TROUT DON'T CARE IF WE CALL their latest meal a Caudatella hystrix or a Doroneuria baumanni, or a mayfly or stonefly, for that matter. So why should we care about learning basic bug identification? Well, if you have determined the size, shape, color, and behavior of an insect that fish are eating, taxonomic accuracy certainly isn't a necessity. Select a fly and a presentation to match, and you will probably catch fish.

Swimmer nymphs are quite slender, and most have three tails, which propel the nymphs through the wafer very well. Rick Hafele photo

Duns and spinners have two tails, and their hind wings are always less than half the size of the front wings. Dave Hughes photo

But suppose you don't hook up. If you knew that the fish were eating a particular type of mayfly, you could look in the appropriate books and find more information that might explain the problem. For example, maybe the duns emerge underwater and should have been imitated with a wet fly. Or say you want to describe what you saw to another angler. You could give them a complicated explanation of the insect's traits, or you could tell them that you fished a size 20 blue-winged olive hatch, making matters a lot simpler.

The more information one tries to communicate about the insects of interest to trout fishermen, the more important identification becomes. Unfortunately, insects are the most diverse group of animals on the planet, and thus pose one of biology's biggest taxonomic challenges. Therefore, the goal here is to present a simplified approach to identifying important aquatic insects, starting with mayflies. The next installment will tackle caddisflies.

You Need a System

The science of naming species and understanding their arrangement and relationships to each other is called 'systematics.' Systematics orders all living things into a hierarchy of seven major tiers we all learned in high-school biology: Kingdom, Phylum, Class, Order, Family, Genus, and Species. The naming and classification of species require extensive study and analysis. And since the diversity of nature tends to defy classification, the process is never-ending.

While learning to identify aquatic insects down to the genus or species can be interesting, it is rarely necessary. The most practical degree of identification includes only three basic criteria:

1. The level of identification should be possible while fishing, requiring only your eyes or a small hand lens. This frequently rules out genus or species distinction, which often calls for a microscope.

2. Practical identification should provide an understanding of how the insect behaves in terms of its availability to fish. For example, do the nymphs swim or dead-drift? Do adults lay eggs on the water's surface or swim underwater? These behaviors provide important information about how you should present a fly pattern that matches the size, shape, and color of the natural.

3. The system should be consistent across different streams and regions. Consistency is a real problem with common names. Depending on where you live, the name 'blue-winged olive' can refer to a variety of mayflies that have different sizes, colors, and behaviors.

Mayfly Types

The order Ephemeroptera, or mayflies, comprises 21 families, about 82 genera, and more than 600 species in North America. Of those, there are about 12 families, 30 genera, and a couple of hundred species potentially important to anglers. Learning to identify the common families and dominant genera and species of mayflies in your local waters is doable. However, the simplest level of identification for mayflies that meets the above criteria requires breaking them down into just four groups: swimmers, crawlers, clingers, and burrowers. Learning to classify mayflies into one of these four groups will help you select an appropriate fly pattern and presentation. This column assumes you have mastered basic fly-fishing entomology and that you already know a mayfly from a stonefly. When you are solid on the basics, it's time to determine whether you are looking at a swimmer, crawler, clinger, or burrower.

Swimmers

If form follows function, then the bodies of swimmer nymphs have the perfect form for swimming. They are streamlined with a torpedo shape that tapers from front to back, and the nymphs dart through the water like little minnows. Most swimmers have three tails as nymphs, though a few species have only two. The edges of the tails are fringed with fine hairs that create a flat, paddle-like surface. Quick up-and-down flicks of these tails propel the nymphs in rapid two- to three-inch spurts. Oval-shaped gills are located along the sides of the first seven segments of the abdomen. Those species that live in lakes and slow streams have larger gills than those that inhabit swift water in streams. The color of swimmer nymphs can vary widely depending on the species, and sizes may range from size 12 through size 22.

Duns and spinners of all swimmer species have only two tails. This trait is shared with clinger duns and spinners, but it can be used to quickly separate swimmers and clingers from crawler duns or spinners, which all have three tails. The head shape of swimmer duns and spinners is round. This trait can be used to separate them from the flat-headed clinger duns and spinners. A few species have no hind wings, but most do, and in those the hind wings are clearly less than one-half the size of the front wings. Like the nymphs, the different species of swimmer duns and spinners vary considerably in size and color.

Swimmers are abundant in many types of aquatic habitats, ranging from swift mountain streams to large, slow rivers and lakes. The most important family is Baetidae, which includes the extremely important blue-winged olives in rivers and the speckle-winged quills in lakes. Of the four main families of swimmers Baetidae, Ameletidae, Siphlonuridae, and Isonychiidae — only species of Baetidae emerge into duns in the surface film. The nymphs of the other three families crawl out of the water near shore to emerge into duns. As a result, the family Baetidae produces more surface feeding activity by trout during hatches than the other three families. Spinner falls can be important to imitate for all members of this group.

A Guide to Quick Mayfly Identification
Nymphs	Swimmers	Crawlers	Clingers	Burrowers
Body Shape	Slender, streamlined body shape.	More rectangular than swimmers and with stout front legs.	Flat and very depressed top to bottom.	Very large when mature (3/4" to 1 1/2" long).
Tails	Three, often fringed with fine hairs. A few species in the family Baetidae have two tails.	Three. Some are fringed with fine hairs; most are not.	Two or three; most with three. Tails are not fringed with fine hairs.	Three tails fringed with fine hairs.
Gills	Small to large oval, plate-like gills.	Oval or forked gills, often lying flat on top of abdomen.	Large, plate-like gills extend out to the side.	Large, forked gills are fringed with fine hairs.
Head	Relatively round head is narrower than thorax.	Round to slightly depressed head that is always narrower than thorax.	Head is flat and wider than thorax.	Relatively round head, usually with well-developed, tusk-like mandibles.
Movement	Fast swimmers.	Poor to moderately fast swimmers, but do not dart quickly.	Clings to rocks in moderate to fast currents. Poor swimmers.	Burrows into substrate. Able to swim well when necessary.

Duns/ Spinners	Swimmers	Crawlers	Clingers	Burrowers
Tails	Two tails.	Three tails. (This is the easiest characteristic with which to recognize adults.	Two tails.	Most species have two tails, but two species have three.
Body Size	Bodies very small (less than 1/16") to moderate (5/8").	Bodies less than 1/16" up to moderately large 5/8".	Bodies generally 1/2" to 5/8" long.	Large bodies (3/4" to 1 1/2", excluding tails.)
Wings	Hind wings are absent or well-developed but always less than half the size of front wings.	Hind wings are absent or well-developed but always less than half the size of front wings.	Hind wings are always present but always less than half the size of front wings.	Hind wings are half as large as front wings.
Head Shape	Head appears round from side.	Head is relatively round.	Head appears flat.	Head is relatively round.
Important Families Genera and Common Names	Baetidae Baetis, Diphetor, Acentrella (blue-winged olives); Callibaetis (speckle-winged quill) Ameletidae Ameletus (brown duns) Siphlonuridae Siphlonurus (gray drake or black drake) Isonychiidae Isonychia (leadwing coachman)	Ephemerellidae Ephemerella (PMDs, Hendricksons, sulphers); Drunella (green drakes, flavs, and BWOs) Leptophlebiidae Paraleptophlebia (mahogany duns, blue quills) Leptohyphidae Tricorythodes (Tricos)	Heptageniidae Rhithrogena (western march browns); Maccaffertium (eastern March browns); Heptagenia (pale evening duns); Epeorus (quill Gordon)	Ephemeridae Hexegenia (big yellow may, Hex); Ephemera (brown Drake, eastern green Drake) Potamanthidae Anthopotamus (golden drake)

Continue Reading "Basic Bug ID, Part I: Mayflies"