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)