Insect Classification

Last week I posted a brief commentary on Pteronarcys californica, a stonefly species found in Alberta. For those of you who are new to the world of fly fishing or entomology, this may be a good example with which to explain how scientists classify and name insects (along with other organisms). Knowing how the different species of insects are classified will help you to understand how the bugs that trout feed on are related to each other, and what to expect in terms of the physical appearance and general behavior of the different groups.

In the west, the stonefly Pteronarcys californica is most often referred to by its common name, the Salmonfly. Many other insects familiar to fly anglers also have common names that appear with great frequency in the fly fishing literature (e.g. Golden Stone, Pale Morning Dun, Blue Winged Olive, March Brown etc.). The problem with common names is that they are not always used in reference to the same species of insect. The term Salmonfly for example is sometimes broadly applied to any large dark stonefly species found on western trout streams; and in eastern Canada “Salmonfly” is sometimes used in reference to a completely unrelated species of mayfly. This can lead to confusion among anglers when discussing hatches, and which insect species are present on specific trout streams.  

To avoid this confusion the scientific community has adopted a standardized system for naming and classifying all living organisms. This system assigns a unique two-part name to each species (binomial nomenclature). The system also includes a hierarchical arrangement that organizes related organisms into various groups (taxa) based on characteristics that are common to members of each group. The groups start as general and broadly inclusive, and become more and more specific until the individual species level is reached. The classification hierarchy in our Salmonfly example would look like this:

Kingdom – Animalia

     Phylum – Arthropoda

          Class – Insecta

               Order – Plecoptera

                    Family – Pteronarcyidae

                         Genus – Pteronarcys

                              Species – Pteronarcys californica

The broadest group is the Kingdom level – if you are an animal (i.e. not a plant or bacteria), you are included at this level under the moniker Animalia. The next level divides all animals into various Phyla (singular = Phylum). Many of the aquatic invertebrates that trout feed on (including our stonefly example) are in the Phylum Arthropoda – this includes animals with hard exoskeletons and jointed legs (e.g. crayfish, shrimp, crabs, insects, zooplankton etc.). Following this, we have the Class level where most arthropods with 3 pairs of jointed legs are placed in the Class Insecta (Insects). From here (at the Order level) some of the names should begin to sound familiar to most fly anglers: The Order Plecoptera includes all Stoneflies, Ephemeroptera would be Mayflies, Trichoptera are the Caddisflies, etc.

As we proceed to the Family level, the members of each group are becoming more closely related to each other from an evolutionary perspective. Once we get to the Genus level, the individual species within each genus are so closely related that they often look the same, share similar habitats, and exhibit similar behaviors – often requiring expert knowledge and a microscope to tell one species from another. There are other levels within the hierarchy but for simplicity I have left some out (e.g. Subclass, Infraorder, Subfamily etc.).

Down at the species level each binomial species name includes the genus name (always capitalized and in italics) followed by the species epithet (all lower case and italicized). The same binomial species name is used by scientists worldwide whenever referring to the same bug (i.e. everyone speaks the same language). Once the full species name has been used in a written article, the genus name is often abbreviated for brevity (e.g. Pteronarcys californica becomes P. californica).

Rhyacophila sp. - A Caddisfly larva in the Order Trichoptera
When the species of a specimen is unknown, it is written as the Genus name followed by "sp."

For many anglers just knowing that an insect they have seen streamside is a stonefly, mayfly, caddisfly, or something else entirely may be as in depth as they wish to go; just tie on a fly of the same general shape, size and color as the natural and start fishing. But for others, part of the enjoyment of the sport lies in knowing the details – knowing why trout are feeding on this bug and not that one, why some insects emerge mid-stream while other crawl out on the bank, and why one style of fly is more effective than another at certain times. Fly fishing at this level is about being observant; putting the various pieces of the puzzle together to form a hypothesis about what the trout are feeding on, then testing that hypothesis, and refining the approach as necessary. This requires knowledge of at least what Family or Genus the bugs that are hatching belong to.