Mayfly Larvae Facts & Information Guide (Ephemeroptera)

Mayfly larvae, often referred to as “nymphs” or “naiads,” are mayflies that are in the immature aquatic stage of their life. They are commonly found in bodies of freshwater, but some can even survive in the slightly brackish waters of marine estuaries. Depending on where you live in the world, they may also be called shadflies or fishflies.

The mayfly experiences three different developmental stages during its life: the egg stage, the larval stage, and the adult stage. The larval stage of a mayfly’s life can last anywhere from a few months to up to three years! When a mayfly is in its larval stage, it can be characterized by its elongated, soft body and sturdy legs that are covered in bristles. They do not have wings at this point in time, and they have gills that are used for breathing underwater.

Mayflies can be found living all over the world! The only places they are not found are the Arctic and Antarctic regions. Mayflies have been around for about 350 million years, and there are over 3,000 different species of mayfly all over the world. In North America alone, there are over 700 species of mayflies! Most of the naiads mature in rivers and streams that are of high quality, so their presence is a good indicator of water quality; they are what’s known as an “indicator species.” As larvae, they tend to feed on detritus but some may eat other insects or plant material as well. 

What Do Mayfly Larvae Look Like?

When comparing aquatic insects with one another, mayfly larvae are pretty easy to identify. Scooping up a jar of sediment from the bottom of a cold river or stream with high water quality will surely turn up at least one or two. Scraping at the bottom of rocks in the water is another surefire way to obtain these larvae. Mayfly larvae are pretty small, and can range in size from ½ inch to 1 inch in size (approximately 3-30 millimeters in length). They have slender, soft bodies that can be either rounded or flattened in shape, depending on the species and the environment.

Nymphs have six robust jointed legs and a series of gills that are located externally. These gills are feathery in appearance. While adults have translucent wings, larvae do not yet have any indication of the appendages. The eyes of nymphs are smaller than those of adults, and their heads are typically flat in shape which helps them stick to rocks in swift currents. Mayfly larvae also have two or three long tail-like appendages coming out of the rear of the abdomen. Most species have three of these “tails.” Those with two tails may get confused with stonefly larvae, but you can tell the difference by looking at their legs – mayflies always have just one small hook at the end of each leg, while stoneflies have two.

Mayfly Larvae Habitats – Where Do They Live?

Nymphs are found worldwide, and throughout North America in freshwater streams and rivers. They can also develop in lakes, wetlands, or ponds, but thrive in cool flowing water. They do not succeed in low-quality polluted water, so their livelihood is threatened by many forms of pollution. All sources of pollution threaten this species, but the greatest are sewage effluents and agricultural runoff.

Within their freshwater habitat, nymphs tend to hang out in the substrate at the bottom. They have gills for breathing under water at this stage in life, and utilize rocks and substrate as cover from predators. They can easily adhere to rocks and have no problems surviving in fast-flowing waters thanks to the helpful hook found at the end of each of their legs.

There are many different species of mayfly, and each one occupies a different microhabitat. Each species thrives under very specific environmental conditions – substrate type, water depth, flow rate, temperature, and so on. Some species are burrowers and they take on a more rounded body type. Other species live above ground and have a more flattened shape; this adaptation helps them thrive in fast-flowing water conditions.

What Do Mayfly Larvae Eat? (Mayfly Diet)

Generally speaking, mayfly larvae are not picky eaters when it comes to their dietary choices! They are opportunistic feeders, eating whatever is edible and available to them in their environment. They are also detritivores as well as omnivores. Most commonly, nymphs will feed on decaying matter in the habitat, or algae that is growing among the substrate. They will also feed on weedy plants at the bottom of the water. Larger sized larvae will sometimes even have carnivorous tendencies and prey on other smaller insects that they find in the environment.

Interestingly, while mayflies in the larval stage will eat nearly anything that is available, when mayflies reach the adult stage they do not feed at all! They exist to reproduce, and then die shortly after.

Are Mayfly Larvae Dangerous or Venomous?

Mayfly larvae are not dangerous, venomous, or toxic! In fact, having mayfly larvae around means that you have pretty healthy water quality and a healthy ecosystem. Any larvae present will simply hang out under rocks, within substrate, on logs, and so on. If disturbed, the larvae will do their best to get away and will not bother you, nor can they bite you.

The Life Cycle of Mayfly Larvae

During its short lifespan, a mayfly will undergo four very different developmental stages. The life cycle of a mayfly begins with a mating ritual. The male mayflies will congregate and form a swarm above a body of water. Females will then fly into the chaotic-looking swarm to begin the mating process.

A male mayfly will then grab a hold of a flying female mayfly, and attach to her using his strong front legs. They will then proceed to mate mid-air while flying around in the swarm! After the copulation event takes place, the male mayfly releases the female from his grip and she lowers to the surface of the water, where the eggs are then laid. The adult mayflies expend all of their energy in this process, and die very quickly after mating.

Egg:

Once the female lays her eggs on the water’s surface following the mating ritual, they will sink to the bottom of the water column and attach to substrate, sunken logs, or plant matter where they land. Depending on the species of mayfly, the eggs can vary greatly in size and shape. They may be oblong, round, or ovaline. After a few days to a few weeks, the mayfly eggs will hatch and larvae will emerge!

Larvae:

After hatching from the egg, the mayfly has entered its larval, or ‘nymph’ stage. During the larval stage, mayflies have extremely robust legs and a feathery set of gills that allows them to be fully aquatic. They live in this stage anywhere from a few days to a few years, and feed on detritus and plant matter. The nymph will undergo a series of molts- each one allowing it to change and grow into the next form. Nymphs can overwinter without any issue, taking refuge beneath rocks or in substrate and going dormant until the spring thaw.

Subimago:

In this subadult phase of life, the mayfly has begun to develop wings. Their genitalia, legs, and eyes are not yet fully developed and they are unable to fly. They also do not yet have the distinctive coloration that serves to attract mates for reproduction. The subimago stage is somewhere in between juvenile and adult!

Imago:

After one final moulting, the subimago mayfly becomes an adult mayfly. The subimago stage is very short lived, as mayflies are no longer even capable of eating when they reach this life stage! Their only job at this point in life is to reproduce. A mayfly may live in this form only for a few minutes, or up to about 24 hours. Adults have large, compound eyes and flexible antennae. They will also have two or three long tails and two sets of wings that are transparent in appearance. When they die, they can form thick carpets that then serve as a feast for bears, raccoons, birds, and just about anything else looking for a protein-rich meal!

How to Attract Mayfly Larvae to Ponds?

Mayflies are very beneficial to have around, so attracting them to your pond is a great move! Not only do they feed on algae and detritus, which helps keep your pond clean and maintained, but they are also a top food source for many species of fish. Sport fish like trout and bass love to feed on mayflies. Ornamental fish like koi and goldfish also enjoy feeding on mayfly larvae! If you have a clean pond that is well oxygenated, it is likely that you already have mayflies living in it.

Mayflies are beneficial to the environment. Not only do they keep fish well-fed, but they are also an important food source for many species of frogs, newts, and birds. If you want to attract diverse wildlife to your pond, mayflies will help you to achieve that.

Because mayflies require high-quality water, your pond may need some cleaning up before it becomes suitable habitat for these insects to thrive. They require well-oxygenated substrate at the bottom of the pond, so it may be necessary to implement a bottom diffused aeration device. Systems that work to diffuse oxygen in the bottom of a pond are highly beneficial. The diffuser works to break up compressed air at the bottom into small bubbles. This creates greater distribution of oxygen within a pond and helps all biological organisms that rely on oxygen to survive throughout the water column. They also prefer some water movement, so an aerator will help to provide this, as well.