A window into a sleeping spider
Jumping spiders cannot move the lenses of their principal eyes. Instead, they aim their gaze by moving long retinal tubes inside the head. In newly emerged spiderlings, the exoskeleton is temporarily translucent—allowing those hidden movements to be filmed from outside.
Researchers used that brief window to observe young Evarcha arcuata through the night. The spiders were not simply motionless. During otherwise quiet periods, their retinas moved in repeated bouts, often accompanied by twitching limbs, spinnerets or the abdomen and by a distinctive curling of the legs.
Regular bouts through the night
The study filmed 34 spiderlings, each between one and nine days after emerging and only about 1–2 millimetres long, from 19:00 to 07:00 under infrared light.
Across the usable recordings, retinal-movement bouts lasted a median of 77 seconds and recurred at median intervals of about 17 minutes. Both the bouts and the gaps between them grew longer as the night progressed.
The different movements were closely connected. Every one of the 135 observed leg curls occurred during a retinal-movement bout. Retinal movement was more common than leg curling, however: only 39.5% of the recorded retinal bouts included a curl.
The researchers also distinguished these episodes from coordinated behaviours such as stretching, cleaning and adjusting a supporting silk line. Those orderly movements probably represented brief awakenings rather than the same active resting state.
Why call it REM-like?
In vertebrates, rapid eye movement sleep combines periodic eye movements with reduced muscle tone and small twitches. The spiderlings showed a striking behavioural parallel: moving retinas, twitching and broad leg curls that may reflect a temporary loss of hydraulic pressure in the legs.
Adult E. arcuata are opaque, so their retinal tubes normally cannot be watched through the body. Even so, adults displayed the same regular twitching and curling patterns while resting. When adults were filmed head-on, rapid retinal movements could also be seen during partial leg curls.
Together, these repeated and coordinated behaviours provide evidence for an REM sleep-like state in a terrestrial invertebrate. They do not prove that a spider's internal physiology matches mammalian REM sleep exactly.
Do jumping spiders dream?
The honest answer is that we do not know. The study measured visible behaviour, not the spider's brain activity or subjective experience. Retinal movement and twitching are intriguing parallels, but neither can tell us what—if anything—the animal experiences during the episode.
That uncertainty is scientifically useful. Jumping spiders are highly visual animals with movable retinas and compact nervous systems. Researchers may eventually test whether visual experiences from the day influence retinal patterns during later rest. Such experiments could probe whether the movements replay visual information without prematurely declaring that a spider is dreaming of flies.
A broader view of sleep
Sleep-like states occur widely across the animal kingdom, but active sleep phases have been studied most thoroughly in mammals and birds. Finding comparable behavioural rhythms in a lineage so distant from vertebrates raises a larger question: are REM-like states ancient, have they evolved repeatedly, or do different nervous systems arrive at similar solutions for different reasons?
One small spider cannot settle that history. It can, however, give researchers an unusually clear view of an active resting state that would normally remain hidden behind an opaque exoskeleton.
Read the study and watch the spiders
- Open full text: Regularly occurring bouts of retinal movements suggest an REM sleep–like state in jumping spiders, published 8 August 2022 in PNAS.
- The paper includes supplementary videos showing tracked retinal tubes, an hour of spiderling rest at 64× speed, leg curling and individual twitches.