Raft spider  (Dolomedes fimbriatus)

Female raft spider
Female raft spider

Facts

Kingdom Animalia
Phylum Arthropoda
Class Arachnida
Order Araneae
Family Pisauridae
Genus Dolomedes (1)
Size Female length: 13-20 mm (2)
Male length: 9-15 mm (2)
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Status

This widespread species is not threatened. It is not listed under any conservation designations (3).

Description

This large brown and white spider has long, sturdy legs and an oval-shaped abdomen. There are striking pale stripes along the sides of both the carapace and the abdomen (3). These stripes are due in part to a row of white hairs (2). Males are similar in appearance to females, although they have smaller abdomens (3).

Range

This spider has a wide but somewhat patchy range in Britain and is widespread in northern Europe (3).

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Habitat

Found in damp, swampy habitats, with patches of water (3), and is especially associated with Sphagnum bogs (4).

Biology

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 (3). The raft spider is able to crawl down water plants if threatened, and can remain below water for around an hour (3).

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 (5). Female spiders belonging to this family make very large egg sacs, which they carry around beneath their body. 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 (3).

Threats

This species is not threatened.

Conservation

Conservation action has not been targeted at this widespread species.

Find out more

Roberts, M.J. (1995) Spiders of Britain and northern Europe. Harper Collins Publishers Ltd. London.

For more on British spiders see The British Arachnological Society:
http://www.britishspiders.org.uk/

Authentication

Information authenticated by Dr Peter Merrett of the British Arachnological Society:
http://www.britishspiders.org.uk/index.html

Glossary

Abdomen: in arthropods (crustaceans, insects and arachnids) the abdomen is the hind region of the body, which is usually segmented to a degree (but not visibly in most spiders). In crustacea (e.g. crabs) some of the limbs attach to the abdomen; in insects the limbs are attached to the thorax (the part of the body nearest to the head) and not the abdomen. In vertebrates the abdomen is the part of the body that contains the internal organs (except the heart and lungs).
Carapace: the top shell of a turtle. In arthropods (insects, crabs etc), the fused head and thorax (the part of the body located near the head) also known as ‘cephalothorax’.

References

  1. National Biodiversity Network Species Dictionary (September 2003)
    http://www.nhm.ac.uk/nbn
  2. Roberts, M.J. (1993) The spiders of Great Britain and Ireland Part 1- text. Harley Books, Colchester.
  3. Roberts, M.J. (1995) Spiders of Britain and northern Europe. Harper Collins Publishers Ltd. London.
  4. Merrett, P. (February 2004) Pers comm..
  5. Arnqvist, G. (1992) Courtship behaviours and sexual cannibalism in the semi-aquatic fishing spider, Dolomedes fimbriatus (Clerck) (Araneae: Pisauridae). Journal of Arachnology 20: 222-226.