An anonymous reader quotes the Scientific American report: Behavioral ecologist Daniela S. Robler, denied access to her lab due to pandemic restrictions, captured local jumping spiders and kept them in clear plastic boxes on her windowsill, planning to test their reactions to 3D-printed models of predatory spiders. However, when she came home from dinner one evening, she noticed something strange. “They were all hanging from the lids of their boxes,” says Robler, a doctoral student at the University of Konstanz in Germany. She had never seen jumping spiders suspended motionless on silk ropes before. “I had no idea what had happened,” Robler says. – I thought they died. It turned out that the jumping spiders were just sleeping – and that Robler had discovered an alternative sleeping habit in the species Evarcha arcuata, which was known to build silken sleep burrows in twisted dead leaves. But the real surprise came when she decided to spy on them all night. […]

Basically the spider just hung there. But then her legs, stomach, and even silk spinners began to sag. Sometimes her legs curled up to her sternum. With each spider recorded by Robler, these strange movements appeared only during isolated bouts lasting just over a minute and occurring intermittently throughout the night. “They were just twitching uncontrollably in a way that’s very similar to when dogs and cats dream and have their little REM phases,” she says. […] Robler and her colleagues wondered if twitching spiders might experience something like REM sleep and perhaps even dream. “We thought, ‘Okay, that would be crazy,'” she says. Then she thought, “Let’s figure it out,” and immediately changed her plans for spider research.

[…] When Robler recorded 34 sleeping spiders, she found that their twitches were accompanied by unmistakable eye tube movements that did not occur during other phases of sleep. […] But it is still too early to say for sure that spiders experience something similar to REM sleep in humans. The researchers first need to confirm that the spiders are indeed sleeping during this phase, showing that they are less responsive to their environment. Robler and her “dream team” of co-authors have already begun those tests. And she notes that leg curling is a particularly striking aspect of the spiders’ fast phase because the posture is usually only seen in dead spiders. Spiders use muscle-supported hydraulic pressure to keep their legs extended, and the twisting can result from the muscle paralysis that characterizes REM sleep. The team’s first findings were published in the Proceedings of the National Academy of Sciences USA.

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