasting on tiny midges. The technical name for these small, two-winged insects is chironomids.
Don't let their size fool you - lots of trophy trout are hooked on these miniatures. Because they're so small, it's not important to get too technical in matching a midge hatch, search your box for the correct approximate size, silhouette and color (in that order).