Black Fly Larva

The black fly larva is the aquatic stage of a small, humpbacked fly whose biting adults are a familiar sight near flowing water in many parts of the world, but the larva itself lives an entirely sedentary, filter-feeding existence anchored to submerged surfaces. Its distinctive club-shaped body and pair of fan-like feeding structures on the head make it well adapted for life in fast-flowing, well-oxygenated streams and rivers.

Black fly larvae attach themselves to rocks, submerged logs, or vegetation using a sticky silk pad spun from a gland near the mouth, anchoring a hooked posterior end into the silk while the body sways freely in the current. From this fixed position, they extend a pair of delicate, fan-shaped mouth brushes into the flow to filter out fine organic particles, algae, and bacteria carried downstream.

Because they require flowing, well-oxygenated water, black fly larvae are frequently used as indicators of good water quality, often occurring in dense colonies on rocks in riffles and rapids. As larvae in a fly's complete metamorphosis, they eventually pupate underwater in a silken cocoon before the adult fly emerges.

• Club-shaped or bowling-pin-shaped body 5-10 mm long, wider toward the rear and tapering toward the head
• Pair of distinctive fan-shaped feeding brushes extended from the head to filter particles from the current
• Single hooked proleg at the rear end used to anchor into a silk pad spun onto rocks or vegetation
• Body typically grayish, greenish, or brownish, often found in dense clusters swaying in the current
• Lacks true legs along the body, unlike crane fly or horse fly larvae
• Distinguished from midge larvae by its club shape and fan-like feeding brushes, and from other stream larvae by its habit of anchoring in place with a silk thread

Black fly larvae are found attached to rocks, submerged wood, and vegetation in fast-flowing, well-oxygenated streams and rivers worldwide, particularly in riffles and rapids where current speed keeps water oxygen-rich and delivers a steady supply of drifting food particles. They are largely absent from still or stagnant water and are most abundant in cooler months and clean, unpolluted streams.

Black fly larvae remain anchored in place for most of their larval life, spinning a small silk pad on a submerged surface and hooking their rear end into it while extending a pair of fan-shaped mouth brushes into the current to filter out algae, bacteria, and fine organic particles drifting downstream. If dislodged, a larva can pay out a silk safety line and reattach itself further downstream, a behavior similar to a spider's dragline. As filter feeders dependent on flowing, oxygen-rich water, their presence is often used by researchers as a sign of healthy stream conditions, and they serve as an important food source for fish and other stream predators.

Females lay eggs in masses on rocks, vegetation, or directly into fast-flowing water near the surface. The larvae that hatch anchor themselves to submerged substrates and pass through six or more instars over several weeks, filter-feeding continuously and growing with each molt. When mature, the larva spins a silken cocoon attached to the same substrate and pupates underwater, with the pupa also drawing oxygen from the surrounding stream through gill filaments. The adult fly emerges from the cocoon and rises to the surface in an air bubble before flying off, and depending on climate there may be several generations per year.