New fly fishers interested in dry fly fishing will sometimes become confused about the nature of aquatic insects and their life cycles. This knowledge is important in establishing the new anglers perspective and expectations. This isn’t intended as an entomology lesson, just some clarity about the insects that you will want to imitate with your flies, whether tied by you or purchased.
There are four major aquatic insects that fly fishers focus on: commonly called mayflies, midges, stoneflies and caddis. Each has reasonably similar life forms and changes, but common vernacular can confuse the novice. This is a brief generalization of these insects, so no Latin will be used! Let’s start with Mayflies. There are approximately 600 species of mayflies in North America; world-wide five times as many. Not all are relevant to dry fly fishing, but clearly certain ones draw the most attention and excitement. The amazing thing about mayflies, that indears us to them, is their lifecycle. They begin as most insects as eggs and they hatch into “nymphs”. There are numerous varieties of nymphs, and some make themselves available to trout subsurface, but some (burrowers, for instance) provide little opportunity for trout to feed on. There are also climbers, clingers and swimmers. Currents move these versions of mayflies about and trout actively feed on them. As water temperatures rise, all mayflies begin a certain level of migration in preparation for their next stage of life. Generally referred to as the “hatch” mayflies (with some minor exceptions) move through the water column to the surface, where they enter metamorphosis: the dramatic change from a subsurface aquatic insect to a winged, air breathing insect. Mayflies are the ONLY insect that has two adult life cycle phases (metamorphosis). The first adult stage is the “dun”. The term has an interesting history, and even entomologists credit fly anglers with the origination of the term. The dun flies off the water towards riverside vegetation and within a short time (hours or days, depending on species) undergo a second metamorphosis for the purpose of mating and egg laying. That phase is referred to by fishers as “spinner”. Of course the hatch of mayflies in their initial adult stage is quite welcome to both trout and anglers, and cause for great excitement during large and prolific events.
Switching to caddis and stoneflies now in order to call attention to a confusing phenomenon that distinguishes the varieties of food opportunities for trout, and demonstrate vernacular that doesn’t always make sense. Stoneflies have three stages, egg, nymph and adult. While nymphes grow large in some species, they are limited as trout food except during migration, unless a current change catches them off guard and delivers a large meal to waiting trout. The migration is when these beasts of aquatic insects move towards shallow water and crawl into open air. The adult stonefly emerges at that time in what might be compared to a mayfly “hatch”, but it does not occur on the water surface. If you’ve walked stream side at certain times of the early summer, you will no doubt see the leftover exoskeleton of stoneflies attached to rocks and branches, just millimeters from the waters edge. The adults dry their wings, and fly off to find mates, females returning to the water to lay eggs. When they’ve completed egg laying, they collapse on the river's surface and present trout the opportunity to feed.
Caddis, in most cases, undergo the same life cycles as stoneflies, although some specific species do swim to the surface and “launch” into full flight from the pupa stage, a metamorphosis unlike either the mayfly or the stonefly, and scientifically considered complete metamorphosis. Most caddis pupa migrate and crawl to the edge of streams then “hatch” into adults, returning after mating to lay eggs.
As flyfishers, we tend to confuse beginners when we talk about stonefly or caddis “hatches”. While mayflies hatch on the surface and can be imitated with dry flies as duns, stoneflies and caddisflies provide dry fly fishing opportunities when they collapse or “fall”! So when anglers are discussing a great day catching fish during stonefly or caddis “hatches”, they are really talking about “falls”. Spinners, the final adult stage of mayflies, also fall and can offer dry fly opportunities, but mayfly hatches are actually that: hatches!
Midges have four stages as well: egg, larva, pupa and adult. This is referred to as complete metamorphosis. The pupa emerges from the water much as a mayfly, and lay eggs on or near the water as well. One thing all these insects share in their life cycles: and it's amazing when you think about it. All females will fly upstream to lay eggs! Of course, to have evolved otherwise would eventually wash all these aquatic insects out of a stream or river system if they didn’t exhibit that behavior.
Fly fishing enthusiasts throughout history have contributed a great deal to our understanding of aquatic insects, but you needn’t know Latin or the multitude of species and subspecies of insects to become accomplished. I would encourage an occasional venture into books by great authors like Dave Hughes, or Rick Hafele to learn more about the fascinating world of trout food! But don’t be intimidated: an Ephemeroptera Baetidae fishes just as well as a blue winged olive! Terrestrials are another topic for another time!
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