Raft Spider (Dolomedes fimbratus)

Raft Spider (Dolomedes fimbratus)The raft spider is, without a doubt, our most impressive spider. Primarily a spider of bogs and fens, it is always found close to water, is semi-aquatic and has an intriguing hunting technique that’s unique among Irish spiders.

With a body length of up to 2.2cm (just under 1 inch) and a leg span approaching 7cm (c. 2.75 inches) this is Ireland’s largest spider by some margin. It is a chunky, heavy-set spider, mostly brown in colour with a distinctive pale stripe running from the head down each side of its fat, cigar-shaped body. Body colour, stripe colour and stripe width are all extremely variable in this species, but the spider’s large size, robust build, general appearance, behaviour and habitat choice are very distinctive. Confusion with any other Irish spider is highly unlikely.

Although widely known as the raft spider in Ireland and Britain, it does not, in fact, make rafts. It does, however, sit motionless at the waters edge, or on the edge of floating vegetation, with its front legs extended to touch the water’s surface. In mainland Europe the species is called the “fishing” spider, a much more descriptive name that reflects the spider’s hunting behaviour.

What the spider is actually doing is using the surface tension of the water to detect minute vibrations made by insects or other creature. When an unsuspecting victim comes into range the spider springs into action, darting across the surface of the water to grab its prey.

Surprisingly for a spider so large, it does not sink. Its broad leg span and special hairs on the tips of its legs allow it to glide across the water’s surface. It can also break the surface tension when necessary, diving under the water in pursuit of aquatic prey or to evade land-based predators. Air trapped under its body hair makes the spider appear silver when underwater.

Raft spiders take a wide range of prey species, including terrestrial and aquatic insects, smaller spiders, dragonflies and their larvae, tadpoles and even small fish like sticklebacks. Although the bite of this large spider is incredibly effective at immobilising its prey, it is completely harmless to humans.

As well as providing rich hunting grounds for the raft spider, water is also essential for its reproduction. Courtship is a protracted affair, involving a very careful approach by the male across the water’s surface. He has every reason to be cautious. Should he inadvertently trigger the female’s instinctive predatory response he could easily become her next meal.

When he finally gets close enough to the female both sexes bob their bodies slowly up and down in a spidery courtship dance. Mating itself only takes a few seconds, after which the male makes a swift exit.

The female spider then makes an egg sack and lays several hundred eggs into it. She carries the sack around with her for about three weeks, periodically dipping it into the water to keep the eggs moist. Then, just before the eggs hatch she builds a nursery web in vegetation close to the water. Here she guards her young spiderlings for around ten days or so until they disperse into damp vegetation nearby.

If conditions are favourable some females may attempt a second, smaller brood later in the summer, although these are generally less succesful. Adult females die in the autumn, soon after rearing their young. Males generally die much earlier in the year.

In Ireland raft spiders are thought to take two years to mature, becoming adults in their third summer. Juvenile spiders hibernate through the winter months and emerge the following spring, although very little is known about this particular part of their life cycle.

(Photo Credit: Nigel Jones via Flickr)

About Calvin Jones

Calvin Jones is a freelance writer, author, birder and lifelong wildlife enthusiast. He is founder and managing editor of IrelandsWildlife.com. He is also the tour leader and wildlife guide on our West Cork based Discover Wildlife tours.
Calvin is also co-author of bestselling digital marketing titles and offers digital business consulting services and training through Digital Marketing Success


  1. Jonathan Wilson says:

    Hi Clavin,

    Was stacking some turf yesterday and came across a large spire which I hadnt seen before. He was very chunky and matches the sizes mentioned above buy was more silver in colour and had almost pink coloured stripes down his body in the same location as the spider in the picture.
    Does the colour variation extend this far in this species?


    • Hi Jonathan,

      Thanks for your comment. I’m not certain of the amount of colour variation you get in raft spiders, and have never been lucky enough to see one for myself, but spiders in general can exhibit pretty prominent colour variation across their range. I don’t think there’s any other spider in Ireland with similar markings, and there are certainly none so large as the raft spider, so it sounds like that’s probably what you saw.

      Thanks again for the comment.

  2. D. mcdermott says:

    Hi Calvin,
    i would like to bring to your attention a spider as i’m concerned is not in the textbooks yet, while footing turn in a bog in kildare. i went to wash my hands in a bog hole (about 12 inches diameter) when a spider approximately 3 inches overall jumped out of the water hole at very fast speed, landed on the edge of the bank between my friend and i.

    its colour was bright blood red, it looked like it wore a coat of Armour as if it was like a type of crustation. the water sat on him like grease on water making him look like he was very waterproof was it a beetle or spider? it did have 8 legs i asked my mate who has worked on the bogs his entire life and asked him “what type of spider was this” and he told my he had never in his life seen anything like it. to me it was like something you would see in the amazon rain forest. it stayed on the bank for about a minute until jumped with tremendous speed back into the water hole.

    if ever there was a time that i felt like a kid and wanted to put it in a jar today was my day. lol although anxious and privileged to have seen such an indescribable wonderful insect/creature i stood back and said “live and let live” the only decision i changed that day was “don’t wash your hands in the bog hole”

    Calvin i haven’t drank for 16 years and my eyes have been open wide ever since when i tell this story to people who know of the bogs many look at me as if i have 10 heads. i understand now that the bogs are which certain stately bodies are now trying to preserve and protect the wildlife.
    i am sure i have discovered an undetected ‘specie’ of spider.

    this was approx 15 years ago and to this this day i still speak of it.
    p.s blessed are those that see 🙂

    i eagerly wait your reply.

  3. Adrian Murtagh says:

    Hi Calvin, seeing as how you know a bit about Irish spiders, you might be interested in this very interesting specimen we came across today in our local bog in east Co. Galway. It appears to be a raft spider as you have described, but the colours on display were very striking, bright yellow bands down his back and a distinct blue-ish tinge on his legs. he must have measured almost 2″ in length as you can see from the photos. The most unusual spider I have ever seen in this country.

    The funny part is it was my ‘townie’ wife who turned over the sod of turf on her very first proper visit to the bog who found him, and she has a rather advanced dose of arachnophobia!



    • Hi Adrian — it’s definitely a raft spider. Cool find! I’ve yet to see one of these guys in Ireland. I understand that their colouring can be quite variable depending on local environment and other variables. Thanks for sharing!

      • Maurico Pipistrello says:

        Dear Calvin, if you are interested of taking good photos, you can do so in our greenhouse in South Co Galway as we are accomodating one at present 😉 About 2-3 inches, deep chocolaty brown with mustardy-yellowish stripes on the back etc.

        Best regards!
        Wren’s Nest Fruit Farm, Arboretum & Wildlife Sanctuary

        • Wow… lucky you!

          Not sure I’ll get all the way up to Galway from West Cork to see him / her unfortunately — but if you have some photos please do share them via the Ireland’s Wildlife page on Facebook (if you use it) or the Ireland’s Wildlife Group on Flickr.

          • Maurico Pipistrello says:

            Thanks Calvin 😉

            Yes, indeed I managed to take few good shots and I posted one up as suggested for you to enjoy on Wildlife page on Facebook and I will also upload few more under my own gallery of Irish Wildlife there as well!

            I will have some images of Basking Shark and Dolphins to go up as well of our last trip to Clare Coast.

            Best luck with finding your own Raft Spider mate 😉

            Wren’s Nest Arboretum, Fruit Farm & Wildlife Sanctuary
            Shanaglish, Gort, Co Galway

    • Came across a Raft Spider identical to what you have photographed. I found him in the workshop and it scared the bejaysus out of me. I never knew they existed. When you don’t know what it is the yellow stripes make you wonder if it is venomous 😀

  4. araknaphobe says:

    I know now to never visit a bog lol

  5. How can this be irelands largest spider with a leg span of only 7cm? I’ve seen house spiders recently, one in particular who had a leg span of 12 to 15cm easily!

    • Hi Paul,

      The raft spider is substantially longer and bulkier in the body than the house spider, has much thicker legs and is generally a heftier beast in all departments. House spiders (Tegenaria domestica) do have comparatively long, quite skinny legs compared to their body, and that often leads people to overestimate their size. They can get quite big, of course, but 12-15cm sounds a bit excessive.

      • Joan Dee says:

        I think length should determine what is the largest. When people generally talk about the biggest/largest spider in the world, the phrase ‘plate-sized’ is used generally and this description describes the overall size from leg tip at one end to leg tip at the other, so our spiders should be described likewise. I don’t mind the average sized house spider, but can’t bear these large Tegenaria’s in the house. I just saw one last night which was 9 or 10 cms in total length, huge in my view and I regard these as the largest spiders in Ireland. I know he was that size because the area of wall he was on is exactly five inches wide beside a fireplace and there was only a cm. of wall either side of the tips of his leg span, so I agree with Paul above, although I also agree 12-15 cm. is probably exaggerated! Unfortunately this one moved before I could take a photo.

  6. Being a bit of a ‘nature buff’, today I got the job of removing a ‘huge’ spider from a man’s bath tub. It turned out to be Tegenaria domestica. The man said it was the biggest spider he had ever seen. Anyway, I took it out the back and, no doubt, it quickly went in the front.
    I once came across the Raft Spider which was minding a few infants…..about 300….in a bog stream. She was perched on some tall vegetation in the middle of a drain and every time I tried to get a photo, she went behind the ‘stalk’.
    What worries me is the fact that most ‘people’ have no idea that this is wildlife in their midst but, they get to vote.

  7. Andrea Murray says:

    I got bitten by a raft spider once. It leaped from the grass onto my descending thumb and it hurt like hell! I had disturbed it from under the water but I didn’t know what it was I thought it was drowning. When I took it out it didn’t move so when I put my thumb down to kind of nugge it well it leaped and bit it. Put it in a glass after that and it was very aggressive, so would i be if I had been taken from home. Anyway this was some years ago, but I found unusual was that it was under the water the whole time. It was just under there for ages. Wouldn’t it drown after ten minutes. It was greenish and had those silver lluminous parts clinging to certain parts of it’s legs from the water bubbles. I could also see every eye in it’s head so it was definitely a raft spider. I was told told to kill it but I didn’t, I let it go which I am glad I did as it isn’t some unusual beast from abroad. The water at the back of the house has since been filled in so no more spider. how many are there in the country? Are there a lot. I only seen one once. Amazing creatures

  8. Hi Calvin,
    I set down some organic ant bait stations for silverfish insects I have seen in my house, it attracted plenty of ants, however last night when I was refilling it I noticed a small brown coloured spider on the bait station, it had a very distinctive single red dot on its back. I assumed it was dead as it didn’t move when I lifted the station up. I thought the spider was quite odd looking but It was late and I was tired so I headed straight to bed with very intention of getting the spider contained in a jar in the morning, That was not to be as the next morning the spider was gone. I have a small child in the house and I am worried this spider if a poisonous form. I have searched the internet but nothing seems to fit the exact description of what I saw. Would you have any insight that might help?
    Many thanks,

    • Sorry Donna, I don’t know really from that description. However none of our native Irish spiders pose any threat whatsoever to people, so unless you have an exotic interloper on your hands you should be safe enough.

  9. Hi seen large raft spider full grown on nephin mountain yesterday large reddish brown really cool spotted by Fionn and Brian

  10. Hi can you send me your email so I can send you a photo of a much bigger spider in my back garden . Brian Tallon, Athlone. briantallon1@Gmail.com


  1. […] dear – don’t go swimming in the ‘bog or fen’ … check out  the Raft Spider. Harmless sure, gives me the willies […]

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