Subterranean Spiders
Hampstead Heath is home to a quarter of the UK’s known spider species, and the Purse web spider Atypus affinis is one of the most unusual. Indeed its scientific name means ‘strange affinities’, as this is the only UK spider belonging to the tarantula group.
Female Purse web disturbed from her web © MG
Purse web spiders have a rather bizarre appearance, with huge chelicerae (mouthparts) contrasting with a dumpy body and short legs. When grabbing prey, their fangs move up and down in common with their tarantula relatives (most other spiders have fangs that move inwards in a pincer motion). Despite these large fangs, purse web spiders don’t have a dangerous bite.
The purse-web spider’s odd proportions suit a life underground. They build a home by digging a burrow in soft soil with their mouthparts and lining it with a tube of silk. They extend the silk tube out of the burrow mouth and close off the end, forming the ‘purse’. They then line the exposed silk with debris to camouflage it, making a structure unflatteringly described as a ’dirty sock’. The spider waits in ambush in its burrow for prey to crawl across the protruding purse web. The vibrations caused by an insect clambering over the silk alert the spider to a meal and it rushes out to grab it. This is when the huge fangs come in handy, piercing through the web to inject the prey with paralysing venom. The upper section of their chelicerae are serrated, allowing the spider to saw through its silk tube and drag the prey through.
The ‘Purse web’ aka ‘Dirty sock’ © Nick Nimbus
Having a secure home with dinner turning up at the door, the female purse web spider has no reason to leave her lair so long as it remains undisturbed. Hence they are very rarely seen. It is up to the mature males to venture out to search for a mate.
Somewhat scrawnier and leggier than the females, males crawl around in the low vegetation in late summer and autumn, trying to find female webs by scent. If he finds a purse web, a male purse web spider will try his luck at courting the female by vibrating his legs and palps (feelers) on her tube web. If she is uninterested, she will tug on the web from inside and he will move on. If there is no response he repeats his message. If she still doesn’t reject him he takes it as an invitation and tears his way into the tube to meet her.
After mating, the pair live together in the burrow until the male’s natural death. Less romantically, the female will eat her suitor’s body to help develop her eggs. When she lays them, she bundles the eggs into a silk lined egg sac suspended in her burrow and will dutifully guard the eggs and hatchling ‘spiderlings’ until their first moult.
Once moulted in spring, the spiderlings leave the burrow en masse, making silk threads onto surrounding stems to get off the ground. The spiderlings then let out lines of silk to catch the breeze and blow away to their new homes, an act known charmingly as ‘ballooning’. Once the spiderlings have set up home, they take 4 or 5 years to mature and the females may live for 8 years.
A male Purse web spider © Siga.
The purse-web spider’s unusual lifestyle and long maturing time make them a sensitive species. They can only thrive in open grasslands and heaths with loose soil for burrowing, and a sunny aspect to keep them warm. They also need to be relatively undisturbed to avoid damage to their webs by trampling.
Purse web spiders are nationally scarce and declining, with many colonies being small and isolated. They therefore must be treated with care, and are an important species on the Heath’s annual management plan.
On Hampstead Heath, Purse Web Spiders live on the sandy slopes of the Vale of Health. This is the oldest known population of these spiders in London (and for a time was thought to be the only one). They are likely to be surviving relics from a time when the Heath truly was heathland. Now the main areas for the spider are carefully managed by the City of London and Heath Hands volunteers by cutting away bramble and small tree saplings. This prevents the ‘spider slopes’ becoming overshaded without disturbing the burrows by digging. Efforts to restore heather growth in surrounding areas will likely also help these spiders.
You can learn more about Heathland and other special habitats on Hampstead Heath here or by coming on a guided nature walk. And you can support Heath Hands’ work to protect habitats for creatures like the Purse Web Spider by donating to our wildlife habitats appeal here.
