IUCN threat status:

Vulnerable (VU)

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Biology

Fen raft spiders are predatory and do not build webs to catch their prey. They hunt from perches at the water's edge, typically sitting with their back legs on a stem and their front legs resting on the water surface in order to detect vibrations set up by potential prey. They can rush across the water to seize prey items, using the surface tension to support their weight. It is this ability to sit on the water surface that has given rise to the name of 'raft' spider. They can also break the surface tension to run down stems under the water to catch prey or escape from predators. Raft spiders are voracious hunters. Adults eat drowning terrestrial insects and many aquatic species, including pond skaters, other species of aquatic spiders, dragonfly larvae and even sticklebacks. In Britain the spiders are thought to live for just over two years, maturing into adults in their final spring. Adults females die at the end of the summer but most males die by mid-July. Courtship takes place on the water surface and is elaborate and protracted, involving slow and careful approaches by the male, accompanied by tapping the front feet on the water surface and a slow bobbing of the body. Females lay several hundred eggs into a silk sac, about 1 cm in diameter, which they carry around under their bodies for around three weeks. During this period they select a site in vegetation above the water surface where they build their nursery web once the young are ready to hatch. Particularly in hot weather, females descend at frequent intervals to dip their egg sac under the water to keep it moist. Nursery webs comprise a large tent of webbing built between 10 and 100 cm above the water. The females guard their young in the web until they disperse into the surrounding damp vegetation, usually after five to nine days. The breeding season lasts from late June to late September, with most females making two breeding attempts. Fen raft spiders hibernate during the winter, from the first frosts until warm weather returns in February or March. Little is known about their hibernation although they are thought to hide amongst leaves in the dense bases of sedge tussocks.

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Source: ARKive

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