Fishflies

What Are Fishflies?

Fishflies are large winged insects (not actually flies) that belong to the order Megaloptera, family Corydalidae. These insects are commonly found near freshwater sources like rivers, streams, and lakes, as their larvae are aquatic.

Are Fishflies Harmful?

Fishflies are not harmful to humans, pets, property, or crops. Despite their intimidating appearance and large size, they are harmless and generally considered a nuisance rather than a threat:

• Swarming: During peak emergence, they may appear in large numbers, particularly near water and artificial lights. This can create slick surfaces from crushed bodies (hazardous on roads or sidewalks), foul odors (often described as “fishy,” especially when they die en masse), and visual disturbances or discomfort from swarms fluttering around lights or windows.
• Attracted to Light: Like many nocturnal insects, fishflies are drawn to bright outdoor lighting, especially near water.
• Bites or Stings: Adult fishflies do not bite or sting. Some species, particularly males, have large mandibles that look menacing, but these are used for mating displays rather than defense or feeding.
• Allergies or Toxicity: Fishflies are not venomous, and there are no known allergens specifically associated with them.
• Physical Contact: If handled, they might flap their wings or crawl awkwardly, which can be startling but not dangerous.
• Do Not Infest Homes: Fishflies do not invade homes in the way that ants, termites, or roaches do. They may accidentally enter buildings if attracted to lights, but they don't breed indoors or cause damage.
• No Structural Damage: They do not chew, bore, or nest in wood, insulation, or wiring.
• Short Lifespan: Adults live only a few days to a week, making any presence temporary.

Fishfly Appearance

Fishflies have a distinct, dramatic appearance that can be startling at first glance due to their large size and unusual features:

• Size: Adult fishflies are large insects, typically 1 to 2.5 inches (25 to 65 mm) in body length, with an even greater wingspan.
• Color: Most are grayish-brown or dark brown, sometimes with mottled or smoky wing patterns that help them blend into tree bark or leaves.
• Wings: Two pairs of long, membranous wings. The wings are clear to grayish, often with dark veins or speckles. At rest, the wings are held roof-like over the body, unlike dragonflies (which hold theirs flat).
• Head and Antennae: Large, rounded head with prominent compound eyes. Antennae are long and thread-like, sometimes feathery (especially in males). In some related species, males have large, tusk-like mandibles (e.g., dobsonflies), though this feature is less dramatic in typical fishflies.
• Body: The body is soft and elongated, somewhat flattened, and segmented. Often fuzzy or hairy in appearance, particularly along the thorax and legs. Legs are spindly and adapted for perching.

Larval Stage (Hellgrammites)

Though not often seen unless you're near water:

• Dark brown to black, with a tough, segmented body.
• Up to 3 inches long.
• Equipped with strong mandibles, lateral filaments (feathery gills), and clawed legs.
• Aquatic, typically found under rocks or submerged debris in fast-flowing streams.

Fishfly Habitat

Fishflies are most commonly found in regions near freshwater bodies such as rivers, streams, lakes, and ponds, especially those with clean, well-oxygenated water. Their distribution is heavily influenced by the aquatic environment required for their larval stage (hellgrammites), which is the longest part of their life cycle.

Geographic Distribution

• Great Lakes Region: Especially around Lake Erie, Lake Ontario, and parts of Michigan, Ohio, Pennsylvania, and New York. Massive seasonal hatches are common here.
• Midwestern U.S.: States like Illinois, Indiana, Wisconsin, and Minnesota see regular populations near rivers and lakes.
• Northeastern U.S.: Common in New England, New Jersey, and upstate New York.
• Southeastern U.S.: Found near rivers and streams in Georgia, Tennessee, North Carolina, and Alabama, particularly in forested or mountainous areas.
• Eastern Canada: Especially southern Ontario and Quebec, often along the St. Lawrence River and tributaries.

Habitat Characteristics

• Water Quality: Fishfly larvae (hellgrammites) require clean, oxygen-rich, fast-moving water—making them strong indicators of good environmental health. You’ll most often find them where water is unpolluted and has gravel or rocky bottoms, such as streams and small rivers.
• Nearby Environment: Adults often emerge in wooded or semi-rural areas near these aquatic sites. They may travel short distances from water to mate, lay eggs, or seek light sources, which is why they sometimes appear in urban areas close to water.
• Artificial Lighting: At night, fishflies are commonly seen in large numbers near streetlights, porch lights, parking lots, and commercial buildings close to water. This behavior can cause significant seasonal swarms, particularly during warm, humid evenings after rainfall.