Scientific name: Insecta plecoptera
Common name(s): Stonefly
You've probably never seen a stonefly. They are secretive, aquatic insects. But, like many aquatic insects, they also have a terrestrial life stage -- a life stage spent flying and crawling around above the water. The pictures on this page show the aquatic life stage or larval stage. It is at this point in the life of a stonefly that you are unlikely to see them.
Stoneflies spend most of their lives as larvae in the water, crawling along the bottom of streams and rivers, and clinging to the underside of rocks and woody debris. Some are predators (they eat other bugs), while others eat plants and algae or decaying organic matter (plant bits). The diet of the stonelfly pictured here consists mainly of plant bits and algae.
The Pteronarcyd stonefly, or salmonfly, is the largest of the stoneflies. They spend anywhere from two to four years in the water until they emerge as winged adults. At their largest, they are almost two-inches long -- and they look pretty scary. Salmonflies tend to hang out in rapidly flowing streams and rivers in areas where leaves and other plant material accumulate.
As adults, stoneflies feed very little, if at all, and live for only a few days to a couple weeks. Unless you're a fly-fisherperson it's unlikely that you've seen an adult. Those who fly-fish know that stoneflies are excellent trout food. Both adults (dryflies) and larvae (wetflies) are used to catch gamefish found in cold, fast flowing water where trout and salmon are found.
After they leave the water, stoneflies tend to hang out on the rocks and vegetation along the streamside. When looking for a mate, they hold on tightly to a branch and rock or tap their bodies, making a drumming sound. The prospective mate hears this sound and wanders towards it's source, sometimes drumming a response call. Eventually, after several calls, the male and female find each other and mate.
Larval stoneflies are important to those at the Pollution Control Agency because they are known as sensitive or indicator taxa. What that means is that they are sensitive to changes in water quality. In particular, stoneflies are sensitive to changes in dissolved oxygen and water temperature. Stoneflies need a high amount of dissolved oxygen (D.O.) to live comfortably. When the D.O. falls or the temperatures rise, stoneflies disappear. The same goes for most species of trout. As a general rule, if there are trout in a stream there are also stoneflies; they have similar environmental and habitat requirements.
In case you are confused, stoneflies are not flies like a house fly is a fly, they don't buzz around and bother you while you're trying to enjoy yourself outside. Rather, they are like dragonflies and are called flies because they have wings.
Common name: Salmonfly (Stonefly)
Family: Perlidae with Acroneuria as genus, and Pteronarcyidae with Pteronarcys as genus
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The creature highlighted on this page was collected by MPCA's Water Quality Lab. This lab samples and analyzes water from around Minnesota.