Fishfly vs. Mayfly: 5 Differences Explained

Quick Take

• These two insects actually share a nickname, and the mix-up goes deeper than most people realize. Untangle the nickname confusion →
• One of these insects spends years underwater preparing for an adult life so short it can't even eat. See the mayfly's brief adult life →
• Massive summer swarms of one of these insects are so dense they show up on weather radar. Explore the radar-visible swarms →
• Spotting one of these insects near a stream tells scientists something important about the water, though not what you'd expect. See what fishflies signal about water →

Mayflies and fishflies are both living fossils. These 2 insects have been around for over 150 million years. Mayflies (order Ephemeroptera) have a fossil record dating back to the Late Carboniferous period, over 300 million years ago. Fishflies (family Corydalidae) are also ancient, but their exact origin in the fossil record is less precisely dated. At a glance, you could easily mistake a mayfly for a fishfly. But these insects are incredibly different. Keep reading to learn more about the main differences between fishflies and mayflies, including their appearance, distribution, habitat and lifecycle, and human connections.

Fishfly vs. Mayfly: Clarification

Although the term fishfly is sometimes used regionally to refer to mayflies, especially around the Great Lakes, true fishflies belong to the family Corydalidae (order Megaloptera) and are distinct from mayflies, which are in the order Ephemeroptera.

For this article, fishfly refers specifically to insects in the family Corydalidae, which belongs to the order Megaloptera. These insects are also known by other names, such as dobsonflies, but their official name is fishfly.

The scientific name for mayfly is Ephemeroptera, an order that contains more than 3,000 species. It’s a Greek word that means short-lived, a nod to their incredibly short lifespan. Nicknames for the insect include drake, fishfly, sandfly, dayfly, and shadfly.

Fishfly vs. Mayfly: Distribution

True fishflies in the subfamily Chauliodinae have a massive presence in North America. Outside of North America, the Chauliodinae subfamily is found in Asia, South America, Africa, and Australasia. Although they are found on multiple continents, the fishfly’s distribution is exceptionally fragmented.

Mayflies are considered cosmopolitan insects and have a much broader range and distribution. Mayflies are found on every continent except Antarctica.

Fishfly: Life Cycle and Habitat

The life cycle of fishflies in the subfamily Chauliodinae follows a complete metamorphosis that spans one to several years, with the vast majority of that time spent underwater. It begins when adult females lay rows of eggs on vegetation or rocks overhanging clean, flowing streams.

Once hatched, the aquatic larvae, called hellgrammites, drop into the water, where they spend one to five years as voracious nocturnal predators. They breathe through specialized respiratory tubes while hunting under rocks and debris. When fully mature, the larvae migrate out of the water onto land during late spring or summer, burying themselves in damp soil, moss, or rotting logs to pupate.

After a few weeks, winged adults emerge for a brief terrestrial lifespan lasting from a few days to a few weeks, during which they focus on finding a mate to restart the cycle.

Mayfly: Life Cycle and Habitat

The life cycle of mayflies in the order Ephemeroptera follows an incomplete metamorphosis that can last several years, though it is famously defined by an incredibly brief adult stage.

Female mayflies deposit their eggs directly onto the surface of freshwater streams and lakes, where they sink to the bottom. The hatched aquatic nymphs spend months to years foraging on algae and organic debris, molting dozens of times as they grow.

Once mature, they float to the surface or climb onto shoreline rocks to undergo a unique winged transition stage called a subimago, making them the only insects that molt after developing functional wings. Within a few hours to a day, the subimago molts one final time into the fully mature imago.

These true adults lack functional mouthparts and cannot eat; they form massive aerial mating swarms, reproduce over the course of a few hours to a couple of days, dying immediately afterward.

Fishflies and mayflies are typically found in aquatic areas, such as streams, rivers, and ponds.

Fishfly vs. Mayfly: Appearance

Fishflies are a rusty or gray color. Their wings will be white or clear. When looking at a fishfly, you’ll notice that they have two pairs of wings extending towards their body’s lower half. Their wings can reach a length of 3 inches. Their long feathery antennae are one of the fishfly’s most distinguishable characteristics.

Mayflies have short antennae and a slender body, which makes their eyes appear much larger, giving them a true bug-eyed look.

Mayflies have large, clear, triangular wings at the front of their body and much smaller, rounded hind wings that can be difficult to see.

The wings are covered with horizontal and vertical veins that create a net-like pattern.

Fishfly vs. Mayfly: Human Connections

Fishflies are considered harmless bugs. Although they can pinch, their pinch doesn’t cause any long-term issues. Fishflies are good news for the environment. Stream ecologists know that healthy fishfly populations are a sign of good water quality.

Mayflies are beneficial insects in both terrestrial and freshwater ecosystems. They do not sting or bite. However, they are known to swarm. Mayflies emerge from lakes and rivers every summer and take to the skies. Scientists are still studying when their emergence starts.

During warm summer months, mayflies swarm the Mississippi River Basin and Great Lakes. Sometimes the swarms can be so thick that they appear on weather radar.