Ever killed a spider or seen a dead one lying around? Aside from the fact that spiders use the tactics of playing dead to trick predators or, to avoid sexual cannibalism, and others to attract (or trick) female spiders to mate, the poor spider could probably be dead. So do you wonder why spiders curl up when they die?
Getting to Know Spiders:
To address the question, we must first learn about spider anatomy and biology. Spiders are arthropods with eight legs, fangs, and spinnerets. The fangs spew poison, while the spinnerets use spiders to create their web.
Anyone who has studied biology knows that the legs have flexor muscles that help contract the legs and extensor muscles that help straighten the legs. The catch is that spiders lack extensor muscles. The hydraulic system means that spiders employ their blood pressure. The blood flows directly from their hearts to their legs, straightening them. In actuality, blood and blood pressure to the extensor muscles’ task.
So, why do spiders curl up their legs, especially when they die? Logic dictates that blood pressure governs leg straightening. Its blood pressure drops as it dies. Thus, it can no longer act as an extensor. Only the flexors remain, which explains why spiders curl up when they die.
Before Spiders Curl Up Legs and Die, They Fight for Survival:
Before we explore why these spiders curl up as they die, let us first discuss how such a small creature can survive in a world of predators.
Spiders can weave sophisticated webs and poison their prey, as you may know. The stealthy abilities spiders have acquired to become the most fearsome organisms on eight legs are far more than that. Here are some of the ways spiders catch their prey without becoming food.
Here are some clever things spiders manage to do in order to survive:
They have a super-powered sense.
Spidey senses were not created just for the purposes of comic books. To compensate for their incapacity to spin webs, jumping spiders have keen eyesight and acute hearing. Scientists assumed for a long time that spiders couldn’t hear because they don’t have ears. Jumping spiders, on the other hand, can “hear” very well—they merely use the incredibly sensitive hairs on their legs to do so, according to a 2016 study. These spiders can also sight astonishingly well, as astronomer Jamie Lomax demonstrated by luring them away from her desk with laser lights like they were tiny cats.
They appear like ants.
It’s not a new discovery that the jumping spider Myrmarachne formicaria fools predators into thinking it’s an ant by replicating its look. However, until recently, it remained unknown how it accomplishes this. The spider pulls off this deceiving maneuver while walking on all eight legs, according to a Harvard study published in the Proceedings of the Royal Society B. It takes 100-millisecond pauses during its performance to bring its front two limbs to its head, like antennae. The changeover occurs so quickly that, from above, the spider appears to be just strolling with its back six legs while elevating its front legs off the ground. High-speed cameras were used by scientists to demonstrate that this was not the case.
They adjust their webs.
Spiders have amazing musical abilities despite their lack of ears. They tune the strands of their webs exactly precisely so they can sense particular vibrations, much like guitar strings. Researchers from the University of Oxford and the Charles III University of Madrid observed garden cross spiders maintaining their webs for their study, which was published in the Journal of the Royal Society Interface. They discovered that altering the tension and stiffness of the silk helps the spiders to detect frequencies they are familiar with. One signal could indicate that prey is nearby, while another could be related to web structural concerns.
They act like bird poop.
Camouflage is common among arachnids, but orb-weaver spiders may have the most memorable disguise. In its juvenile stage, the spider will enclose itself in the center of its web with a thick, white substance. Its whitish abdomen blends in with the “decoration,” giving the impression that the spider is buried in a spatter of bird droppings. The unappealing appearance usually convinces predators to search elsewhere for a more digestible meal.
They throw nets.
When it comes to webs, bigger isn’t always better. Consider the web-casting spider: Its silken net for capturing food is small enough to fit between its limbs. The spider excretes a pale “target” onto the forest floor and then hangs above it, sometimes for hours, waiting for an insect to come along and trip a “tripwire” connected to the ground. When that time arrives, it doesn’t waste time lunging for its target and wrapping it in its web. It then bites and paralyzes its prey before launching into the feast.
Their hands can fire like tiny barbed spears.
If everything else fails, tarantulas have their spear-like hairs to fall back on. When a tarantula feels threatened, it will use its “urticating hairs.” It can discharge barbed hairs at its target like a shower of tiny throwing stars by rubbing its back legs against its abdomen. As many tarantula pet owners have discovered the hard way, you don’t have to be a predator to trigger this protection mechanism.
They somersault.
When most spiders need to flee a harmful situation, they rely on their eight limbs to scamper them to safety. The golden wheel spider wraps up its body and rolls downhills to make an even faster escape. This spider is native to the Namib Desert in Southern Africa, where steep, sandy dunes abound. When coiled into a ball, the spider can achieve tumbling speeds of 3.2 feet per second.
They make bubble submarines and scuba suits.
Spiders have adapted some very smart techniques to survive underwater for extended periods of time, even without gills. The diving bell spider spins web balloons that absorb dissolved oxygen from the water around it while filtering out carbon dioxide. Using this homemade scuba suit, the spider can stay underwater for an entire day before needing to come up for air. Then there are wolf spiders, which have a much more dramatic survival strategy. A 2009 study discovered that marsh-dwelling wolf spiders appear to drown after being submerged for extended periods of time. They flicker back to life when placed on dry land, however. They avoid death by slipping into a coma underwater.
Reasons Why Spiders Curl Up When They Die:
Flexor Muscles
Spiders do not employ muscles to lengthen their legs, resulting in this spidery leg-curl of death. Instead, they harness the power of fluids. A succession of little tubes is what composes the legs of spiders. The joint where these tubes attach to the body, known as the “hip,” functions exactly as you’d expect. It has extensor muscles to lengthen the legs and flexor muscles to curl them in. So, paired muscles that oppose one another.
For example, your biceps and triceps allow you to flex and extend your forearm. The remaining joints of spiders’ legs, on the other hand, only contain flexor muscles. Spiders pressurize their joints to straighten them up by pumping them full of hemolymph, the spider equivalent of blood.
Suffering from rigor mortis
When a spider dies, however, its body suffers from all of the regular side effects of death, including rigor mortis. That is when a corpse stiffens as its muscles contract. It occurs because a dead body ceases to produce adenosine triphosphate, or ATP, the energy source that fuels our muscles.
To modulate muscle movement, ATP interacts with calcium ions and a few particular proteins. It is required to have ATP in order for muscles to relax. Because a dead body no longer produces ATP, the muscles quickly run out of it. They get trapped in clench mode until they disintegrate sufficiently to end rigor mortis.
No more pressured hemolymph.
Being dead also means no more pressured hemolymph for a spider. As a result, there is nothing to counteract the flex of those legs, which is why they are constantly curled.
Do Spiders Curl Up When They Sleep?
While we’re on the subject, I’m sure many of you will come up with fresh questions about spiders that you’ve never considered.
Spiders do not sleep like humans, but they do have daily cycles that include activity and repose. Because spiders lack eyelids, they can not close their eyes and fall asleep like humans. But spiders have a mechanism that decreases activity and metabolism to preserve energy. We can compare here. You all have laptops and know how to put them to sleep. When you hit a computer button, it comes on again. And while sleeping, it did not lose any energy, i.e. battery.
Spiders are the same. They go into sleep mode and when they sense movement in their network, they wake up and act. Spiders don’t fold their legs to sleep. If it’s colder, they can flex their legs to remain warm. Do spiders curl up while they sleep? Spiders curl up their legs when they sleep, but not like when they’re dead.
Playing Dead: Can Spiders Curl Up Their Legs and Make it Look Like They Die?
As mentioned above, spiders are clever creatures. Too clever, they can trick predators, their prey, and even female spiders. Read on to find out.
Spiders play dead to trick predators.
When confronted with a predator, some spiders pretend to be dead. Spiders such as house spiders, harvestmen (daddy longlegs) spiders, huntsman spiders, and black widow spiders, when attacked, are known to play dead.
Spiders curl up and do not actually die, but only want to attract or trick a female spider to mate.
The females are not thrilled when male nursery web spiders “pay” for mating with useless gifts.
These gifts are decoys: useless flowers, cotton, or ant husk fragments wrapped in silk to distract the female while the males get their way. Spiders polygamously mate in the wild. A single present may not have been sufficient, but multiple gifts may have had an impact on their (or their children’s) health. Males with gifts are more likely to be accepted by females and have longer copulation than males without gifts.
Another peculiarity of gift-giving spiders is playing dead. The female drags the male along as she tries to flee with her tasty fly bite. Thanatosis is the name given to this spider’s activity. As a result, when the female stops dragging him, he is awakened and takes her for another mating.
Pretending to die permits males to continue mating and transfer more sperm. There was no possibility for the male to undergo thanatosis. Hence, extend sperm transmission, indicating that females recognized gift content and restricted sperm transfer by those males.
Spiders curl up pretending to die to avoid sexual cannibalism.
Spider sex is intricate and kinky.
Take, for example, wolf spiders. Sexual cannibalism is a behavior observed in them. For males to avoid being devoured, they have created their own tactics. A dying bug or an egg sac from another female are examples of this. The gift could help the female regulate her appetite or act as a physical shield for the male.
A study in 2006 focused on a particular male wolf spider behavior to avoid sexual cannibalism: pretending to be dead! And it works for male wolf spiders. Males who employed thanatosis were completely successful in copulating. Whereas, non-thanatosis males were only 58% successful.
Female pre-copulatory sexual cannibalism improves female reproductive success while negatively affecting male reproductive success. This sexual battle predicts cannibalism and male reproductive success. Male Pisaura Mirabilis spiders demonstrate unusual death feigning behavior (thanatosis) during courtship before mating with potentially cannibalistic females, according to our findings.
Although thanatosis is a typical anti-predator tactic, it is uncommon in sexual selection. When a girl approaches him with a present, it’s not uncommon for a man to “drop dead.” The male collapses and remains motionless in thanatosis, holding the present for both lovers. When the female consumed the gift, the male “awoke” tentatively and began copulation.
Males who feigned death had more copulation, but only for a short time. We believe that death feigning evolved as a male mating tactic in conjunction with nuptial gift-giving in order to avoid sexual cannibalism by the female spider.
Fun Fact: Spiders Curl Up When They Die, But They Don’t Bleed
Before we end, here’s one fun fact about spiders:
Spiders also don’t bleed. You probably saw a spider lose a leg with no trace of fluid flowing out. Since spiders have an open circulatory system, no blood vessels are involved. A small wound might be lethal to the eight-legged creature. Spiders have a specific mechanism that shuts off the damaged leg, preventing loss of pressurization.
Conclusion:
Spiders are arthropods with eight legs, fangs, and spinnerets. They have a super-powered sense of hearing and the ability to catch prey without becoming food. Spiders are clever species. To avoid predators, they will play dead. The jumping spider Myrmarachne formicaria fools predators into thinking it’s an ant. The spider pulls off this trick while walking on all eight legs. Tarantulas can fire tiny barbed hairs to protect themselves from predators.
Spiders can trick predators, their prey, and even female spiders into mating. When confronted with a predator, some spiders pretend to be dead to trick predators. Some spider sex is intricate and kinky, others play dead to avoid sexual cannibalism.
