Entomology is a thirty-cent word that fly anglers like to throw around. To folks who don’t use it often, ‘entomology’ creates a big impression for its thirty cents. But at the heart of it, it’s just the study of bugs. Fortunately for most fly fishers, the study does not have to be very intense. On the positive side of things, though, many fly fishers have learned to speak a bit of Latin, even if only to say hexagenia limbata. See, you’re impressed already.
The reason you should learn a bit of these bug basics is this: fish that fly anglers target eat bugs. It’s the reason that some very big, very smart fish will try to eat a hook with fur and feathers tied to it. Most, though not all, of the bugs that fish eat live parts of their lives underwater. Learning about these aquatic insects is one of the more wondrous aspects of fly fishing. The marvel of rivers and lakes and the bug life they produce is one of the reasons that we fly fishers often act like children. (Other reasons are not nearly as acceptable as this one.) Professional entomologists, university professors and the like, tell us that thousands of species of insects live at least some of their lives in the water we fish. The good news for fly anglers is that we don’t need to be knowledgeable professionals, except a in few categories that have similar body shapes and characteristics. Before we look at the groupings, let’s have a quick primer on basic insect parts.
Review of Basic Insect Parts
All the aquatic insects that are important to fly fishers have some common body parts. Most of the insects that interest fly anglers can be identified by comparing the shape, size, and color of the parts. Most aquatic insects have a number of stages through which they grow. These stages are called the life cycle. While all the aquatic insects important to fly anglers have life cycles, entomologists recognize two types of life cycles, which they call complete and incomplete metamorphosis. Complete metamorphosis has four stages and an incomplete metamorphosis has three stages. Because fish eat aquatic insects in all their stages except the egg, a fly fisher’s success can sometimes depend on knowledge of the stages and how to best imitate the insect’s life stage on which fish are feeding. Aquatic insect life cycles have some or all of the following stages: larva (nymph), pupa, and adult. These stages are in addition to the egg, which has no importance to the fish or the fly fisher.
Aquatic Insect Groupings
Professional entomologists group insects using an accepted scientific method call taxonomy, which is the science dealing with the identification, naming, and classification of organisms. Fly anglers have common, easily remembered names for the groups of insects that are important in the world of fly fishing. For most of the fly fishing world, the most important groups of insects include mayflies, caddis, and stoneflies.
Probably no other group of insects represents fly fishing as well as mayflies. While hundreds of individual species exist, they have enough physical characteristics in common to allow fly anglers to easily recognize them as a group. With only subtle differences in body shape and size, their primary difference is color.
Even though the mayflies are more prevalent, particularly in fly fishing for trout, caddis are even more widespread throughout the year. Most experts assess the caddis as having over 2,000 individual species. As with the mayflies, fly fishers have to know only general descriptions and color differences. Most caddis are very similar in shape, with size and color being the major variations.
Stoneflies are bigger than most other insects, sometimes reaching two inches in length. They have pincer-like antennae, and a thick, armor-looking exoskeleton. (Exoskeleton is entomologist-speak for the outer, brittle skin that many of the aquatic insects have. It’s kind of like the shell on a crawfish.) They are, however, harmless.
Many other insects, most of them aquatic, also play a role in fish diet and are therefore important to the angler. These insects are not as important in our area as the single groups above, so we’re clumping them together. They include midges, dragonflies, damselflies, and terrestrial insects.
Midges are an important group of aquatic insects. Many midges are present in northern Michigan, but trout and other fish generally ignore them because there are other insects on which to feed. Midges are very small insects, and many visitors to the river have noted the number of ‘gnats’ swarming above the water. Many times the gnats are really midges.
Dragonflies are large insects, but are not as dense in numbers as the mayfly and caddis. They can be important food items for fish at times. They are particularly important in still water, such as ponds and lakes. They are similar in appearance to damselflies, though damselflies are smaller. Fish will eat either larva or adult stages of the dragonfly’s incomplete life cycle.
Damselflies are well known predators of mosquitoes. Almost any fly angler who sees them welcomes the shiny-winged, often brilliantly colored damselflies. Damselflies are at times important food items to fish in both their larva and adult stages. They have incomplete life cycles and are most often important in ponds, lakes, or slow-moving streams.
Some of the most exciting fly fishing occurs when bugs begin falling into a stream or lake. When grasshoppers become plentiful, beetles are over-populating streamside banks, and caterpillars drop from branches, fish often focus on them more than aquatic insects. Other less populous terrestrials, such as jassids, ants, and moths are also good for fishing and for the fly fisher who pays attention. A few terrestrial flies should be in all fly anglers’ boxes. Carrying a half-dozen is a good idea. In fact, carrying an additional one or two is not a bad idea. That would be, you know, an extra terrestrial.
Anglers aren’t limited to imitating aquatic or terrestrial insects. Many organisms, some of them larger than insects, serve as food for trout and other fish. Fly anglers have learned to effectively imitate many of them. Some of the other food items include scuds, sowbugs, crawfish, minnows, leeches, and frogs.
Scuds and Sowbugs
Scuds and sowbugs are crustaceans, much like crawfish, only smaller. Scuds generally are about the size of some mayfly and caddis larva, most often about one-eighth to three-eighths of an inch in length. Both organisms are found in rivers and still water, but appear in tailwaters (rivers below dams) quite often.
Crawfish are familiar to most fly anglers. A number of species exist, and some of them become quite important as food sources. Smallmouth bass, in particular, are known to use crawfish as one of their principal food items. Crawfish, however, should not be dismissed as food items for other species. It’s one more reason for a fly angler to know the organisms present in the water being fished.
Minnows and Small Fish
Minnows and small fish often look the same to the fly angler, even though minnows are a type of organism in their own right. The good news for the fly fisher is there is no need to distinguish between the two. A smaller trout and a minnow can be imitated with the same flies. Certainly that is something that all fly fishers should be prepared to do since many fish prey on other small fish and minnows. While streamers imitate many types of organisms, they are most often tied and fished to imitate the small fish. That fact alone should be enough to convince an alert fly angler to be prepared to imitate the minnows and small fish that are found wherever we find large fish.
Leeches and Frogs
This is a category of food items that is often ignored by fly anglers. It should not be ignored. In still waters, and some major rivers, both leeches and frogs are food items that the fish snack on. Bass in particular often target frogs, but leeches are food items wherever they appear.