Seaweed flies are 5-12 mm long and dark brown to black. The dorsum of the thorax is flattened, and the body and legs have numerous bristles. They occur along the seashore where seaweed (kelp) has washed on shore in large amounts. The larvae breed in the seaweed, and the adults often swarm over it in large numbers, and they are sometimes carried inland where they can be nuisance pests around houses. Shore birds may be attracted to and feed on the flies gathering at the seaweed. Other flies are associated with accumulations ofseaweed, including species ofFucellia (Muscidae), Thoracochaeta (Sphaeroceridae), and Helcomyza (Dryomyzidae).

Seaweed fly, Coelopa frigida Adults are 3-11 mm long, slightly flattened, and dull black or brown. The head and legs have strong bristles, and the thorax has long bristles at the sides only; the wings are clear to slightly brown. Full-grown larvae are about 16 mm long and yellowish white; there are fine spines on the anterior margins of segments 1-3. Anterior spiracles have 16-17 lobes; posterior spiracles are widely separated and surrounded by setae. There are star-like spines above the anal lobes. Larvae feed in aggregations (as many as 600 per liter) in piles of wet seaweed (Laminaria, Fuscus) on the shoreline in protected places. Seaweed along the tidemark dries quickly and is usually not infested. Eggs are laid in 3-5 batches directly on the surface of decaying seaweed; fecundity is about 80 eggs. Hatching occurs in about 3 days. Development takes about 28 days and the pupal period lasts about 14 days. Under laboratory conditions of 25 °C, development takes about 12 days. Adults either remain on wet seaweed during cold weather or hover 2-3 m above this substrate in large swarms. Sometimes they travel or are carried by wind inland away from the shore, and may be a nuisance in commercial and residential areas. Adults are attracted by the odor of the chemicals trichlorethy-lene and carbon tetrachloride, which are sometimes used in the process of commercial dry cleaning. This fly is found along most of the European coastline from Biscay to the Arctic, and also in eastern and western North America and Asia.

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