Increased response latency in Araneus diadematus (Araneidae) during putative sleep in the wild

Sleep is a seemingly universal behaviour across the animal kingdom, yet for the majority of species, experimental evidence of sleep is still lacking. The recent report of REM sleep-like behaviour in a jumping spider has highlighted the potential for spiders to become a model organism to study invertebrate sleep. While behavioural evidence of potential sleep states in spiders is strong, a crucial piece of evidence is lacking: a shift in arousability during sleep compared to awake states. Targeting a spider exquisitely suited for conducting experiments in the wild, we collected arousal threshold data for the diurnal orb-web spider Araneus diadematus. Our field experiments revealed significant differences in response latency between daytime and night-time. Using a sound stimulus of 400 Hz with increasing amplitude that robustly triggers an antipredatory response (raising of the front legs), we tested both immobile and active spiders during the day and during the night. We found that immobile spiders during the night had a significantly longer response latency to the stimulus compared to immobile spiders during the day. There was no difference in the response latency between active spiders at night and active spiders during the day. Overall, our data demonstrate a shift in arousability during periods of night-time immobility in support of a putative sleep state in A. diadematus . Our study, along with recent work on jumping spider sleep-like behaviour showcases not only the suitability of spiders for sleep research, but the need for more research into nonmodel systems where sleep remains largely unexplored.