image of an Alderfly
Scientific Name
Sialis spp.
Sialidae (alderflies), in the order Megaloptera (alderflies, dobsonflies, fishflies)

Adult alderflies are usually black, dark brown, or gray. They look a lot like stoneflies but are more closely related to fishflies and dobsonflies. They are generally poor fliers. At rest, the 2 pairs of heavily veined wings are held folded over the back in a rooflike (usually not flat) shape. Antennae are about half the length of the body. Alderflies lack ocelli (small, simple eyes), while fishflies and dobsonflies both have ocelli on their heads, between the compound eyes. The fourth tarsal segment (one of 5 the beadlike components of the foot) is inflated and 2-lobed.

Alderfly larvae are aquatic and look a lot like fishfly larvae, but they are usually much smaller. They lack gill tufts below the abdomen, and the abdomen tip has only 1 tail filament that points straight back from the body. They are usually fairly light-colored.

Similar species:

  • Adult stoneflies have a pair of cerci (similar to antennae) at the abdomen tip; alderflies lack cerci.
  • Adult fishflies have ocelli (simple eyes) and tend to hold their wings flat, not rooflike; they are usually larger; they lack the inflated fourth segment on each foot.
  • Dobsonflies are larger yet, and their forewings are usually more than 2 inches long, while those of alderflies are much shorter.
Adult length: usually to no more than 1 inch; varies with species.
Where To Find
Adult alderflies are usually found amid the vegetation lining the aquatic habitats their larvae require. They sometimes are attracted to lights at night. Alderflies don’t live long as adults: they mate, lay eggs, and expire soon afterward. Different species prefer different types of aquatic habitats. The larvae of many alderflies prefer slow-moving, detritus-littered waters with silty bottom sediments, such as ponds and lakes, while others require clearer, faster-flowing, better oxygenated streams.
Alderflies generally do not eat during their brief time as flying adults. In their larval stages, they are omnivorous or predatory, depending on the species.
Alderflies are one of two families in the insect order Megaloptera (the name means “large wings”). The other family in this order comprises the fishflies and dobsonflies. In the past, megalopteran insects were considered part of a much larger order Neuroptera (“nerve-winged” or “net-winged” insects); today Neuroptera is a smaller group, including lacewings, mantidflies, antlions, and owlflies.
Life Cycle
Like many other insects with aquatic larvae, most of an alderfly’s life is spent in water, growing through immature stages, with only a brief period experienced as a winged adult. Females lay eggs on leaves, branches, or rocks overhanging water, and the young move into the water soon after hatching. As aquatic larvae they eat, grow, and molt. They crawl out of water to pupate in a sheltered place; sometimes in the soil, sometimes in rotting wood or behind loose bark.
Adult alderflies are harmless. The larvae, like many other aquatic invertebrates, provide food for growing fish and are therefore important for healthy fisheries.
Adult alderflies are eaten by a variety of insectivorous animals such as flycatching birds and spiders. Alderfly larvae eat smaller invertebrates or organic detritus and are eaten by larger aquatic organisms, such as crayfish and fish. They are also eaten by the larvae of their close relatives, the fishflies, which are larger.
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Similar Species
About Land Invertebrates in Missouri
Invertebrates are animals without backbones, including earthworms, slugs, snails, and arthropods. Arthropods—invertebrates with “jointed legs” — are a group of invertebrates that includes crayfish, shrimp, millipedes, centipedes, mites, spiders, and insects. There may be as many as 10 million species of insects alive on earth today, and they probably constitute more than 90 percent all animal species.