DS0D0648-Raft-Spider-or-Swamp-Spider,-Dolomedes-fimbriatus

            Raft Spider (or Swamp Spider), Dolomedes fimbriatus.

Found in damp, swampy habitats, with patches of water, and is especially associated with Sphagnum bogs.  This spider is widespread on southern heathlands, but very scattered elsewhere in Britain, north to Scotland. It is widespread in north-western and central Europe.

Although this species is widely known as the 'Raft Spider' in Britain, it does not make rafts. It is more appropriately called the 'Fishing Spider' in Europe, as it hunts by sitting on vegetation next to pools of water, characteristically with the first two pairs of legs held together at an angle and resting on the surface of the water. This allows it to sense the presence of tadpoles, insects and even small fish, which it hauls out of the water. This species also hunts on land amongst vegetation, and on moss. The Raft Spider is able to crawl down water plants if threatened, and can remain below water for around an hour. During courtship, males signal to females by making regular surface waves on the water by jerking their abdomen up and down, and waving their legs in the air in a characteristic fashion. Females are very aggressive towards males, and in some cases they eat prospective mates. Female spiders belonging to this family make very large egg sacs, which they carry around beneath their body, as in this photograph. These vary in diameter from 8-15mm and are held in the chelicerae and often attached to the spinners as well. The abdomen is curled around the sac and so often only the carapace is visible. When the time for the spiderlings to emerge approaches, the female deposits the egg sac on a leaf and spins a protective silk 'nursery web' around it. She then opens the egg sac slightly, and stands guard over it until the spiderlings emerge. 
 
As adults, the two European Dolomedes species are usually striking and distinctive in appearance. As well as being very large as adults, females 18-23 mm and males 13-18mm, they are characterised by a white, cream or yellow stripe along the sides of both the carapace and abdomen which contrasts with the dark body. 
However, unbanded adults also occur, particularly in D. plantarius populations, and can be more difficult to distinguish from other spider species, as can both banded and unbanded juveniles. At all growth stages there is considerable variation in body colour as well as the colour and width of the stripe. Body and stripe colour may be the same or different between the carapace and abdomen. Stripe width and boldness is also very variable and some individuals have paired white spots along the abdomen.
Their bodies are essentially cigar-shaped and powerful in appearance, tapering towards both ends. The legs and body are short and hairy and when the spiders are under water, air trapped in the body hairs makes them appear silver.
In common with other spider species, the sexes are indistinguishable until they are subadult. As well as clubbed palps, the mature males have a broader, slightly shiny carapace, narrower abdomen and longer legs than the females. Their overall body length is usually several millimeters less than that of the females. Adult females are most conspicuous and unmistakable when they are carrying their bulky egg sacs. 
   

Taken on: January 3, 2021

Tags:

''swamp

spider''

''dolomedes

fimbriatus''

spiders

''british

wildlife''

insect

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