Fish flies are aquatic insects best known for their dramatic seasonal emergences near rivers and lakes, where they can blanket buildings, streetlights, and parked cars in staggering numbers. The name itself causes confusion, though, because “fish fly” is used for two very different groups of insects depending on where you live. In entomology, a fishfly (typically one word) is a soft-bodied insect in the order Megaloptera, related to dobsonflies and alderflies. But across the Great Lakes region of the United States and Canada, “fish fly” is the colloquial term for mayflies, which belong to an entirely separate insect order. Understanding which insect people are actually talking about clears up most of the contradictory advice and lore surrounding them.
Two Insects, One Name
The naming overlap is not trivial. If you live near Lake Erie or Lake St. Clair and complain about fish flies piling up under your porch light every June, you are almost certainly talking about mayflies, order Ephemeroptera. These are the insects that emerge in clouds so thick they show up on weather radar and make roads slippery. They have delicate, upswept wings, long tail filaments, and famously short adult lives, sometimes measured in hours.
True fishflies, on the other hand, are members of the family Corydalidae within the order Megaloptera. They look nothing like mayflies. Fishflies are bulkier, with broad wings that fold flat over the body at rest, and many species have feathery or comb-like antennae. They are less likely to form the apocalyptic swarms that make headlines, though they can still appear in large numbers near clean streams. They are closely related to dobsonflies, which are famous for the males’ oversized, pincer-like mandibles. Fishflies have smaller, less dramatic mouthparts but share the same general body plan. Research into the internal anatomy of Megaloptera, including fishflies, has helped clarify how the three major lineages within the order, Corydalinae (dobsonflies), Chauliodinae (fishflies), and Sialidae (alderflies), relate to one another.
The Fishfly Life Cycle
Like all Megaloptera, fishflies spend the vast majority of their lives underwater as larvae. The larval stage can last one to three years depending on the species and water temperature, while the winged adult stage lasts only a few days to a couple of weeks. This lopsided life history is a common pattern among aquatic insects: the larval phase is the “real” life, and adulthood is essentially a brief reproductive sprint.
Fishfly larvae are aquatic predators that live on the bottoms of streams and rivers, hunting smaller invertebrates among rocks and leaf litter. They breathe through lateral filaments along their abdomen, which function as gills, and they are equipped with strong mandibles for capturing prey. Their role in stream food webs is considered significant but still surprisingly understudied. A 2019 review noted that the ecological significance of megalopteran larvae is likely underappreciated, as their contributions to the food webs of stream and river bottom communities remain “largely unquantified.”1PubMed Central. Bionomics and Ecological Services of Megaloptera Larvae (Dobsonflies, Fishflies, Alderflies) They serve as both predators of smaller organisms and prey for fish, making them a link between the invertebrate community on the stream bed and larger animals higher up the food chain.
When a fishfly larva is fully grown, it crawls out of the water and pupates in soil, under rocks, or in rotting wood near the stream bank. The pupal stage is relatively short, usually a few weeks, after which the adult emerges with functional wings. Adults of most species do not feed at all, or feed only minimally on liquids. Their mouthparts are reduced compared to the larvae’s. The entire purpose of the adult stage is to find a mate and reproduce before dying.
How Fishflies Find Mates
The mating behavior of fishflies is less spectacular than you might expect from insects that spend years preparing for adulthood. In at least one well-studied species, the dark fishfly (Nigronia serricornis), males patrol the edges of streams and fly toward females that are perched in low vegetation along the bank. There is no elaborate courtship display or male-to-male combat. The first male to reach a receptive female mates with her, and later arrivals simply inspect the pair briefly and leave. Researchers described this as a “scramble competition” system, where success depends on speed and search efficiency rather than fighting or display.2BioOne. The Scramble Competition Mating System of the Dark Fishfly (Nigronia serricornis) (Megaloptera: Corydalidae)
After mating, females lay their eggs on surfaces overhanging or adjacent to water, such as rocks, bridge supports, or vegetation. The eggs are deposited in masses, sometimes covered with a white, chalky substance. When the larvae hatch, they drop or crawl into the water below to begin their long aquatic childhood. This egg-laying habit explains why fishflies are so closely tied to waterways. You will not find them far from a stream or river unless they have been attracted to artificial lights.
Those Massive Swarms Near the Great Lakes
The insect events that make the news, where millions of winged creatures coat gas stations and need to be shoveled off sidewalks, are almost always mayflies, not true fishflies. The distinction matters because the biology and the practical response are different.
Mayflies emerge synchronously in enormous numbers, a reproductive strategy that overwhelms predators through sheer volume. Their adult phase is the shortest of any insect group, with some species living less than 24 hours after their final molt. They do not bite, they do not eat, and they do not sting. They exist as adults solely to mate, lay eggs, and die. The piles of dead mayflies that accumulate under lights and on bridges are a nuisance but completely harmless.
True fishflies can also emerge in notable numbers, but they rarely approach the biblical-plague densities of a mayfly hatch. Their emergence windows tend to be somewhat more spread out, and their adult lifespan is longer, a week or two rather than a single day. Both groups are strongly attracted to lights at night, which is why they tend to show up on the sides of buildings, under streetlamps, and on illuminated signs near water. If you are dealing with an insect invasion near a lake or river in late spring or early summer, the insects with long tail filaments and delicate upright wings are mayflies. The ones with broader wings held flat and no tail filaments are more likely fishflies or a related megalopteran.
Are Fish Flies Harmful?
Neither true fishflies nor the mayflies commonly called fish flies bite, sting, or transmit disease. They are not attracted to food, do not infest homes, and pose no threat to pets or children. The only real problems they cause are aesthetic and slippery. Large emergences near lights can leave thick layers of dead insects on walkways, roads, and building facades. When wet, these layers become genuinely slippery and have been known to cause traffic hazards on bridges.
Fishfly larvae, despite being active predators underwater, are harmless to handle. Some people who fish or collect aquatic invertebrates for bait pick them up, and while the larvae can give a mild pinch with their mandibles, it is nothing like the painful bite a dobsonfly larva can deliver. Adults are even less threatening. If one lands on you, it is resting, not attacking.
From an ecological standpoint, both fish flies and mayflies are overwhelmingly beneficial. Their larvae help process organic material in streams and serve as food for fish and other wildlife. Large emergences transfer nutrients from aquatic ecosystems to terrestrial ones, as the bodies of dead adults fertilize shoreline soils. The presence of healthy fish fly or mayfly populations is also a useful indicator of water quality, because both groups are sensitive to pollution. A river that supports big mayfly hatches or abundant fishfly larvae is generally a cleaner river.
Dealing With Swarms Around Your Home
If you live near water and experience seasonal fish fly or mayfly emergences, the most effective single thing you can do is manage your outdoor lighting. These insects navigate by natural light sources and are overwhelmingly attracted to artificial lights at night. Switching exterior lights to yellow or amber “bug lights” reduces their attraction, though it does not eliminate it entirely. Turning off unnecessary outdoor lights during peak emergence weeks makes a bigger difference than most people expect. Some lakefront communities have experimented with timing streetlight shutoffs during the worst nights of mayfly season.
Beyond lighting, there is not much to do except wait. Swarms typically last a few days to a couple of weeks, and the dead insects can be swept or hosed away. Insecticides are not practical against these emergences, partly because the sheer number of insects makes spraying futile and partly because killing aquatic insects at scale would damage the ecosystem they support. The fish, birds, and bats that depend on these insects as food are part of the reason your local waterway remains healthy.
For vehicles, some people recommend parking in a garage or covering their car during peak emergence. The bodies of dead mayflies can etch automotive paint if they are left baking in the sun for days, thanks to the slightly acidic chemistry of their decomposing tissues. A quick wash within a day or two prevents any lasting damage.
Where True Fishflies Live
The subfamily Chauliodinae, the true fishflies, is found across every continent except Antarctica, though their diversity is concentrated in temperate and subtropical regions. In North America, several genera are found in streams from coast to coast, with species diversity highest in the Appalachian region and the Pacific Northwest.
Population genetics research on the dark fishfly has revealed a lot about how these insects recolonized North America after the last ice age. Analysis of genetic variation across eastern populations showed that the species expanded northward from southern refugia as glaciers retreated, following three distinct migration routes. One moved from Tennessee into the upper Midwest and Ontario. A second radiated eastward from Pennsylvania. A third tracked the Atlantic coast from North Carolina up into New England and then into New York.3PubMed. Phylogeography of postglacial range expansion in Nigronia serricornis Say (Megaloptera: Corydalidae) Genetic diversity was highest in the south and decreased moving northward, a pattern seen in many organisms that expanded out of ice-age refugia. Nearly half of the total genetic variation occurred between populations rather than within them, suggesting that fishfly populations in different watersheds are fairly isolated from one another, which makes sense for an insect whose larvae are confined to specific streams.
This kind of population structure also means that local fishfly populations can be vulnerable to localized threats. A pollution event or dam construction that wipes out larvae in one stream system cannot be easily replenished by immigrants from neighboring watersheds, because the adults are weak fliers and short-lived. Conservation of fishflies, like conservation of many aquatic insects, comes down to protecting water quality and stream habitat at the local level.
An Ancient Lineage
Fishflies are not a recently evolved group. Fossil evidence and phylogenetic analysis place the origin of the fishfly subfamily Chauliodinae deep in the Mesozoic era. A study combining fossil and living species into a single evolutionary tree identified the Middle Jurassic genus Jurochauliodes as the earliest-branching fishfly lineage, making the group at least 165 million years old.4PLOS ONE. Early Evolution and Historical Biogeography of Fishflies (Megaloptera: Chauliodinae): Implications from a Phylogeny Combining Fossil and Extant Taxa The analysis recovered fishflies as a single natural group defined by shared wing-vein characteristics and specialized larval breathing structures. From that Jurassic ancestor, fishflies diversified into three main evolutionary branches that gave rise to the genera found today across the Americas, Africa, Asia, and Australasia.
The broader order Megaloptera, which includes fishflies, dobsonflies, and alderflies, is one of the smallest insect orders, with only around 400 described species worldwide. By comparison, beetles have over 400,000. This modest species count belies a deep evolutionary history. Megaloptera are among the earliest-diverging lineages of insects that undergo complete metamorphosis, the developmental pattern that includes a distinct larval, pupal, and adult stage. Their persistence across hundreds of millions of years, through mass extinctions and continental drift, speaks to the resilience of the basic aquatic-larva-to-short-lived-adult life plan.
Why Fishflies Are Considered Water-Quality Indicators
Aquatic biologists have long used the presence or absence of certain insect larvae as a shorthand for stream health. Fishfly and dobsonfly larvae, along with mayfly and stonefly nymphs, are among the groups most sensitive to degraded water conditions. They need relatively clean, well-oxygenated water with intact substrate, meaning a natural mix of rocks, gravel, and organic debris on the stream bottom. Sedimentation from runoff, chemical pollution, and reduced oxygen levels all tend to eliminate megalopteran larvae before they eliminate hardier organisms like certain worm and midge species.
This sensitivity makes fishfly larvae useful in bioassessment surveys, where researchers sample the invertebrate community in a stream and use its composition to judge overall water quality. Finding fishfly larvae in a stream is generally a good sign. Their absence from a stream where they historically occurred can flag a decline in habitat quality, sometimes before chemical monitoring picks up the change. Because fishfly populations are genetically structured by watershed, as the population genetics work on the dark fishfly demonstrated, losing a local population is not something that self-corrects quickly.5PubMed. Phylogeography of postglacial range expansion in Nigronia serricornis Say (Megaloptera: Corydalidae) Recolonization depends on adults from other streams finding and ovipositing in the recovered habitat, which can take years or decades given their limited dispersal ability.
For anglers, the presence of fishfly and dobsonfly larvae also signals productive fishing water. These insects are a natural food source for trout, bass, and other stream fish. Some fly-fishing patterns are tied to loosely imitate hellgrammites, the common name for dobsonfly larvae, and the similar-looking fishfly larvae. Whether you think of them as indicators, fish food, or a seasonal nuisance that covers your porch light, fish flies in either sense of the name are a sign that the waterways near you are doing something right.