Guide to Alderflies and Alderfly Larvae 2023 (Sialidae)

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Alderfly Facts & Information Guide 2023 (Sialidae)

Snipe fly resting on wood
The most common alderfly in Europe is the mud alderfly, pictured above. Jochum Kole, CC BY 3.0, via Wikimedia Commons

Alderflies are a diverse group of winged insects that are attracted to freshwater ponds. They spend most of their lives as larvae, which maintain a fully aquatic habit. They belong to the order Megaloptera, alongside fishflies and dobsonflies, and are altogether composed of approximately 66 living species. The most common one in Europe is the mud alderfly (Sialis Lutaria), which has widespread populations in Britain.

Like other Megalopteran species, these short-lived flies are considered primitive in form. They are among the first few insect groups to have evolved to include a pupal stage in their life cycle. Winged adults emerge from this brief stage, but are unfortunately regarded as weak fliers with the sole purpose of finding a mate. At present, there are no known threats to alderflies, though many species have gone extinct and have a rich fossil history.

Alderfly larvae are a favorite meal for many aquatic predators, including economically important fish species. For this reason, anglers create “flies” that mimic the appearance of mature, highly active alderfly larvae. They go to extra lengths to imitate every detail, as “flies” that look just like the real thing are remarkably effective. The next time you want to have freshly caught trout for dinner, you’ll know exactly how to lure one in!

Alderfly, mud alderfly
Aquatic invertebrate (larvae); Winged invertebrate (adult)
Worms, crustaceans, dead invertebrates (larvae); Adults do not feed
Lakes, ponds, rivers, streams
2 – 3 years
1 – 2 cm (larvae); 1 – 2.5 cm (adult)
Most species have not yet been assessed

What Do Alderflies and Alderfly Larvae Look Like?

Alderfly larva
Alderfly larvae have 3 pairs of jointed legs, allowing them to maneuver expertly in search of prey. André Karwath aka Aka, CC BY-SA 2.5, via Wikimedia Commons

Adult alderflies are similar in appearance to flyfishes, but are much smaller. On average, they reach a length of just 2.5 cm (1 inch). Their cigar-shaped bodies are partly covered by 2 pairs of membranous smoky-looking wings, with a distinct nerve-like network. Generally, the hindwings are broader at the base when compared to the front wings, and have a wingspan of just 1 cm. Adults have 7 pairs of pointy appendages that extend from a soft, tapered, brown/black abdomen. A pair of long, beadlike antennae arise from their broad dark-colored heads, which have compound eyes but lack ocelli.

Understandably, aquatic alderfly larvae look very dissimilar to their adult counterparts. Mature larvae are flat-bodied, segmented, and have strong mouth pincers for eating a variety of underwater treats. Their upper bodies are equipped with 3 pairs of jointed legs, allowing them to maneuver quite expertly in search of prey. Extending upwardly from the abdomen (segments 1 – 7) are lateral filaments that serve as gills. Another hair-like filament, called a sensillum (used to detect external stimuli), extends from further down the abdomen. As an added measure of camouflage, their dorsal regions are typically more dark-colored than their underside regions.

Alderfly and Alderfly Larvae Habitats – Where Do They Live?

Alderfly on a branch
Alderflies are often found in riparian forests. Quartl, CC BY-SA 3.0, via Wikimedia Commons

Adult alderflies are usually found in vegetative areas next to bodies of fresh water, particularly from late spring to early summer. Adults are active during the warmest part of the day, although they may also travel short distances at night.

Alderflies are commonly associated with the Alder group of plants, to which they owe their common name. Often found in riparian forests, Alder plants include lakeside trees and ornamental shrubs that are said to improve soil fertility and reduce erosion. The Alder tree is especially known for having water-exposed roots and for dropping leaves that quickly decompose and sink. These provide nutrients for various aquatic invertebrates, including insect larvae!

Alderfly larvae tend to gravitate towards muddy or detritus-lined bottoms of shallow freshwater systems, such as ponds and streams. The larvae of some species prefer slow-moving water and silty sediments, above which they are often found foraging for food. Others may require highly oxygenated water in faster-flowing streams, with crevices or stones that provide refuge along the bottom.

What Do Alderflies and Alderfly Larvae Eat? (Alderfly Diet)

Annelid worm
Alderfly larvae feed on a wide range of macroinvertebrates, including annelid worms. Alex Popovkin, Bahia, Brazil from Brazil, CC BY 2.0, via Wikimedia Commons

Alderfly adults don’t spend their precious time looking for food, as they have just a few days of adulthood to mate. Only their larval forms are equipped with the mechanisms to forage for food. Depending on the species, alderfly larvae are vigorous omnivores or carnivores. They feed on a wide range of macroinvertebrates that dwell on the pond floor. These include annelid worms, larvae of other insects, mollusks, and crustaceans.

When food is scarce, they become opportunistic scavengers that won’t hesitate to feed on dead invertebrates. In cases where populations of alderfly larvae are high and they must compete with one another to feed, cannibalism can occur. This is certainly unfortunate for the unsuspecting victims, but is a necessary means to survival that can separate the strong from the weak. Unfortunately, many survivors inevitably become prey for larger fish and crayfish.

The Life Cycle of Alderflies

Female alderfly laying eggs
Some alderfly species lay their eggs on overhanging leaves and branches. James Lindsey at Ecology of Commanster, CC BY-SA 2.5, via Wikimedia Commons

Like most insects, alderflies have a complex life cycle, with adulthood being the shortest phase of their lives. They start off as batches of eggs that are laid by female alderflies on the water’s calm surface. Some species lay their eggs on overhanging leaves and branches. Oftentimes, these batches of eggs are notably large, as many females will choose the same surface for egg-laying. After some time, these eggs hatch into aquatic larvae that naturally fall or crawl towards the water’s surface in search of food.

The entire alderfly larval stage lasts 1-2 years, during which the larvae occasionally molt. They develop more and more anatomically complex parts as they grow through stages, which are technically referred to as instars. At maturity, they have jaws that are powerful enough to break the exoskeletons of small crustaceans and the shells of tiny mollusks. Once they’ve molted enough times (10 – 12 times for most Megalopterans) and have stored enough energy, they are ready to pupate. At this stage, mature larvae of the largest alderfly species can reach a length of 2.5 cm (1 inch).

To pupate, alderfly larvae exit the water column in search of sheltered areas – usually in soft soil, under loose logs, or in the gaps of rotten wood. Once they have chosen an appropriate space, they become enclosed in a protective shell that may accommodate only small movements. The pupae phase lasts for 3 – 4 weeks, after which alderflies emerge as winged adults.

Adults are ready to mate as soon as they exit their pupal shells. As alderflies are solitary in nature, they make use of pheromones to attract potential mates. Unfortunately, they have just 2 – 3 days to do so as they are unable to sustain themselves with food. The males die shortly after mating, and the females follow suit after depositing their eggs. 

How to Attract Alderflies to Ponds

As adult alderflies live for such a short time and emerge during a limited timeframe each year, they may be quite challenging to attract. Though they are one of the most widespread aquatic insects, they are partial to only good quality ponds. They may, at times, visit your pond as they are known to travel short distances between waterbodies. Ensuring that they find it fitting enough to mate and lay eggs there is another story.

Exactly which features would attract pairs of alderflies is generally not known. However, your chances of rearing your own alderfly populations are undeniably increased if your pond is in good health. Clean, clear water and naturalized pond bottoms and edges would perhaps draw in more alderflies, as they are likely to scout for conditions that would best suit their larvae. A mature pond, with emergent aquatic plants and thriving edge plants, would likely attract them as well.

Trees with branches extending to above the water’s surface, such as those commonly found in riparian forests, would perhaps be more effective. They are known for laying clusters of eggs on these, so the more overhanging structures, the better. A few night lights around your pond may also draw them in. The only drawback to this is you may end up attracting some unfavorable pests!

Are Alderflies and Alderfly Larvae Beneficial?

Alderflies, their larvae, and even their eggs and pupae would certainly be beneficial additions to an ornamental pond set-up. In fact, they are important indicators of a healthy pond ecosystem.

Each of the alderfly’s life stages becomes a vital part of the natural food web. Their eggs may serve as food for some of your pond’s inhabitants and wild visitors. Alderfly larvae will likewise become food for larger fish and pond invertebrates, while they play their predatory part in maintaining a well-balanced pond bottom community.

Their pupae and adult forms can become protein-rich treats for visiting pond amphibians. Ideally, a fraction of these would hopefully stay alive long enough to lay eggs! And so, the wonderful yet harsh cycle of life continues; hopefully sometime soon in your very own pond.

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