The science of spider paws
Hairy feet help hefty spiders climb walls with the aid of van der Waals force, writes Tim Wallace.
Were you to feel one – or eight, more likely – touching your skin, the chances you would squeal with delight are not high. But pictures of spider paws have, of late, become popular on social media, eliciting the type of gushing – So cute! How adorable! – usually reserved for kittens.
An outbreak love for spiders – usually high scorers by dread-to-weight ratio – is no doubt something committed arachnophiles will welcome. But serious questions remain.
How common are cute furry paws in the spider world? Why do spiders even have them? And if you felt one (or eight) on your leg, would it still be more sensible to exclaim “eek” rather than “aww”?
The most-shared pictures of cute spider paws all belong to tarantulas, of which there are about 900 species. The two image aboves are of the pinktoe tarantula (Avicularia avicularia), a South American native that grows up to 15 cm (about 6 inches) in length. The paws aren’t really paws, in the way cats and dogs have paws, but just dense pads of hair.
These dense pads of hair around a spider’s claws are known as ‘claw tufts’. They are common to tarantulas and related spider groups.
“Claw tufts occur in a number of spider families that can run up vertical surfaces like walls and glass,” explains Robert Raven, head arachnologist at the Queensland Museum. He is particularly fond of a related group of furry-pawed Australian spiders in the Barychelidae family – brush-footed trapdoors known as ‘silverbacks’ (genus Idiommata).
“They are magnificent and gentle creatures, not so feisty as the tarantulas,” he enthuses. Growing to more than 20 cm in size, they are often mistaken for tarantulas. Their ‘feet’ and the underside of their legs are covered with dense hair that flashes iridescent in the right light.
Hefty spiders like the tarantulas and brush-footed trapdoors have hairy “feet” for a reason: the profusion of hairs creates the adhesion mechanism the spiders use to climb surfaces – van der Waals force, the electrical attraction between molecules that very close to each other. Each individual hair in the tuft is covered by hundreds of thousands of smaller hairs, called setules, that can only be seen with an electron microscope.
Based on studies of a jumping spider (Evarcha arcuata), a team of Swiss and German calculated its eight “feet” have about 600,000 setules, whose contact with a surface cumulatively creates an adhesive force allowing the spider to carry 173 times its body weight. Which is important when you are a hunter weighing just 15 milligrams. Large spiders need more hairs – and setules – for adhesion. The picture below shows the setules on the hairs of a brush-footed trapdoor spider; the scale of 10 microns, a hundredth of a millimetre, is about one-tenth the thickness of a sheet of standard office paper.
“Yes, van der Waals forces are most of the explanation” for why spiders have ‘furry paws’, says Raven. “However, there are two ways to go with this: if you are big you can get those beautiful big, dense iridescent pads as with the Idiommata; or you can get lighter, thus needing less, as in the case of huntsman spiders, which are among the few spider groups – along with jumping spiders – that can run upside down on ceilings and dangle off them with a toad in their fangs.”
Spiders have extremely diverse feet, Raven says. Originally they had two long claws and one tiny ‘thumb’ claw, and none had dense hairy pads. According to their evolutionary adaptations, some groups have lost the third claw and developed pads.
Orb-weaving spiders generally have three claws with an ‘auxiliary’ thick, serrated bristle that helps them hold onto their silk. Redback spiders have a row of spines on the tip of their fourth legs, used to comb out swathes of silk when wrapping their prey.
Hairy legs are also valuable to spiders. The final segment of a spider’s leg is called a tarsus, and its hairs – known as the tarsal comb – fulfil a lot of important functions. “The tarsi, being the most advanced point of spiders, are rich in sensory organs,” Raven explains. The hairs or bristles sense heat, movement and environmental conditions such as wind. They are ‘auditory’ as well as chemically sensitive.
Which brings us, finally, to whether ‘aww’ or ‘eek’ would be the most rational response to a closer brush with those adorable feet featured in our first few images. That depends on how you feel about spiders generally, and big spiders more particularly.
Some people love tarantulas, and keep them as pets. Whether one should pet them or not is another question. While they do pack venom to subdue their prey, tarantulas aren’t particularly venomous and don’t pose a threat to humans. They tend to bite things too big to eat only when riled. If they do bite, however, the sheer mechanics of their size means it is going to hurt.
The mightiest of the furry-pawed spiders is the Goliath birdeater (Theraphosa blondi), the world’s largest spider by mass (weighing up to 175 grams) and body size (12 centimetres in length). As its name suggests, it is plenty big enough for one to think it could catch and eat a bird, though in fact it mainly feeds on large insects and the occasional frog, lizard or mouse. Though opinions about its suitability as a pet are mixed, it is relatively harmless and you could theoretically enjoy having one pad up your leg. However, you definitely want to avoid contact with its abdomen hairs, known as urticating bristles, which the tarantula uses for defence. They work like poison darts, causing extreme irritation when injected under the skin.
Bottom line: luxuriate in spider paws with reckless abandon from afar, stroke furry spider legs with appropriate caution, and definitely avoid the temptation to give a tarantula a belly rub.