Lurking under rocks and chunks of bark by day, flattie spiders (Selenopidae) come into their own once the sun goes down. From atop their lairs, the arachnids wait patiently in the dark for a tasty insect to wander past before pouncing. ‘I am interested in all aspects of flattie spider biology’, says Sarah Crews from the California Academy of Sciences, USA, who travels the globe identifying new members of the family. Describing the spiders’ speedy reflexes, Crews recalls how Yu Zeng was impressed by the arachnids’ agility when he first saw them being fed while he and Crews were graduate students at the University of California, Berkeley, USA. ‘He suggested we film some [spiders] capturing prey with hi-speed cameras’, she says. However, Crews and Zeng were unable to analyse the spiders’ fleet footwork at the time and had to wait several years before their paths crossed again, when they could film more spiders in order to pin down the details of the arachnids' speedy manoeuvres.
Crews travelled to Central America and the Caribbean to collect several species of flatties and describes how gathering them can be difficult: she has to cautiously lower a plastic vial over the spiders, taking care not to pinch their splayed legs, ‘Which are usually bigger than the diameter of the opening’, she explains. In the lab, Zeng and Crews ensured that the spiders could not escape from the combat arena by coating the top with slippery Teflon before encouraging crickets to approach their predators from all directions. The duo then filmed the confrontations at speeds of 1000 frames s−1.
Analysing the spiders’ movements, Crews and Zeng were amazed to see that instead of simply grabbing the crickets when the insects wandered in front of them – like most other predators – the spiders wheeled around to grasp the crickets, even when the insects were located behind the spider. So, how were these spiders able to strike out over the full 360 deg range, while other species are limited to lashing out at victims that are located in front of them?
Slowing down the high-speed movies, the duo could see that the spiders performed what looked like a handbrake turn. They essentially anchored the foot closest to the prey to the ground and pulled themselves towards the appendage while extending the legs on the opposite side of the body to begin pushing the body around. In addition, they curled the remaining legs close to the body to allow it to spin more easily. If necessary, they repeated the leg extension phase of the manoeuvre as many as three times to complete the turn, spinning at up to 3000 deg s−1 while pulling accelerations of up to 4.5 g and wheeling around in less than one-tenth of a second to embrace their victims. And when the pair analysed the timing of strikes aimed at crickets that were up to 5 cm away, they were impressed to see that that spiders speeded up the attack to capture the furthest crickets, and the smallest youngsters turned faster than the older, larger spiders.
Zeng attributes the spiders’ agility and their ability to take on prey from any direction to their legs, explaining: ‘The eight legs point in different directions to form a ready-to-fire system that can immediately generate the strikes without reconfiguration’. And the scientists are now keen to find out how the spiders are able to detect victims approaching from the rear and how they manoeuvre tight turns when walking.
