Raft Spider (Dolomedes fimbrata)
If you don't like spiders look away now!
What an amazing little animal this is? The body of the female Raft Spider can be up to 20mm, over half an inch, which means those eight legs are over an inch long and the whole beast over 2 inches long from front to back tips.
The Raft Spider, as its name implies, is only found near water, inhabiting swampy areas that have larger clear pools amongst them. Although widespread they are much commoner in the south and are, because of their particular habitat requirements, quite local.
The marshy areas around Arne RSPB in Dorset is as good a place to see them as anywhere. Look for lilly pads on the many 'dragonfly' ponds, then look for the spider waiting on it with its front four legs touching the waters surface. When they detect an insect struggling in the water they run across the water and strike.
Seen in Spring and summer, to my mind they are worth a visit to Arne alone!