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Alder Fly Sialis lutaria

Nowadays the Alder Flies are often considered being a complete order on their own. It is called Megaloptera, which means Megawinged. It thus is one of the smallest insectorders in Europe, represented by some 2 to 6 species in most Western European countries. All our Alder Flies are brown with big wings and clearly marked black veins. And like in the other Neuroptera these make out an intricate pattern. Of the British species only one is well known: the (Common) Alder Fly.

The Alder Fly is a darkbrown flying insect. It has a rather robust body and big wings, which are folded over the body like a roof top when the animal is at rest. The wings do contain a lot of veins. These veins are very striking indeed, for they are very well marked and black in colour. When resting the animal is very similar to the Caddis Fly. It has bare wings, though. The antennae are long and build up of numerous segments. The animal is a very bad flyer. When it flies, the flight is always slow and short. The animal rather crawls or climbs. The adult Alder Fly measures around 20mm in length. Depending on the temperatures in winter and spring, the adults appear in April or May, often appearing in great numbers at about the same time. The adults are seen in rapidly declining numbers to the end of June. Males are short lived and usually die within a day or two after leaving their pupa. Females live a bit longer and maybe around for three to four days. Even though they sometimes take some water or other fluids, adult Alder Flies do not eat. Females lay their eggs on the stems of plants above water. She lays them in tightly connected rows. On the plants the greyish brown eggs are visible as rectangular bundles. Depending on temperature the eggs will hatch within three weeks, usually all at the same time. The larvae fall into the water, or crawl to the water if they fall on land. The larvae prefer muddy, soft grounds and love living among detritus. They are real carnivores, even eating their own kind, given the chance. Their main enemies are the larvae of damsel flies and dragon flies. The larvae remain in the water for 2 years. During this period they moult no less than 10 times! When it is time to pupate the larvae comes out of the water and digs a small hole in the ground. It may also pupate between plant roots or leaf litter. The pupa is not made within a cocoon. Pupation may be extremely fast: after 4 days only the new adult enters life. Usually pupation takes longer, but rarily over 4 weeks. The Alder Fly is common over much of Britain and continental Europe. In some years it appears in enormous numbers in spring time. The animal spends most of its life under water, but it does not have many demands. Any standing or slowly floating water will do: a river or canal, a swamp, a lake or a small garden pond.