One of the key differentiators about fly fishing that sets it apart from other styles is the emphasis that is placed on the fish’s food sources. It’s not just about getting the general size and action of a wounded baitfish, as it often is in other types of fishing. It is about identifying the exact species, size, and lifestage of the predominant food source… which by the way, is regularly changing. It's no secret that this can be intimidating, and it would be foolish to think that you'll become an expert overnight.
Entomology, as it's called, is a study that you could devote years to learning. With that said, a foundational understanding of the major insect families is sufficient for most anglers, especially at the beginner level. The more experience you gain, and the more time you spend around the sport, the more knowledge you'll accrue. So don't stress if there are things you don't understand. We'll cover the basics in this post, and after absorbing it, you'll be far ahead of many anglers when it comes to learning the important insects of fly fishing. Let's get started.
A staple in both fish’s diets and angler’s fly boxes, mayflies are an essential insect to understand. They have a unique life cycle that gives them ample opportunity to become a meal for a hungry trout, and are prevalent across multiple seasons in many areas of the world.
There are many subspecies of Mayflies that vary in color, size, and some behavior, but generally you can split them into four subcategories (based on their underwater behavior as nymphs).
There are burrowers (which burrow into the bottom of rivers and lakes), clingers (which cling to twigs, rocks, and other underwater material), crawlers (which move around stream and lake bottoms), and swimmers (which, you guessed it, are good swimmers)
Regardless of their subcategory, all mayflies spend the majority of their lives underwater, making them the subject of many nymph imitations. When it is time to emerge into their adult forms, mayflies either crawl onto streamside rocks and vegetation or move to the water’s surface where they emerge in the surface film. These mayflies in particular are vulnerable to predation, and we imitate them with emerger patterns. Once they emerge, they become duns (or subimagos for you science nerds), which fly to shoreline objects and molt once more, becoming sexually mature adults, or spinners, or imagos. Once the adults mate in a swarm above the water, the spinner fall occurs, leaving hordes of dead insects on the water’s surface, a perfect meal for hungry gamefish.
Another incredibly important and prevalent insect for fly fisherman is the stonefly. Stones come in many varieties, but most fall into one of two categories: the light colored (almost yellow) and the dark colored (almost black) varieties. As far as insects go, stoneflies tend to run on the larger side, with some varieties, like the salmonfly, getting well-above an inch in size.
Found mostly in cold, fast water, stoneflies require highly oxygenated water and cool year-round temperatures, making them a staple in many northern, fast-flowing rivers. They have multi-year lifecycles, and spend the majority of their lives crawling and clinging to the bottom of the river. Being that they can’t swim, stoneflies are excellent clingers and can be found in shockingly fast water… but, if they do happen to get swept away, their flailing legs paddling in the current make them an easy target for big trout. Thus, stonefly nymphs are a great imitation, and because they are found in the stream all year long, they are reliable across different seasons.
Another benefit of this multi-year life cycle is the margin for error it buys us as anglers. Because there will be stoneflies of different sizes in a river at any given time (some are older than others), we don’t typically have to match the size of the hatch as closely as we might for mayflies. This is not to say that the trout won’t hone in on a stoneflies of a certain size, but your presentation would not necessarily be unrealistic if it differed slightly from the prevailing insect size. With that said, color is of even more importance, since yellow stones and black stones are easily distinguishable by their color, and if the fish are keyed in on one and not the other, you want to make sure you have the right tools for the job. For that reason, I keep a few sizes of stonefly nymphs in my fly box at all times- some imitating yellowish stones, others in darker colors. .
Easily recognizable by their impressive larval cases, caddisflies are the architects of the insect world. In their larval stage, they construct casings for themselves from silk and whatever available material they can find on the stream bottom. These casings can range from the mundane, almost inanimate forms in the photo above, to elaborate casings made from tiny pebbles, underwater vegetation, or whatever creative substance is readily available. Nonetheless, it is important to note that the casing is separate from the larva itself. In reality, the larva is a small, worm-like creature that is imitated by nymphs that could often pass as scud patterns- they both have a thin, almost hunched profile in the water. When the conditions are right, the larva, which have turned to pupa within their cases, will rise to the surface, either via a tiny gas bubble, or by swimming, depending on the species. Compared to other insects, caddisflies hatch fairly quickly, leading to what is known as the “caddis rise”. These splashy, often acrobatic rises will sometimes accompany a caddis hatch, as trout race to catch the insects before they leave the surface film. While such rises are hard to miss, it can be easy to overlook what the fish are actually rising on. There is usually a slight delay as the caddisfly emerges from its pupal skin, which takes place in the surface film. Thus, fishing an emerger pattern (which sits in the surface film) can be a terrific strategy, since many of your fellow anglers will be throwing high-riding dries to capitalize on the rising fish. Late in the hatch and in the hours that follow, crippled and spent caddisflies offer an easy meal to any trout that is not already stuffed with emergents. Dead-drifting a sunken dry will often provoke these fish, though the rises will be calmer in the residual aftermath. Understanding the three distinct stages of the hatch that matter to trout is of the utmost importance- you don’t need to know every subspecies by its latin name, having a conceptual understanding of what those insects are going through is essential to getting the most out of hatch fishing.
Maybe it’s because these little guys are so tiny, but it seems that many fly anglers overlook midges. For those who aren’t familiar, midges are a group of more than 1000 species of extremely small, non-biting flies. They often create swarms, and can be an incredible nuisance to those who don’t appreciate them as a staple food source for fish. And a staple food source they are. If you’ve heard people talk of the incredible fishing during a chironomid hatch (most of us have), they are referring to a type of midge. Spending much of their lives burrowed in the bottom of the lake or stream, midge larvae are small, worm-like creatures. When they do venture from their burrows, they make an easy meal for trout and other species lying in wait downstream. TIny nymphs with a very thin profile are best for this type of imitation. Often, the best midge larva imitators are size 20 and smaller, and are little more than a thinly wrapped hook. They can take on a range of colors, from black, to dark olive, to red. Dead-drifting these within a few inches of the bottom is by far the most effective way to fish them, since it is rare that you’ll find a larval stage midge in the upper reaches of the water column.
As a midge hatch progresses, pupae begin struggling to break the surface (known as the meniscus). For nearly all aquatic insects, the surface tension of the water’s surface creates an imposing barrier, and is the reason that insects often struggle in the surface film as they attempt to molt into adults. In this state, they are vulnerable to predation, making for fantastic emerger and dry fly fishing. With midges in particular, though, this part of the hatch can be tricky for anglers. Midges, besides being incredibly small and difficult to see, also emerge with an air-bubble and form a distinct curved shape as they shed their skins and puncture the surface film. As a result, anglers throwing dry fly imitations of their adult form are often befuddled by their lack of success, when in reality the fish are feeding on something that looks quite different in the surface film.
With all the focus that is put on the more quintessential fly fishing insects, terrestrials are often forgotten. Perhaps it's because they don’t have distinct hatch cycles, or rhyme with the words bayfly, bonefly, or baddis. Don't get it twisted, though. Terrestrials are no joke. The term terrestrial is somewhat of a catch-all, referring to bugs that live primarily on land, not in water. The big players in the category are crickets, grasshoppers, earthworms, ants, and spiders, but we could also loop in frogs, and even mice. The latter two aren’t bugs at all (duh), but in certain areas can be major food sources for large gamefish (don’t believe me? Google “mice in trout stomach” and prepare to be amazed). The terrestrial category is broad, and some people have different stances on what exactly constitutes a terrestrial. Some contend that it is strictly land insects, while others use it much more broadly, using it to describe other, more niche food sources, such as leeches, dragon and damselflies, scuds, and more. For the purposes of this post, let’s get past the semantics of categorization, and focus on the idea that alternative food sources actually makeup a fairly large portion of trout’s diets.
Grasshoppers and ants are two I’d like to spend some time on, because unlike some of the others I mentioned, these insects can be found in great density at certain periods, often enough to create hatch-like selectivity. For ants, the fall is when they come into the savvy angler’s lexicon, as flying ant migrations can bring great numbers of ants to particular areas, leading to what is known as an “ant fall” (think of a spinner fall, but with ants). In the summer, grasshoppers can be found in huge numbers too, particularly in areas with fields and high grass. A thin stream through a secluded meadow is what comes to mind. On particularly hopper-heavy days, fish will line up along the banks to catch them, abandoning normal lies. As an angler, it is important not to have tunnel vision towards the more traditional insect hatches, because often there are other insects in play.