Biology of D.plantarius (Clerk)


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The fen raft spider, Dolomedes plantarius , is one of two species of the genus Dolomedes (fam. Pisauridae), that live in Europe. It is a large species, with adult females reaching body lengths of up to 23mm, spanning 7cm including their legs. Most individuals are also strikingly marked, with black or brown bodies and white or cream stripes along the sides of the cephalothorax and abdomen.

The species is essentially aquatic, inhabiting lowland wetland and fen areas and depending on a constant supply of neutral to alkaline water at each stage of its life. The adults are usually found in the littoral zones basking or lying in wait for their prey, with their back legs on emergent riparian vegetation, and their front legs on the water surface. Numerous chaetae on the legs equip the spider with an excellent vibratory sensory system, allowing them to detect the motions of aquatic invertebrates, which form the main constituent of their prey.

The chaetea on the legs enable them to utilise the surface tension to dart or glide across the water surface to grab their prey. They can also break the surface tension and run down stems underwater to catch prey or escape predators. The primary prey species are invertebrates, including smaller species of aquatic spiders, pond skaters and dragonfly larvae. The spiders are also known to catch small vertebrates such are fish and tadpoles, although such prey probably comprise only small proportion of their dietary intake.

Water is also essential for courtship and breeding. In the UK, females are thought to breed when they are two years old. The sensory system plays a role in mate finding and courtship. Courtship is very protracted, involving numerous careful approaches across the water surface to the female by the male, carefully tapping his legs on the meniscus. When the male and female are very close, they both bob their bodies up and down slowly. Mating itself takes only a few seconds.

The spiders of the Pisaurid genera, including D.plantarius , are also known as nursery web spiders. After mating, the females produce an egg sac, which they carry for approximately three weeks, dipping the sac under water every few hours to keep the eggs moist. Immediately prior to hatching, the female constructs the nursery web, a tent of silk up to 25 cm across, attaching it to emergent vegetation between 10 and 100cm above the water. The young develop in these webs, guarded by the mothers, usually for around a week, until after their second moult, when they disperse into the surrounding vegetation.

In the UK, raft spiders normally take two years to mature, becoming adult in their third summer. Juveniles hibernate during the winter, although little information is available on hibernation requirements. Although most nursery web building is concentrated in July and August, the breeding season is protracted. Most adult females are thought to make two breeding attempts during the summer, usually producing a larger egg sac at the first attempt. Second attempts have a much higher failure rate: by late summer the females are usually in poor condition, often with one or more legs missing, and weather conditions are usually less favourable to successful completion of breeding. The adult females die at the end of summer. The adult males die much earlier, with latest UK records in late July. There is no evidence that they are cannabilised by the females.


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