Last Sunday morning, I loaded my kayak and fishing tackle and headed to the Juniata River to enjoy the beautiful spring day. After some great days of trout fishing in recent weeks, I wanted to catch a few smallmouth bass.
A chunky smallmouth smacked a crankbait on the second or third cast, and I felt justified in anticipating lots of bass action the rest of the afternoon. But nearly an hour later, I was still trying to land my second bass of the day.
As I changed lures and tactics in hopes of finding what the fish were in the mood for, I couldn't help but notice a steady parade of large, yellow mayflies that drifted by the bow of my boat. In my early days as a fly-fisherman, I studied mayflies intensely, and I am still fascinated by these beautiful and important aquatic insects.
Currently, about 500 different species of mayflies occur in North America in both the flowing waters of rivers and streams and the still-water environments of lakes and ponds. Over the past ten years or so, I have marveled at the abundance and variety of mayflies I've observed on the Juniata River because many mayflies are quite sensitive to pollution, so their presence is often an indication of good water quality.
Mayflies are one of the oldest forms of insect life, with fossil records documenting their existence more than 200 million years ago. But the life cycle of mayflies is even more remarkable than their longtime presence here on Earth.
With a few exceptions, most species of mayflies have a lifespan of almost exactly one year, beginning when the adult female mayflies lay their eggs in the waterway from which they came. The eggs hatch into tiny nymphs that live and grow primarily on the stream bottom.
Different families of mayfly nymphs typically occupy different types of underwater habitat. Some cling to rocks in areas of fast-moving current, while others burrow into the stream bottom for shelter, while still others are able to swim like tiny minnows. Because mayfly nymphs can be present in great numbers in so many areas of a stream or lake, they represent a most important link in the food chain for many species of fish and other aquatic creatures.
After living under water for an entire year, the nymphs of a given species finally reach maturity and undergo a quick and amazing transformation into their adult stage. In simplest terms, these underwater bugs leave the water and turn into winged adults that now breathe air and can fly.
And most species make that transformation in a period of seconds. Some species of nymphs leave the stream bottom and swim to the water's surface to emerge, while others crawl to shore or onto rocks, logs or other streamside objects to hatch. During this process, the outer skin of the nymph splits open, and the winged adult literally crawls out of its own skin.
Fly-fishermen call the newly hatched adult mayflies "duns," while biologists refer to them as "subimagos." Mayflies are completely benign insects. They don't bite, sting, damage crops or bother humans in any way. In fact, adult mayflies live only a day or two and eat nothing at all during that time because they have no digestive system. Their only function is reproduction.
Once they hatch, mayflies usually retreat to streamside vegetation, where they rest before undergoing one more transition - a final molt. Mayflies are the only insect that molts in the winged stage, which fishermen call "spinners" and scientist call "imagos." Male and female spinners generally meet in swarms over the water and mate in flight.
The females then deposit the fertilized eggs into the water to produce the next generation of its kind. Their mission completed, the females die soon after and often fall back into the water themselves.
The butter-colored mayflies I watched hatching last weekend on the river were a species we fly anglers commonly call "Light Cahills." I scooped up several of them with my paddle and photographed them while is sat in my kayak.
I also noticed many nymphal shucks floating near the surface as well, so I plucked a few of them from the water as well. To my amazement, two of what I thought were empty shucks were actually nymphs is the process of hatching, and I was able to watch and photograph those mayflies emerge in the palm of my hand.
So while the bass fishing turned out to be a disappointment that day, I was treated to a wonderful natural show that has been happening for millions of years. I'm not about to complain about that.