The herky-jerky movements of spider legs look unnatural and creepy to human eyes.
Many psychologists believe that our primordial instincts of “arachnophobia” may well arise how we view the way spiders scurry and scuttle about at night.
The motion of their angular, jointed legs evokes shudders, more so than their orb-like eyes, arced fangs or fear of venomous bites.
For example, check this out:
Admit it, you shuddered a bit, didn’t you?!
Maybe this is the reason why spiders with lots of hair covering their bodies and legs are preferred as pets, while those with naked, spindly legs – like in the video – are loathed? Who knows?
What does go unappreciated, often, is that spider legs are an amazingly cool mode of locomotion, unlike most others in the animal kingdom.
Basic Animal and Human Locomotion – How Are Spiders Different?
Most animals possess limbs with what are called flexor and extensor muscles, as do we. Flexor muscles are what allow us to pull our limbs “in” towards our bodies – so for example, let us tuck our arms and legs in.
“Extensor” muscles, on the other hand, are what allow us to reach out or step forward – that is, we use them to thrust our limbs outwards.
Since most animals possess these two types of muscles, their movements forward are smooth, with knees and ankles rolling smoothly as the various muscles deploy – this is what our eyes are used to.
Spiders, on the other hand, look like they are walking on stilts, or coming to a brief halt and then lunging or thrusting as they move or leap forward or upward (more on this later).
This is because all the segments of spiders’ legs do not have both types of muscles. Each segment does have flexor muscles, which can draw their spindly legs inwards, but extensor muscles are typically present only at the topmost segment where the hip joint connects each leg to the cephalothorax – the bulbous and hard middle dome of the spider’s body.
The way spiders move is a natural wonder that human brains took hundreds of thousands of years to develop – hydraulics.
Hydraulics: The Science Behind Spider Movement
Hydraulics is a process whereby power is generated, controlled and transmitted using pressurized liquids.
The science and engineering involved is captured under the area of “Fluid Mechanics”, which is the counterpart of “Pneumatics” – the study of how the pressure from gases can generate and transmit power.
The human body uses hydraulic power to drive blood and fluids through our vascular system, as well as through our erectile tissue. In engineering, hydraulic pressure is used for dam design, pipe flow, fluid circuitry, and a whole host of other uses.
As a practical matter, you can see man-made machines every day – think of dump trucks and skid loaders that are controlled and operated through the use of hydraulics.
Hydraulics is a natural phenomenon in the world, though humans first started harnessing its powers about 2500 years ago.
Irrigation channels and water clocks in Mesopotamia and Ancient Egypt employed hydraulic power, as did the grand system of canals, dams, mills and underground aqueducts that commenced in Ancient Persia under King Darius I.
The Greeks, Romans, Chinese, Indians, Arabs etc. all used various forms of hydraulics throughout the next 2000 years, but the science and engineering did not start getting formalized as a discipline till late in the Renaissance period, when the likes of Benedetto Castelli (a student of Galileo Galilei) and Blaise Pascal began to write the theory behind it.
We are all used to seeing a dump truck or a garbage truck’s back portion tilt with the same pipe mechanism coming into view, but we can see how hydraulic principles are driving the entire thing in this video:
If this looks cool, think about other machines you use, like a gasoline pump, dishwasher or hydraulic brake system, that uses fluid mechanics. And after that, let’s view spidey movements through a new lens, shall we?
How Spiders Use Hydraulics
Hydraulics is the key, but let’s first look at both the exoskeleton and the musculature of a spider. All of a spider’s legs are joined to the central bulb-like “shield” atop their body, the cephalothorax. Spider movements are inextricably linked to this exoskeleton.
To move, the spider increases the pressure in the cephalothorax, sending blood flowing to its extremities, causing them to stretch outwards.
The hydraulic mechanisms are employed instinctively by spiders, increasing and decreasing pressure in tiny fractions of a second – as unthinkingly as we use our muscles – to enable them to skitter about.
Small spiders (weighing under 3 grams) use mostly a hydraulic catapult to move around – showing how powerful the mechanism actually is! Larger spiders (those over 3 grams) will use a combination of a hydraulic catapult and muscle contractions to move.
There are at least two other ways that spiders use hydraulics. First, most of us know that spiders have prodigious jumping ability – the spring-like motion made famous by Spiderman movies.
As a practical matter, jumping spiders are able to leap more than 50 times their own body lengths – a feat that Olympic athletes would never even dream of accomplishing.
The trick to it all is that they are not constrained by the quality and capacity of their muscles – jumping spiders can rush a significant amount of blood to their third and fourth limbs, boosting pressure and giving them the ability to spring.
The Actual Mechanism of Jumping
Most spiders have four pairs of legs. Each pair has a specialized task to aid movement – especially jumping. Even though their four front legs tend to be markedly longer, spiders actually use their back legs to launch their leaps.
During forward motion, the front two pairs pull towards the rear by flexing inward.
The third leg pair acts as a pivot – not unlike the pole used by an athlete to vault over a bar. The fourth leg pairs create the “spring” or push by extending from hydraulic pressure.
The other part of the hydraulic mechanism is the leg itself. Unlike our legs, which have three distinct and uniform parts, each supported by a muscle and with a joint in between, the spider leg consists of seven tubular sections across three distinct regions.
The hip joint allows for left and right, as well as up and down, movement. This joint has both extensor muscles and flex muscles. By contrast, the femur-patella, and tibia-metatarsus have only flex muscles and only allow up and down movement.
This means that while the upper part of the leg and the hip joints can extend the limbs out, the lower parts are unable to do so.
They work through pure hydraulics – hemolymph fluid (the spider’s version of blood) is pumped from the body and fills the lower femur-patella and tibia-metatarsus joints, which in turn pressurizes a bellow like structure that extends the leg.
This evolution has a significant side effect. The flex-only musculature means that the spider’s grip on its prey remains strong no matter what – while the hemolymph expands to fill the gaps between muscle fibers to both extend and stiffen the legs at the appropriate time.
Jumping spiders or Salticidae make up about 6000 species – about 13% of all spiders. They tend to move furtively but are capable of sudden jumps when hunting or as a matter of necessity, such as clearing long gaps or to escape from a threat.
They have an interesting physiognomy, marked by their eight (four pairs) of eyes, especially prominent being their anterior median (quite literally, front and center) pair.
Jumping Spiders – Phiddipus audax and Paraphiddipus Aurantis
The Phiddipus audax is one of the best-known jumping spiders. Notice the prominent eyes.
And here is the Golden Jumping Spider (Paraphidippus aurantius), with the gorgeous golden hair, almost fur, and the owl like anterior median eyes – the ones right behind and adjacent seem to stand out more due to the background color.
Entelgyne Spiders have a Different Use for Hydraulics
A different species of spiders, the Entelegyne, use hydraulics for a different purpose.
Scientists have concluded that over eons of evolution, this species of spiders have replaced the function of the genital bulbal muscles normally found in spiders with expanding membranes (hematodochae) that aid hydraulic movement.
This allows for expanded bulbal rotation and movements of higher complexity, which may enhance the lock between male and female spiders during mating.
It may or may not resonate what performers these simple, very commonly found, arachnids are.
Now that we have discussed the interesting role of hydraulics in aiding spider “movement”, let’s review a few aligned topics, which may or may not be obvious to the casual observer.
Spiders Are Extraordinarily Vulnerable to Injuries and Debilitation from a Fall
The mechanism itself is cool, but counterintuitively, these critters that seem to be able to leap tall buildings are really vulnerable to injuries, even death, if you drop them from a height onto a hard surface. Why?
Spiders Have No Bones
Spiders are arachnids. That means that they are invertebrates and do not have internal bones to buttress their limbs or other parts of their body.
The main external “frame” of the spider is the bulbous exoskeleton covering the middle of their body. The exoskeleton is made out of chitin and protein.
Chitin is a fibrous substance made of polysaccharides, what our fingernails and hair are made out of. On the good side, it keeps growing even if portions break out. On the bad side, the material tends to be brittle and can crack if a directed force lands on the shell.
The exoskeleton, being the one thing that holds most of the spider’s body together, also limits the size to which the arachnid can grow.
Naturally, since the span of the spider is determined by the length of its limbs, the exoskeleton holds the key to how far those limbs extend.
This structure leaves multiple points of vulnerability. Not only is the abdomen of a spider soft and unguarded, there are no internal bones to withstand the injury if the exoskeleton gets pierced. Similarly, the legs can actually get detached, either in whole or in segments.
Damage Extent Depends on Weight and Other Factors
Since the impact of dropping something on a hard surface usually depends on the force of gravity (specifically, the terminal velocity at the point of impact).
It is easy to posit that the damage to a spider when dropping to the ground will depend on a few things – the weight of the spider, the height from which it falls and the angle of the fall.
Smaller, lighter spiders will tend to be less damaged, whereas it is not unusual for a heavier tarantula to be inadvertently dashed to its death by a pet owner. It’s not a pleasant demise, the typical impact could split the tarantula’s abdomen open, making it bleed to death.
Spiders can also have parts, or all, of its leg detached from its exoskeleton, but there’s an interesting story on that – as recounted below.
Locomotion Can Come to a Halt with an Injury
The other thing that can cause serious harm to a spider’s ability to move is a puncture of the exoskeleton.
Such a puncture will cause fluid to leak or the pressure within the exoskeleton and the limbs to drop precipitously. Thus, the spider’s movement will be severely inhibited if its exoskeleton is pierced.
This, by the way, is why you see a dead spiders legs curled inwards – their flexor muscles have gone into rigor mortis and with the hydraulic system permanently switched off, there is no countervailing force to stretch out any of their legs.
Spiders can Regrow their Legs … Really
Like we said before, one of the vulnerabilities that spiders exhibit is the possibility that their legs come detached from their exoskeleton. While this is a huge deal which impedes their locomotion, there is a silver lining.
Spiders have the ability to regrow an appendage – similar to the regeneration of deer antlers, starfish limbs and the tails of certain lizards (e.g., the Bearded Dragon and the green iguana).
One thing, though. The spider can only grow back legs at a time when it molts and re-forms its exoskeleton – since its limbs grow out of that shell. Thankfully, spiders molt several times over the course of their lives.
When the new limb grows, it is usually thinner and perhaps shorter than the one that had been severed – it may take two or three molting cycles till the leg regains size and strength comparable to the lost limb.
If a spider is old and has no more than one molt cycle left, it may not ever regain its strength and endurance, but c’est la vie!
Biomimicry: How Scientists are Designing Ways to Imitate Spiders and Benefit Us
A study of spider movements is a fascinating study combining biology, anatomy, physiology, hydraulics and pneumatics – almost a one-stop shop for a junior scientist learning how things can work.
You can design a hydraulic arm or a similar machine using simple hydraulic kits.
Scientists have taken the concept many steps further. For example, they have created a small robot, made from a 3D printer, that uses hydraulics and pneumatics to jump and crawl – their legs are controlled through hydraulic pumps in the abdomen of the mechanical “spider”.
Another design, by Fraunhofer Institute for Manufacturing Engineering and Automation in Stuttgart, Germany, shows not only the ability to balance the robot in uneven terrain, but they are even empowered with the ability of jump – the highly rigid but light body can be propelled through the air with pneumatic legs that can bend, extend and launch the body frame.
Uses for a Spider Robot or Arachno-Bot
Think of the number of ways that such a spider robot can be used. Here are some examples of how they can be used, ranging from very dramatic to prosaic:
- Deployed in a collapsed building, or for that matter after a chemical explosion or a nuclear blast, to dig through the rubble and find survivors
- Aid rescue operations after natural disasters such as earthquakes, tsunamis or volcanic eruptions.
- Sent into a hostage situation or behind enemy lines, equipped with sensors and/or weapons – any situation that is dangerous for humans to participate in.
- Probe dangerous mines or collapsed shafts or go spelunking into narrow caves.
- Defuse bombs and land mines – spider robots can go into more impassable areas than commonly used robots with tracks.
- On a less dramatic note, they can be used to crawl through clogged drains and discover the cause of the blockage, probe caves, go into crawl spaces as an advance guard or even be deployed to guard properties.
The Franhaufer Arachno-bot design is such that it was relatively cheap to produce given the efficacies of 3D printing and efficiencies in the assembly process. As such, they could be disposed of after one targeted use in hazardous terrain.
We hope that the fun facts above have woken you up to what wonders of nature spiders really are. As we already know, most spiders are not venomous and/or super aggressive.
Many of them are spectacular creations, favorites as pets (like the dozens of colorful tarantulas prized by their owners) while in general, their ability to leap and jump at the level of an Olympic athlete only adds to their other extraordinary ability to spin a web with material as strong as steel.
Add in the fact that these arachnids help rid us of pesky pests like mosquitoes and flies. Surely, we can make the case to treat them with respect – rather than be creeped out by their weird, ungainly gait as the scurry and leap across our sights.
If you want to learn more about various insects, then checkout our site categories, we have a bunch of articles there that are totally worth reading:
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