Biologists categorize all living things using a hierarchy that reflects how closely they're related to each other--a sort of "family tree" for every organism on Earth. This taxonomic hierarchy is the easiest way to keep track of the thousands of insects in our streams, which is why I've used it to strucure the Aquatic Insects section of this website. It may take a little getting used to, but it quickly gets easy.

Basic Definitions


Every point in the hierarchy is called a taxon. These taxa are arranged on different taxonomic levels. The best-known levels are:

  • Kingdom

  • Phylum (plural Phyla)

  • Class

  • Order

  • Family

  • Genus (plural Genera)

  • Species


A high taxonomic level like kingdom is very general (like "Plant" or "Animal") while a low level like genus is quite specific (like Hexagenia or Ephemerella). Our well-known groups of mayflies, caddisflies, and stoneflies are orders of the class of insects.

A genus may contain several species, a family may contain several genera, and so on. The taxa contained within a larger taxon are called its subtaxa. For instance, the mayfly species Ephemerella subvaria and Ephemerella dorothea are some of the subtaxa of the genus Ephemerella.

In a misguided attempt to simplify things, some writers mistakenly call every taxon a "species." The word is sometimes used that way in everyday language ("That's a funny species of haircut ya got there, Earl.") but in entomology it just makes things more confusing.

Tip for Troutnut.com


All the aquatic insect knowledge on this site is organized taxonomically. So if you're looking for something and you can't find it on the species page, try the genus that species belongs to, or the family that genus belongs to, etcetera.

Italics and Notation


Because there are so many species, sometimes several share the same name. For example, there are eleven different species of mayflies, stoneflies, and caddisflies with the species name californica. To avoid being confused, when we write a species name we write the genus name with it: Capnia californica, Hydropsyche californica, and so on. It's the same reason we humans are called Homo sapiens and not just sapiens.

Sometimes the genus name is omitted, or abbreviated with a single letter, when it's very clear which genus is being discussed. Hexagenia limbata becomes H. limbata or just limbata, for example. This convention is common in some books but rarely used on this site.

The species name itself is never capitalized, but other taxon names are. This universal old convention seems kind of strange, but it works just fine. Another convention is that genus and species names are usually written in italics, but other taxon names are not. Ephemerella and Ephemerella subvaria are italicized, but the family Ephemerellidae is not.

Omitted from Troutnut.com


There are so many insect taxa that navigating this site would be really confusing if they were all on the menus. In the interest of thoroughness and simplicity, I've placed the uncommon subtaxa of each taxon into a "not included" list below the main list. If a taxon is not included here, you'll probably never need to know about its details for angling in the continental U.S.A.

Evolutionary Complications


Biologists do not toss living things into different taxa at random. Instead they carefully arrange things to reflect the most recent common ancestry.

Any two subtaxa of the same taxon are supposed to be more closely related to each other than to the subtaxa of a different taxon. For example, all mayflies in the order Ephemeroptera share a more recent common ancester with each other than they do with the nearest caddisfly of the order Trichoptera. Within the mayflies, Hexagenia limbata and Hexagenia atrocaudata, two species in the same genus, probably evolved from a more recent common ancestor than Litobrancha recurvata, which is in a different genus of the same family (Ephemeridae).

Figuring out these relationships is a tricky task. The next page details some of its complications.