Fishfly Larva

Fishfly larvae belong to the family Corydalidae in the order Megaloptera, the same group that includes the larger dobsonflies. They are elongated, somewhat flattened aquatic insects with a tough, segmented exoskeleton, powerful biting mouthparts, and a row of filamentous gills running down each side of the abdomen. Because they resemble dobsonfly larvae (commonly called hellgrammites), the two are frequently confused, though fishfly larvae are generally smaller and lack the branched, tufted gills found at the base of each dobsonfly proleg.

These larvae are voracious ambush predators that hide beneath rocks, logs, and leaf litter in flowing or still water, striking at other invertebrates that wander too close. Their presence is considered a good sign of water quality, since fishfly larvae are sensitive to pollution and low oxygen and tend to disappear from degraded streams. After a long aquatic larval period, the fully grown larva crawls onto land to pupate, eventually emerging as a large, soft-bodied winged adult that lives only a short time.

• Elongated, somewhat flattened body, usually brown, olive, or mottled gray to blend with stream substrate
• Strong, sickle-like mandibles used for gripping prey
• A row of simple, unbranched filamentous gills along each side of the abdomen (dobsonfly/hellgrammite larvae have bushier, tufted gills)
• Eight pairs of lateral abdominal filaments and a pair of hooked terminal prolegs used for anchoring in current
• Adults are large, soft-bodied insects with long, net-veined wings held roof-like over the body and comb-like or feathery antennae, distinguishing them from the smooth antennae of dobsonflies

Fishfly larvae live in clean, well-oxygenated streams, rivers, and the rocky margins of lakes and ponds across North America, tucked beneath stones, submerged wood, and organic debris. Adults are found near the same waterways from spring through summer, often resting on streamside vegetation or attracted to lights at night.

The aquatic larvae are nocturnal ambush predators, feeding on mayfly and caddisfly larvae, midge larvae, and other small invertebrates they encounter on the streambed. Adult fishflies feed little if at all, spending their brief lives seeking mates near water, usually after dark. Their long larval stage makes them an important link in freshwater food webs, both as predators of smaller invertebrates and as prey for fish and other larger animals.

Females lay flattened masses of eggs on rocks, bridges, or overhanging vegetation above water; newly hatched larvae drop or wash into the stream below. Larvae undergo complete metamorphosis, molting through many instars over one to three years while living entirely underwater. When mature, the larva leaves the water and burrows into damp soil, moss, or rotting wood along the bank to form a pupal cell. After pupating for several weeks, the winged adult emerges, mates, and dies within days to a couple of weeks.