Losing limbs is routine for arachnids, but how does limb loss affect their ability to run? Researchers led by the Department of Biology's Tonia Hsieh and Suzanne Amador Kane, Haverford College, and published in The Journal of Experimental Biology, that immediately after losing two legs tarantulas mix up limping (alternating between balancing on two and four limbs) and running on six legs, like ants, to move as fast as they had when all eight legs were intact.

It might be hard to imagine, but dropping a limb or two is routine for spiders. If moulting goes wrong or a leg gets stuck, the pragmatic arachnids simply detach the limb just beyond the body joint. Then it regrows within a month when they are young. But how do impaired arachnids cope when suddenly relieved of a couple of limbs? Capturing dinner and evading predators could become an issue. However, going down from eight to six legs might not be a problem if spiders are adaptable. Maybe they relearn how to manoeuvre on just six legs and get better at recovering after losing limbs multiple times; some insects are even capable of trotting on fewer limbs immediately after the loss. Curious to find out, Associate Professor of Biology Tonia Hsieh, Brooke Quinn, CST '20, a PhD candidate at Brown University, and Sarah Xi, a high school researcher in the Hsieh Lab, investigated how well Guatemalan tiger rump tarantula (Davus pentaloris) spiderlings recover their mobility when relieved of two limbs. Publishing their research in Journal of Experimental Biology, Hsieh and colleagues reveal that tarantulas do not relearn how to run after losing limbs. Instead they mix up a combination of different walking styles to compensate and run as fast as they did before losing their limbs.

Quinn and Xi gently adhered the front right and rear left legs to a piece of card, waited for the young tarantulas to detach their limbs and then filmed the animals immediately from above, capturing every detail of the arachnids’ footwork as they scampered away. Then they allowed the animals to regrow their limbs, filming them running with eight intact limbs, before encouraging the tarantulas to shed their limbs once more and filming their manoeuvres again. But with over 43,000 movie frames and more than 800 strides to analyse, Hsieh needed help to get to the bottom of what the tarantulas were up to, so she teamed up with physicists Suzanne Amador Kane and Kris Wu from Haverford College and mathematician Michael Ochs from The College of New Jersey. "Suzanne is an incredible out-of-the-box thinker," says Hsieh, adding, "I don’t know of anyone else who could have come up with the novel approach for this analysis and then written up code to execute it."

Immediately after losing their limbs the tarantulas seemed to pick up running as fast as they had before, which was impressive. In addition, they recovered the ability just as quickly after the second amputation, so they weren’t learning how to run on just six legs better after the second loss. Apart from splaying their legs wider, twisting their bodies a little more to one side and weaving increasingly, they were still as mobile. However, when Amador Kane, Wu and Ochs took a close look at the spiders’ footwork, it was evident that the arachnids are remarkably adaptable.

Hsieh explains that spiders with a full set of legs should alternate between having four feet (the first and third on one side, the second and fourth on the other) in contact with the ground at any one time. And six-legged tarantulas should, in theory, have two options: alternating between having four and two legs in contact with the ground – as if limping when on two alone – or alternating between using three legs touching the floor at a time, like an ant. In practice, the spiders rarely followed these rules. Tarantulas with all eight legs sometimes left a leg lingering on the ground as the next four descended, or raised the fourth limb early, leaving just three in contact. The spiders that were down two legs alternated randomly between limping forward on two and running like an ant on sets of three legs to keep moving fast. And the tarantulas seemed to favour their hind legs, which are mainly used for propulsion, keeping them grounded for longer when down to six limbs.

So, tarantulas don’t relearn how to run after losing limbs, and they bend the rules – alternating between limping and running like ants – to continue scampering without missing a beat.

The research article is freely available at https://journals.biologists.com/jeb/article-lookup/doi/10.1242/jeb.250243