Fishfly Larva Identification Guide

Key Visual Features

• Elongated, segmented body typically 1-2.5 inches (25-65 mm) long at maturity
• Brown to dark brown coloring, somewhat less heavily armored in appearance than a dobsonfly larva
• Paired lateral filaments running down the sides of the abdomen, each often bearing small tufted gills at its base
• Curved, moderately sized mandibles at the head, used for grasping prey, generally less oversized than a hellgrammite's
• Six legs clustered near the head for crawling and gripping surfaces
• Paired anal prolegs at the rear tip of the abdomen, sometimes with small hooks, used to anchor to rocks or debris
• Somewhat flattened body shape that allows it to wedge into crevices

Where and When You'll See It

Fishfly larvae are found under rocks, logs, and debris in streams, rivers, and slow-moving water with moderate current, often in slightly siltier or slower habitats than the fast riffles preferred by dobsonfly larvae. Their larval stage can last one to two years, so they can be encountered throughout much of the year, especially when rocks and submerged debris are overturned during a stream survey. They stay hidden under cover during the day and are more active in searching for prey at night.

Similar-Looking Bugs

• Dobsonfly larva (hellgrammite): very similar overall shape and size, but typically lacks the small tufted gills at the base of the lateral filaments that many fishfly larvae have, and tends to have a more heavily armored look.
• Alderfly larva: smaller, with a single unbranched tail filament instead of paired anal prolegs, and simpler filaments without gill tufts.
• Stonefly nymph: has two long tail filaments (cerci) rather than hooked prolegs, and a more streamlined body without side filaments.
• Diving beetle larva: has larger, more prominent sickle-shaped jaws and lacks the fringe of lateral filaments along the abdomen.

Quick ID Checklist

• Paired lateral filaments along the abdomen, often with small tufted gills at their base
• Paired hooked prolegs at the rear tip (not a single tail filament)
• Brown, moderately armored, segmented body up to about 2.5 inches long
• Curved jaws at the head, smaller than a hellgrammite's
• Found under rocks and debris in streams or slow rivers with moderate current