Slingshot spiders certainly live up to their names. Weaving webs in the shape of a satellite dish with the middle caved in, the finishing touch is a single silk thread, running from the middle of the web to a nearby location to anchor the structure in place. This line – the tension line – works like a bungee cord that the spider stretches to extend the parabolic shape and then releases to make the web spring forward, trapping insects in its path to ensure the spider's next meal. For many years, scientists have been interested in slingshot spiders, but relatively little was known about how the spiders work the tension line to make their webs bounce. To learn more, Symone Alexander and Saad Bhamla from the Georgia Institute of Technology in Atlanta, USA, decided to take a closer look at the catapulting spiders.
In the Peruvian Amazon rainforest, Alexander and Bhamla searched among dead branches and leafy plants to locate the spiders and their webs in their natural habitat. With high-speed cameras trained on the spiders, the researchers snapped their fingers close to the webs to trigger the spiders into releasing the tension line and catapulting their webs forward.
After analysing 15 videos of four spiders releasing their webs, the team found that the spiders used the springiness of their webs to achieve impressively high speeds. During the slingshot motion, the web reached a whopping acceleration of up to 1300 m s−2, which is roughly 130 g forces or about 10 times the top acceleration experienced by a cheetah at the start of a standstill sprint. From this massive acceleration, the web reached a top speed of 4.2 m s−1 in just 6 ms – only a fraction of the time it takes to blink an eye – allowing the spiders to catch unsuspecting insects as the web shoots forward.
Nimbleness is key here, with the spider making use of all eight of its legs – and then some. As the spider sits on the back side of the web facing the tension line, its four hindlimbs hold onto the web while all four forelegs grab the tension line. To set the web in motion, the spider lets go of the tension line, which releases the web, allowing it to shoot forward. However, with all of its legs already on duty, the spider allows the tension line to slide between its pedipalps – small leg-like appendages near the jaws – which allows it to quickly grab a hold of the line to restart the process anew.
But how does the spider engineer its ballistic web to reach such impressive speeds? Analysing the springiness of the spiders’ web lines, the team found that the silk was particularly efficient at converting stretch into bounce, even outperforming other biological and man-made materials designed to be elastic. So while the spiders may be exceptionally imaginative to use their webs so creatively, credit must also go to the springy silk that puts bounce into the structure.
Alexander and Bhamla provide new insight into how the slingshot spider uses its web like a catapult to catch insects. The team is also curious to learn more about the molecular structure that makes the silk so springy in the hope that we can use the lesson to engineer materials with superior springiness. As it currently stands, only slingshot spiders get to take advantage of their silk to go on their whippingly fast rides.