Alderfly

Alderflies (genus Sialis) are small, drab members of the order Megaloptera, sharing ancestry with the much larger dobsonflies and fishflies but lacking their imposing size and jaw structures. They are named for their habit of resting on alder and other streamside shrubs as weak-flying adults.

The aquatic larvae are found in the silty or muddy sediments of slow streams, ponds, and lake edges rather than the fast, rocky riffles favored by dobsonfly larvae, giving alderflies a somewhat different ecological niche within the same insect order.

Adults are short-lived, clumsy fliers usually seen resting on vegetation near water in spring, and they are frequently used by researchers and educators as an indicator of the broader health of slow-water freshwater habitats.

• Small, dark brown to black insect with a single pair of held-together, smoky, heavily veined wings that appear as one when folded roof-like over the body
• Head and body somewhat flattened, with moderately long antennae
• No elongated mandibles; jaws are small and inconspicuous in both sexes
• Weak, fluttery flight compared to fishflies and dobsonflies
• Larvae are elongated, brownish, with seven pairs of tapering lateral filaments along the abdomen and a single long tail filament (unlike the paired terminal hooks of hellgrammites)
• Lookalikes: small fishflies, though alderflies are smaller and duller with a single terminal larval filament

Alderflies occur across much of North America, Europe, and Asia in association with slow streams, ponds, and the silty margins of lakes. Larvae burrow into soft sediment or hide among detritus and roots, tolerating lower oxygen conditions than many dobsonfly larvae. Adults appear in spring, typically clustering on alder, willow, and other waterside vegetation close to their larval habitat.

Adult alderflies rarely feed and live only a short time, mainly to reproduce, and they are weak fliers that stay close to the water where they emerged. Larvae are active predators and scavengers, feeding on small aquatic invertebrates and organic detritus within bottom sediments, contributing to the breakdown of organic matter and serving as prey for fish and other predators in slow-water ecosystems.

Females lay tightly packed rows of eggs on vegetation or structures overhanging water. Hatchling larvae drop into the water and burrow into sediment, developing through several instars over about one year. When mature, larvae crawl out onto land to pupate in damp soil near the shoreline, emerging weeks later as short-lived winged adults that mate and lay eggs to renew the cycle.