JUDY JESSOP: Aquatic Insects Are Graceful and Fascinating to Watch
I stand on the shady bank of a creek mesmerized by the sheer beauty of liquid motion as a fly fisherman teases the creek's surface.
Thigh-deep in the swift water, he whips his shimmering line in slow graceful curves. His lure (a finely tied bit of deer hair, filament and hook) skips and dances on the surface of the water -- both lure and motion intended to mimic the very insects that dance in the sunlight.
Surely you have seen these fairy-like insects, at least in their adult stage when they flit about, often in swarms over water. But did you know that these graceful insects spend most of their lives in the water?
These critters, along with caddisflies and stoneflies, are among the most important groups of aquatic insects found in fresh and brackish water. In their larval stage, they are a vital link in the food chain. They eat algae and other aquatic vegetation, thereby collecting the energy stored in these tiny plants and making it available to fish, birds and other animals that, in turn, eat these insects.
Any fly fisherman will attest to the value of mayflies, caddisflies and stoneflies as food for game fish. It is the reason that anglers tie lures to look like these flies in their different stages of development.
Much like canaries in the coal mine, these insects are also very important to scientists that monitor the quality of our water.
Different species of these insects have different tolerances to pollutants: stoneflies are the most sensitive to water quality, while many species of caddisflies can survive in streams affected by agricultural and even sewage treatment runoff. Which of these insects is present or absent in our rivers, streams and lakes provides important information on just what types of pollutants are present as well as the level of the pollution.
Fascinating Life Cycles
All of these aquatic insects have fascinating life cycles.
Mayflies spend most of their time in the larval stage of development and are called nymphs (one of the stages anglers mimic), molting 20 or more times as they grow. They are equipped with gills and are best identified by their three tails. In fast-moving creeks, they have evolved to be flat and often cling under rocks, eating the algae and vegetation growing there.
In slower-moving, silt-bottom streams, mayfly nymphs live in U-shaped burrows collecting algae and other tiny plants by filtering the slow-moving water.
When a mayfly is nearly mature, it goes through a special molt, emerging from the water as a winged insect, which is not yet an adult (anglers mimic these also, calling them duns). Duns have very tiny hairs on their wings and are smaller than adults. This stage is very brief. It usually lasts just long enough for them to dry off and fly to a nearby tree, where the mayfly molts again into a true adult.
Adults (known by fisherman as spinners) have long slim bodies, two or three extended thread-like tails, and wings that are held upright when at rest.
The adult phase is very brief. Mayflies have no mouth parts, for they do not eat as adults. Their sole purpose is to mate and lay eggs. Males swarm together while females visit these swarms and choose a male.
Pairs leave the swarm, flying off to mate. The female then lays eggs in water, a few at a time as she flies, while dipping her abdomen along the surface. The eggs sink to the bottom -- the cycle begins again.
Caddisflies are aquatic insects closely related to moths. Like moths, they spin silk cocoons to pupate. But during their aquatic life as nymphs, many caddisflies use their ability to spin silk in many other ways.
Hiding in Cases
Many of these insects weave homes for camouflage and protection. There are tube-case makers that spin either straight tubes or circular ones (which look much like snail shells). Some weave pebbles into their cases, while others use plant material such as leaves or twigs.
They hide in their cases and only slip out to grab plants or other prey. The cases can be beautiful. There are jewelers that grow pebble-weaving caddisflies and provide colorful stones -- the caddis nymphs use these stones for their case-building, and the company fashions the cases as earrings, necklaces and other types of jewelry.
Tortoise-case makers build a dome with a strap, which allows them to carry their home with them, on their back, as they move about looking for food. These cases may also have pebbles or sand woven in. When the nymphs are ready to pupate, they remove the strap, fastening the case to something solid.
Still other caddisflies are free-living, often using their silk-fashioning nets to capture or filter food from the water. It is only when it comes time to pupate that these free-living nymphs build modified cases for this transformation into adults.
Adult caddisflies look very much like small, fragile moths -- except they have hair-like antennae and do not have well-developed mouths (no coiled, tube-like mouth parts) as moths do. But they are able to sip liquids, and like most aquatic insects, their adult phase is short and consumed with mating and reproduction.
Stoneflies, as adults, have a whole different way of finding a mate. These flies have long, fragile wings that extend just over the length of their bodies. They are weak flyers, so they do not travel far. Instead they often communicate by drumming.
A male will drum by beating his abdomen on a hard surface. If the drumming attracts a female, she will drum in answer. They will continue this communication until they find one another and mate. The fertile eggs are laid in or near fast-moving water. The eggs will take several weeks to hatch, often lying dormant through summer and hatching as the water temperature cools.
The hatching nymphs are either plant shredders or predators. Some species even start out eating plants and become predators in later stages of development.
These aquatic insects may go through 10 to 30 instars (stages between molts) before they mature into adults, which (depending on the species) can take from a few months to three years. They also require cool water temperatures. In the south, they tend to be active in winter when water temperatures are most suitable.
Large Diversity of Stoneflies
This habit is much appreciated by the fly fisherman, for it provides an aquatic insect to mimic during the cooler months of the year when most insects are dormant.
Stoneflies require oxygen-rich, moderate- to fast-moving water and are the first to be affected by pesticides or other pollutants in streams.
Many streams in the Sandhills have a large diversity of stoneflies. The reason? Our streams often have a sandy bottom, which helps cleanse the water, and, in addition, our rivers have consistent water flow throughout the year.
The major limiting factor for aquatic insects in this region is acidity. In the smaller upper reaches of our creeks and rivers, the acidity is high, a condition that is detrimental to these critters. However, as water volume increases downstream, the acidity decreases and the number of insects goes up.
The area of the Lumbar River near Wagram, for example, has low acidity and very good water flow. This means high oxygen levels, and the quality of the water must be very good because the diversity of aquatic insects is incredible.
Stoneflies can be highly specialized, for example, in the lower Little River a species of stoneflies has been found that live nowhere else in the world. If you think that fish hang around snags just because of the shade and protection, there are more important reasons. Snags provide habitat for aquatic insects, stable material on which to build their homes and to gather food. The fish hang out in such areas because there is so much potential for a rich insect meal.
If you are wondering if a stream is clean and healthy, just pull up an old snag or a large rock and take a close look. If the water is relatively free of pollution, you will find aquatic insects peering right back at you.
If the stream has such critters lurking among rocks and snags, then there is a good chance that the fish will be there as well. Whether it is a lazy July morning when the mayflies are swarming, or a chill early-spring day when stoneflies are emerging from the water about to molt into adults, fish are ever ready to take advantage of an easy meal.
Fly fishermen who do their homework know which insects fish are finding particularly plentiful, so it just doesn't matter what time of year it is -- with a well-chosen fly, as mimic, these anglers will be having success as well.
Judy Jessop can be reached at firstname.lastname@example.org.
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