In fact two spiders qualify for this title, but they are so similar that only experts can tell them apart: Dolomedes plantarius and Dolomedes frimbiatus – the Raft Spiders. The females can span an impressive 7 cm, from leg-tip to leg-tip, with a body of up to 22mm. The males, as usual in spiders, are notably smaller, with a body of 10 -13mm.
Chocolate brown, with creamy white stripes, Raft Spiders live in watery habitats, typically fens and marshes. They are found at the water’s edge, resting their legs on the surface. Vibrations in surface tension inform them about what’s happening in their vicinity. They can run across the water to catch insects such as bluebottles or larger prey such as dragonflies, as shown in the spectacular photo above taken by Helen Smith, who is monitoring the recovery of D.plantarius, the Fen Raft Spider, by far the rarer of the two species. Check her website for more information.
Particularly astounding is the Raft Spider’s fishing technique: stirring the water with its front legs to attract small fish to the surface. The spider can submerge to catch the fish, an ability that also comes in handy for escaping from predators. Underwater they turn silvery as bubbles catch in their hair.