Sialis

Sialis is a genus of aquatic insects in the order Megaloptera, commonly known as alderflies. It belongs to the family Sialidae and includes numerous species distributed across the temperate regions of the Northern Hemisphere. Adults are small to medium-sized, with soft bodies, two pairs of wings that are usually clear or lightly colored, and long antennae. They typically rest with their wings held roof-like over the abdomen.

Sialis species undergo complete metamorphosis. Eggs are laid on vegetation or debris above or near water. Larvae, or naiads, are aquatic and predatory, living among stones and vegetation in streams, ponds, or wetlands. They possess well-developed jaws and gill filaments along the sides of the abdomen. After several molts, larvae crawl to damp ground to molt into pupae, then emerge as winged adults. Adults have a short lifespan and are not strongly feeding; when they do feed, it is usually on nectar or honeydew, and some may not feed at all.

Species occur in Europe, Asia, and North America. Sialis lutaria, the common alderfly, is widespread in Europe and parts of Asia. Alderflies are often found near clean, slow-flowing waters; larvae are important predators and serve as bioindicators of water quality.

Sialis is the type genus of the family Sialidae, one of the two extant families in Megaloptera, the other being Corydalidae (dobsonflies and fishflies).