Stonefly, an insect whose immature forms are commonly found on the undersides of stones in streams, rivers, waterfalls, and lakes. Adults are usually found on land near freshwater. There are about 3,000 species. Stoneflies are found throughout the world except in polar regions.

The immature forms, or naiads, eat aquatic plants or insects. After one to three years, naiads develop into adults. Adults typically have two pairs of wings, are brownish or yellowish, and are one-half inch to 2 1/2 inches (1 to 6 cm) long. Adults of most species do not eat; adults of some species eat plants or algae. Adults live only two to three weeks. Many types of fish eat naiads, and both naiads and adults are used in fishing as bait.

Stoneflies make up the insect order Plecoptera.