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Today on Animal Fact Files we’re discussing stoneflies. Though they have the name "fly" in their common name, these aren't true flies. Like dragonflies, adult stoneflies have two sets of wings, totaling four wings in comparison to the two wings found on true flies. There are more than 3,000 living stonefly species around the world. They occur on every continent except Antarctica, however they have a preference for cold, oxygenated water with an observable current. Stoneflies are not tolerant of polluted water, so they're an excellent indicator of clean water. The name stonefly gives a hint to their preferred habitat. These insects prefer streams, creeks, and even lakes with rocky bottoms, though there are some species who live in moist leaf litter in swampy habitats.

Stonefly nymphs can easily be mistaken for mayfly nymphs, another type of clean water indicator insect. While not always foolproof, stonefly and mayfly nymphs can be told apart by their tails and their gills. Mayflies have external gills that run along the sides of their abdomen and have three appendages at the tip of their abdomen; stoneflies have only two of these appendages and their gills are found along other parts of their bodies. As adults, mayflies hold their wings up while stoneflies fold theirs against their body, similar to a dobsonfly.

Stoneflies undergo incomplete metamorphosis meaning there is no pupal stage. Adults gather along the water's edge to feed and mate. Some males may drum their abdomen on the bank to entice females to them. A female lays over 1,000 eggs directly in the water. These are sticky and cling to vegetation and debris so they aren't carried off by strong currents. Stonefly eggs and nymphs are fully aquatic. Nymphs emerge from the eggs and remain in this stage for weeks to years depending on the species. One to three years is typical for these invertebrates to live in their nymph stage. During this time, the nymph feeds, grows, and molts. As nymphs, they may shed their exoskeleton more than 30 times! When they’re ready to shed their final time as nymphs, they’ll climb out of the water onto the shore and the adults will come out. Most species emerge from the water during spring and summer though there are also species that emerge in winter. These are known as winter stoneflies if you can believe that. As adults, they live a few short weeks, reproduce, and then die.

Stonefly nymphs are largely herbivorous and take food in the form of algae, detritus, and aquatic vegetation; however, a few species are carnivorous and eat other invertebrates like mayflies and midges. Adults may also eat vegetation, though some don’t have a functional mouth at all! They remain close to the water during their short adult phase. Although they have well developed wings, like crane flies, they aren't storing fliers.

In size, stoneflies can reach over two inches (5cm) in length, though some species don't even reach half an inch (1.3cm) long. In adults, their wings are longer than their abdomen. Stoneflies, especially their nymphs, provide food to many aquatic predators such as trout and bass.

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