The question of whether spiders curl up when they sleep often arises from observing inactive spiders. While the curled-up posture might seem like peaceful slumber, the true nature of sleep in arachnids is complex. This position is generally an indicator of physiological distress, which is why the two concepts are often confused. Understanding how scientists define sleep in invertebrates and examining the reasons for the curled posture reveals a significant difference between true rest and a state of emergency.
Defining Sleep in Arachnids
Scientists define sleep in invertebrates using behavioral criteria, since they lack the complex brain structures required for EEG measurements used in vertebrates. Primary markers include a prolonged period of immobility or behavioral quiescence. This state must be reversible upon stimulation, distinguishing it from paralysis or a coma.
Another defining characteristic is an elevated arousal threshold. This means a sleeping spider requires a stronger stimulus or more time to react compared to an awake spider. The state must also show homeostatic regulation, where sleep deprivation is followed by a compensatory increase in the amount of sleep. This regulatory mechanism suggests a biological need for the rest period.
The adoption of a species-specific posture is also a factor, though this varies widely across arthropods. In arachnids, a sleep state is a regulated behavior with clear physiological consequences. This differentiates true sleep from simple, non-regulated immobility.
Why a Spider Might Be Curled Up
The observation of a spider with its legs drawn tightly inward is strongly associated with a deceased spider. This posture, known as a flexed posture, is a mechanical consequence of the spider’s unique hydraulic system, not a sleeping habit. A spider’s legs do not contain extensor muscles, which are needed to push the limbs outward.
Instead of extensor muscles, spiders rely on hemolymph, their internal body fluid, which is pumped into the legs under pressure to extend them. The legs possess flexor muscles, which draw the legs inward. When a spider dies or is severely dehydrated, the internal hemolymph pressure drops significantly. Without the opposing hydraulic force, the flexor muscles contract the limbs into the characteristic curled-up position.
This flexed posture indicates a loss of hydrostatic pressure and can signal death or severe dehydration. A spider may also enter a state of torpor due to cold or lack of food, slowing its metabolism drastically. In these instances, the posture is a sign of physiological failure or extreme inactivity, not a regulated state of true sleep.
Observing Active Rest Behaviors
Recent scientific observations show that true spider rest looks very different from the curled-up misconception. Studies on the jumping spider Evarcha arcuata documented a specific nocturnal resting behavior where the spider suspends itself from a silk thread. While in this immobile, pendant posture, the spiders exhibit periodic bouts of activity that align with the criteria for an active sleep phase.
These bouts are characterized by brief, involuntary movements, including rhythmic twitching of the legs and abdomen, and slight spasms of the spinnerets. In young spiderlings, whose exoskeletons are temporarily translucent, researchers observed clear movements of the retina. These retinal movements occur concurrently with the limb twitching.
The combination of periodic limb movements and rapid retinal movements strongly suggests a state similar to Rapid Eye Movement (REM) sleep found in vertebrates. These REM sleep-like bouts are short, lasting 30 to 90 seconds. They increase in duration and frequency throughout the night, supporting the idea of a regulated sleep state involving distinct physical behaviors.