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Sialis spp.


The body of an adult alderfly is brown or clack in color, robust, and about 1 inch long. The dark, lacy wings are almost always folded in a tent-like manner along the length of the body. Alderflies can be distinguished from other Neuropterans by the lack of "tail bristles."

adult alderfly on alder leaves
adult alderfly

Distribution and Habitat

Alderflies can be found on every continent except Antarctica,but appear to be most common in regions with temperate climates.

Adults are found close to ponds and slow-flowing streams where there is plenty of silt. Larvae are water dwellers.

Life Cycle

The female lays up to 2,000 brown, cigar-shaped eggs on plants or stones near water. These eggs stand on end in flat masses of 200-500 eggs each. They hatch in about two weeks and the larvae make their way to water.

female alderfly laying her eggs
laying eggs

The alderfly larva is divided into segments, each with a pair of jointed appendages. Those on the head form vicious jaws; three pairs on the thorax form legs; and those on the abdomen are gills. The larva spends most of its time under stones and pebbles, but can swim freely by undulating its body. When the oxygen level in the water is too low, it will undulate its body in order to create a water current over the gills. The larval stage lasts about two years, during which time it grows to about an inch in length. It then leaves the water and digs into the soil or waterside debris to pupate. The pupal stage lasts about three weeks.

larval alderfly
alderfly larva

The emergence of adults is synchronized, so that large numbers of both sexes are present at the same time. The adults' sole purpose is to mate and produce eggs, after which they die.


The larval alderfly preys on on any small animal that comes its way, using its pincer-like jaws. The adult alderfy does not eat.

Scientific Classification

phylum Arthropoda
class Insecta
order Neuroptera
family Sialidae
genus Sialis

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The Robinson Library >> Science >> Zoology >> Insects >> Order Neuroptera

This page was last updated on March 23, 2018.