Fly anglers often make it more difficult than it needs to be. I know this because I have been guilty of making knowing the insects more complex than it needed to be. This is my best attempt to cover the major "bugs" that fly anglers need to know - the mayflies (order Ephemeroptera), caddis (Trichoptera), stoneflies (Plecoptera), and midges (order Diptera; family Chironomidae). If you've not read part 1 of the angling entomology posts, that will help get you acquainted with these orders.
Before we get into each order, there is a bit of terminology about life histories shared by some of these orders. These insects all undergo metamorphosis but mayflies and stoneflies undergo an incomplete metamorphosis (hemimetabolism) and have egg, nymph, and imago or adult stages. Whereas caddis and midges undergo complete metamorphosis (holometabolism) which involves a pupal stage before the imago (adult) stage. Understanding these stages, how insects transition from one to the next, and for different hatches, which stages or transitions are most important, I think is an important step in any fly angler's learning process.
You do not need to know all the species but if you were to figure out the half dozen or so of the best hatches in your area, you would have a pretty good working angling entomology knowledge and would almost certainly improve your fishing success.
If you have gotten deep into angling entomology, mayflies are almost certainly where you got your start. Mayflies have an incomplete metamorphosis but it is a bit of a unique version of hemimetabolism. They begin as a fertilized egg which develops into a nymph or more technically a naiad which is the life stage that all (?) aquatic insects spend the vast majority of their lives. Nymphs go through a number of molts (ecdysis) as they grow and get larger. One important thing for fly anglers to know is that as they grow and get closer to emerging, their wing pads become more developed and darker. Tying some of your nymphs with black or peacock wing pads is a wise idea and one I am a fan of.
As nymphs (or naiads), mayflies are much more diverse than they are as sub-adults (sub-imagos or duns in fly fishing speak) and reproductive adults (imagos or "spinners"). Nymphs are generally broken into four categories based on where and how they live and feed as nymphs - swimmers, clingers, crawlers, and burrowers - whose names inform us of their behavior.
Swimmers - well, they swim, so they mostly - but not always - live in slower currents and are often associated with vegetation. The best known of these are Isonychia (slate drakes), Baetis (Blue-winged Olives or BWOs), and Callibaetis (speckled spinners) which are best known from lakes.
Clingers - again, the name sort of gives it away; these nymphs are dorso-ventrally flattened and the "cling" to rocks. They tend to be wide but from top to bottom, very thin. They can live in the fastest of currents. These are largely - but not entirely - in the family Heptageniidae. The species you maybe familiar with are the March browns, both eastern and western, light Cahills, and quill Gordons.
Crawlers - These are many of our early or mid- season hatches - the sulphurs (Ephemeralla dorothea and, in places, some other species), Hendrickson (Ephemerella subvaria), Tricos - those small "white-winged black curse" mayflies (Tricorythrodes spp.), and pale evening and pale morning duns (Ephemeralla spp.). Crawlers are mostly riffle-dwellers but maybe not in the very fastest of water that is home to clinger nymphs. Some crawlers are more at home in slower waters of runs and pools.
Burrowers - obviously these make burrows and they are some of our largest mayfly species. To make these burrows, these species need relatively soft, small, and stable substrates which often occur in downstream reaches of our trout streams or in larger trout rivers. These are often our "drakes" a not so scientific name for larger mayflies that include the Hexagenia spp., Eastern Green Drake (Ephemera guttulata), and brown drake (Ephemera simulans) along with the white mayflies (Ephoron spp.) of late summer.
Knowing about these mayfly nymph types tell you a lot about their habitats, where you should expect to find them, and a bit about how to select flies that will match those nymphs. Find a good hatch chart (see resources at the end) and that will help with the when part of the equation.
From: WildTrout.org; Mayfly Time! ("The mayfly" hatch in Europe is Ephemera danica).
In transitioning to adulthood, nymphs emerge to become what we fly anglers call "duns" but more technically, they are a sub-imago, basically a "pre-adult" stage. This happens in different places and different rates of speed for different species. For others that are fans of the Sparkle Dun fly pattern, the trailing shuck represents the nymphal shuck that remains temporarily attached to the emerging dun which makes this intermediate stage particularly susceptible to trout. Mayflies then undergo one more metamorphosis to become imagos or spinners in the fly anglers' vocabulary. At this point they are reproductively mature. Quite typically the spinner has wings that are clear and less distinctly veined. After reproducing, most spinners will fall to the water in that classic "spent wing spinner" style. At both the dun and spinner stages, mayflies do not feed. As imagos (spinners), their abdomens are full of eggs or sperm rather than digestive system "parts" and their job is to reproduce, nothing more.
To simplify this for the angler, nymphs vary a bit in shape but honestly, fishing a clinger or a more flattened crawler imitation probably makes no difference to the fish. After all, perdigons and other nymph imitations that look little like real nymphs work really well. One of the best reason to learn some of the major hatches in your area is that for different species, different stages are more or less susceptible and thus some are better to imitate than others. For example, blue-winged olives often hatch during overcast and rainy times of days and as swimmers, they are likely swim quickly to the surface - look for splashy rises of trout trying to "catch up" to these swimming nymphs. At the surface as they emerge from a nymph to a dun, they make "get stuck" between stages, particularly when air temperatures are low, and this makes them particularly susceptible to trout. For other species, like Tricos, Ephorons, and Hexagenia, spinners are typically the most important stage to imitate. And for the burrowing mayflies, they are generally only available to fish when they are emerging but at his time they can be incredibly important.
Like mayflies, stoneflies undergo an incomplete metamorphosis but their life history is even more simple than are mayflies as there is no sub-imago stage. Stoneflies go from egg to nymph to adult. Stoneflies tend to crawl from the rocks they live on and under in riffles to the banks or rocks and wood that break from surface of the water. Then they split their nymphal skins and become adults.
Stoneflies are of relatively little importance in Wisconsin. The three ways I have fished stoneflies in Midwest streams are fishing large Pteronarcys nymphs through riffles rapids in large northern streams, the yellow sally hatch in June, and the winter stoneflies. Yellow Sallies are a June hatch and run about a size 14 and can be dependable on some streams. The winter stoneflies which tend to be small (#18 to #22), dark, and hatch during the warmest part of a winter day can be fun. They are probably not hugely important to the angler but it is always nice to catch a trout on a dry fly in January and February. In general, stoneflies are much more common on freestone streams, particularly in the western US.
Arguably, caddis are the most important of the aquatic insect groups (orders) on most Wisconsin trout streams. I know I fish caddis dry flies more than any other patterns; terrestrials would be second. The nice thing about caddis is while there is a ton of diversity, imitating them is generally pretty simple and straight-forward.
Caddis and midges undergo a complete metamorphosis which means that like butterflies (order Lepidoptera), they have a pupal stage (egg, larva, pupa, adult). Probably the most interesting thing about caddis are that they have a diversity of different types of larva. While caddis are best known for the diversity of cases that different species make, there are also net-spinning and free-living caddis. In any case, all caddis have simple "worm-like" larva that are identified by their 2 anal hooks. A number of species use the silk that they produce to make a case from sand, pebbles, organic matter, or other materials - each case type is unique to the species. There are a ton of different "peeking" or cased caddis a patterns to imitate cased caddis which occasionally get dislodge - often by trout that will try to eat them off the rocks. Hydropsyche caddis are a common net spinning family of caddis that are abundant in many streams. As adults, these are often the size 14 (or so) tan caddis that typically hatch from mid-May to Mid-June. And the most common free-living caddis you are likely to come across in most places are the Rhycaphilia caddis or "green rock worms".
From the larval stage, they pupate which some times takes place mostly in their case but almost always finishes with pupa emerging through the water column and finishes with them breaking from their pupal stage and into their adult form at (or near) the water surface. This short-lived emergence does make them very vulnerable and fishing a pupa, wet fly, or soft hackle with a downstream rising presentation can be a ton of fun.
Dry fly fishing is often to imitate adults but sometimes the pupa or the pupa to adult transition is also imitated. Recently hatches adult caddis may float on the surface for a bit while their wings dry and their exoskeletons harden but compared to mayflies, this tends to be more condensed. Then as they return to the stream to lay their eggs, they are imitated by dry flies or in some cases as soft hackles or wet flies as some caddis dive underwater to lay their eggs. If there is a "secret" to fishing egg laying caddis patterns - whether dry or wet - it is that they move and you can imitate that with twitches. I fish a CDC and Elk dry fly most of the time and I don't much worry about color - natural CDC works just fine for any caddis I have encountered.
For more about caddis entomology; posts from Winona Fly Factory: Parts 1, 2, 3, and the last post.
Midges (order Diptera, family Chironomidae)
Last but not least are the midges which are very important hatches, particularly in the winter. Many anglers have a love-hate relationship with midges (maybe its more hate than love). The problem with midges is that they are small which makes them difficult to imitate effectively (and to thread onto a tippet). Probably the best known midge pattern, the Griffith's Gnat - imitates a mating cluster of midges which gets around the issue of them being really small; although Griffith Gnats tend to be tied in #18 and #20 most of the time.
Midges undergo a complete metamorphosis quite similar to caddis flies. They begin as eggs, develop into a 'worm-like' larva where they live in sediments or on and between rocks, they emerge as a pupa which is very short-lived, and then transition into an adult midge. All three of the (non-egg) stages are easily imitated with really simple flies. The larva are often imitated by something like a zebra or a simple thread midge and you should have some in black, tan, gray, and maybe red. The pupa have a more distinct thorax and often the gills - represented with something white - are quite prominent. Lastly, the adults can be imitated with very simple patterns - a simple body, hackle tips or synthetic material to imitate the wings, and a hackle collar is what most adult midge fly patterns use. Or imitate their mating clusters so you can get away with a larger fly.
I am going to sneak crane flies in here too as they are related - they are both dipterans (true flies). For more on crane flies, which are mostly but not entirely in the families Limoniidae and Tipulidae, I wrote a post in praise of crane flies.
The Wrap-up
Knowing a bit about the life cycles of the major aquatic insect orders helps make you a better angler. If you do not care to learn a bunch of entomology (how much do I need to know?), carry imitations in a few different sizes, shapes (up-wing and down-wing), and in a few different colors or shades. There is the old adage that you should match - in order - size, shape, and then color. Some anglers put behavior as the first in the list - a post for another day. In any case, you do not need to try to imitate every species because a few general patterns in a few sizes. Most of our Driftless hatches are #14 on the large size to #20 on the small side and a #16 or #18 is what I fish most often. No need to make it more complicated than it needs to be - caddis, mayflies, and craneflies are what you're most likely to encounter from about April until the end of the season.
My choices of simple dry fly patterns are:
Caddis: CDC and Elk, Henryville Special would be my #2 fly
Mayflies: Parachute patterns in a diversity of colors; parachute Adams
Stoneflies: Biot-wing stonefly or a small stimulator
Midges: simple midge and Griffith Gnat
Craneflies: Coulee Cranefly
Wisconsin Hatch Charts
Driftless Wisconsin Hatch Chart (one I did many years ago; Silver Doctor Fly Fishing)
Wisconsin Emergence Calendar and Emergence Database (Patrick Hager, Wisconsin Fly Fishing)