Raft spiders are large, semi-aquatic hunters that can stand and run on the surface of freshwater. In Britain, the name usually refers to Dolomedes fimbriatus, a brown spider found on bogs, wet heathland and pond margins. A close relative, the fen raft spider (Dolomedes plantarius), is rarer and closely associated with lowland wetlands.
Their size can be surprising, but their behavior is much more interesting than the idea of a dangerous bite. These spiders use the water itself to detect prey, dive when disturbed and construct silk nurseries for their young.
Raft spider facts at a glance
- Common name: raft spider; the related D. plantarius is called the fen raft spider or great raft spider.
- Scientific names: Dolomedes fimbriatus and Dolomedes plantarius (two distinct European species).
- Family: Dolomedidae, following a taxonomic revision published in 2025; older sources may say Pisauridae.
- Body length: roughly 9–20 mm for D. fimbriatus; female D. plantarius can reach about 23 mm.
- Habitat: freshwater edges, bogs and wet heathland; habitat preference differs between species.
- Food: mainly invertebrates, with occasional tadpoles or small fish.
- Human bite risk: low; defensive bites are possible, but these spiders do not pursue people.
How big is a raft spider?
A typical adult raft spider may have a body about 1–2 cm long, measured without the legs. For D. fimbriatus, The Wildlife Trusts gives a body-length range of 0.9–2.0 cm (9–20 mm). Females are often the more substantial-looking spiders.
The fen raft spider can be bigger. Adult females of D. plantarius may reach around 23 mm in body length, with a leg span approaching 7 cm (about 2.8 inches). Body length and leg span are different measurements, so a spider spanning several centimeters is not necessarily that long in the body.
Size alone does not identify a species: sex, age and individual variation all matter.
How to identify a raft spider
Look for an elongated brown to dark-brown body, long legs and a habit of resting on vegetation beside freshwater. Many individuals of both British species have a pale cream, white or yellowish stripe along each side of the front body section and abdomen. The body can appear almost velvety because of its fine, water-repellent hairs.
The stripes are useful clues, not a definitive test. The two species overlap in color and pattern; some fen raft spiders have very faint bands or none at all. For reliable separation, specialists may need to examine an adult’s reproductive structures or use genetic evidence. A photograph alone may not resolve a difficult identification.
Smaller waterside wolf spiders can be mistaken for raft spiders. Rather than relying on one marking, record the spider’s approximate body length, location, habitat and behavior. For a broader approach, see Spiderpedia’s spider identification guide.
Where do raft spiders live?
In Britain, Dolomedes fimbriatus favors wet heathland, acidic bogs and mires. It can appear beside pools or in damp vegetation, sometimes away from open water. Adults may wait on a plant with their front legs touching the water surface.
Dolomedes plantarius has a different ecological niche in Britain. It depends on suitable lowland fens, grazing-marsh ditches and other unpolluted, slow-moving or standing water. Emergent plants provide places to hunt and support its nursery webs. Loss of wetland habitat and reliable water supplies is a major conservation concern for this species.
Both spiders are European species. Elsewhere in the world, related members of Dolomedes are often called fishing spiders, but the appearance and habitat of those species can differ.
How do raft spiders walk on water and catch prey?
A raft spider does not need to build an orb web to feed. It waits near the water’s edge, using sensitive hairs on its legs to register tiny movements in the water and nearby air. Surface ripples can reveal the position of an insect or other potential prey.
Water-repellent hairs and the surface tension of water support the spider’s weight. When a suitable target comes within reach, it can run across the surface, grip the prey with its legs and use venom delivered through its fangs to subdue it.
Most meals are invertebrates: aquatic insects, pond skaters, small spiders and insects that fall into the water. Some Dolomedes also catch tadpoles or small fish, but these are not their principal food source. A raft spider can also break the surface and move beneath the water, either during a hunt or to escape danger.
Despite the name, these spiders do not need a floating leaf as a permanent raft. They can use water directly as a hunting platform.
Do raft spiders bite humans?
Raft spiders possess venom, but they are not considered a major medical danger to people. Their venom is adapted for prey. A spider that is trapped against the skin or picked up may defend itself by biting, but an undisturbed raft spider normally tries to escape rather than confront a person.
A confirmed bite could produce pain and local redness or swelling, although well-documented human bite reports for individual raft spider species are limited. It would be misleading to promise a particular level of pain or to identify a spider from the appearance of a skin mark alone.
What to do if you think you were bitten
- Wash the affected area gently with soap and water.
- Use a clean, cool compress if there is pain or swelling. Avoid applying ice directly to the skin.
- Avoid scratching the area and watch for worsening symptoms or signs of infection.
- Seek medical advice for significant or worsening pain, spreading redness, fever or other concerning symptoms. Get emergency help for breathing difficulty, swelling of the throat or fainting.
These are general first-aid precautions, not a diagnosis. See the NHS guidance on bites and stings or consult a healthcare professional for advice relevant to your location.
Do raft spiders build webs or care for their young?
Raft spiders use silk, just not a conventional prey-catching web. After mating, a female carries a rounded egg sac beneath the front of her body, gripping it with her mouthparts. As the young approach emergence, she builds a three-dimensional nursery of silk among sturdy wetland plants.
The mother guards the nursery while the young spiderlings remain together. This combination of active hunting and maternal care makes raft spiders particularly interesting relatives of other nursery-web spiders. Read more about how nursery webs work in Spiderpedia’s nursery web spider guide.
Are raft spiders endangered?
It depends on the species and place. The ordinary raft spider, D. fimbriatus, is widespread in Britain but can be scarce locally. It should not be confused with the much rarer fen raft spider, D. plantarius.
The fen raft spider is assessed as Vulnerable in Britain and is specially protected under the Wildlife and Countryside Act 1981. Wetland restoration, habitat management and carefully planned translocations have helped support its British populations.
If you encounter a suspected fen raft spider, photograph it without picking it up, disturbing its nursery or damaging surrounding plants. The wetland is as important to protect as the spider itself.
Frequently asked questions
Can raft spiders swim or stay underwater?
Yes. They can move below the surface and take refuge among submerged stems. Fine water-repellent hairs help trap a layer of air around the body while underwater.
Are raft spiders aggressive?
They are effective predators of small animals, but that does not mean they are aggressive toward humans. Their usual response to disturbance is to retreat or dive.
Can a raft spider eat a fish?
Yes, a sufficiently small fish can be caught by a large raft spider. Aquatic and terrestrial invertebrates are more typical prey.
Is every large waterside spider a raft spider?
No. Other fishing spiders, waterside wolf spiders and nursery-web relatives can look similar. Geography and detailed morphology are important, and sometimes a photograph cannot establish the species.
References and further reading
- The Wildlife Trusts — Raft spider (Dolomedes fimbriatus)
- British Arachnological Society — Water spiders
- Fen Raft Spider Conservation — Identification
- Fen Raft Spider Conservation — Biology
- British Arachnological Society — Conservation
- World Spider Catalog — Dolomedes (family Dolomedidae)
- Morris, Hazzi & Hormiga (2025) — Molecular phylogenetics of nursery web spiders
- NHS — Insect bites and stings
