There are so many midge species that it would be irrational to try to catalog them. However, a few general statements can be made.
There are river-dwelling and lake-dwelling species, but their emergence is similar: the larva (which is worm-like) changes into a pupa, and the mature pupa rises to the water's surface. For a midge that's the rub: since midges are usually very small, the surface tension is a barrier to the pupae and they hang suspended and vulnerable right at the surface. That is where trout often take them--just below the surface.
Because midge pupae often hang just below the surface for long periods, the current will eventually concentrate them into backeddies and current seams. You can also find them in slow, flat runs, and sometimes even in fast water.
Midge pupae face the same hurdles in lakes, but the action can be almost anywhere. Lake midges can be much bigger than your typical river midge. Some are best matched with size 10 or 12 hooks. Despite that large size, however, the natural insect is still very slender, a fact that fly tiers need to be aware of.
Adult midges form mating swarms, and midges locked in amorous embrace often fall back to the water. Trout are not romantics and will eat these mating clusters, so a dry fly such as the Griffiths Gnat, Renegade, or even a very small Elk Hair Caddis will sometimes take trout.
Midge hatches can be tricky to match. Because so many insects hatch at one time, trout become very selective, and you need to carefully match the size, color, and appearance of the natural insect. Because there are so many species, this can be difficult. When fishing in a river, seine the current and examine the seine for pupae, emerging adults, and shucks of the natural insect; match the size and color. In lakes, you can often find shucks and hatching midges on the surface. Again, examine carefully and match the size and color.
Articles About Midges
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Midges Redux Midge adults can be maddingly difficult to match, especially in winter. Two simple patterns can help you. Jeff Morgan
Winter Midges Most winter anglers aren't using what trout eat the most of. Jeff Morgan
Shuttlecock Midges Early spring offers some outstanding opportunities for dry fly midging--if you've got the right patterns. Check this out! Jeff Morgan
Tying Midge Pupa Patterns--Part 2 Jeff's Fab Five pupa patterns. These flies incorporate the four principles outlined in part 1 of this article. Jeff Morgan
Tying Midge Pupa Patterns--Part 1 Spring-time is midge-time. The right patterns can give you the most productive fishing of the year. But most commercial patterns don't measure up. How come? And what can you do about it? Jeff Morgan
Midges in Lakes and Rivers Tiny but numerous, midges are a staple for trout in lakes. They can also be important in rivers. Learn the three factors that govern midge fishing. Rick Hafele and Dave Hughes