There are spiders that encase their bodies in thick armour

The tetrablemmids are unique among spiders in the steps they take to protect themselves from attack, and they have been around for 100 million years

In the steamy tropical forests of the Late Cretaceous, a male spider stirs beneath the scaly bark of a prehistoric conifer. He scuttles confidently along the tree trunk despite his tiny size – he is just a few millimetres long. Perhaps his unusually heavy suit of organic armour explains his self-assurance.

Yet it offers him no protection when, without warning, he is engulfed by a dribble of resinous sap running down the trunk. Within minutes the spider is dead. The sap dries around him and becomes hard amber.
About 99 million years later, this tiny spider has been unearthed, still encased in his tomb of amber, in modern-day Myanmar, also known as Burma. His species, Electroblemma bifida, is the newest addition to a remarkable and strange family of spiders known as the tetrablemmids.
Each of the 161 members of this group boasts several toughened layers of exoskeleton covering its body, including distinctive overlapping plates running down its bulbous abdomen.
"All spiders have hard plates covering the upper and lower parts of the front of the body," says Paul Selden, director of the Paleontological Institute at the University of Kansas in Lawrence, who was part of the team that discovered E. bifida. "But few have hard plates on the abdomen."
Exactly why this small group of spiders have encased themselves in such formidable armour while the majority of other arachnids have not is an enduring puzzle among arachnologists. However, new research is starting to provide clues. The armour may offer more than mere protection.
Like all arthropods, spiders' bodies are contained within a hard "sclerotised" exoskeleton. In most spiders, the toughest part of the exoskeleton is the carapace, which sits on top of the front section of the body; the cephalothorax. The carapace helps to protect the spider's delicate eyes, mouth and other sensory organs.
They really look like little tanks 
On the underside of the cephalothorax is another toughened plate called the sternum. The carapace and sternum are joined by thinner, less tough material, through which the legs connect to the body.
Behind the cephalothorax lies the abdomen. In most spiders this is covered in a much thinner and more flexible cuticle material made of chitin, which allows the abdomen to expand and contract during feeding.
Armoured spiders differ from this standard plan in two key ways.
First, the carapace on their cephalothorax is usually far thicker and is fused to the sternum plate, leaving no vulnerable join that a sharp point could creep between.
These voracious insects swoop down onto their prey to deliver a paralysing sting 
Second, they have tough armoured plates covering their abdomen, each hardened with extra sclerotin. Beneath this sclerotin, the cuticle membrane is also thickened – about three times thicker than the cuticle of soft-bodied spiders. It is like having a layer of chainmail on underneath a suit of armour.
"They really look like little tanks," said Christian Kropf, an arachnologist at the Natural History Museum of Bern, Switzerland, and the world's leading authority on tetrablemmids. "[The armour] is astonishingly thick when you compare it to spiders of the same body size."
Many of these tiny spiders measure just 2mm long and less than 0.5mm wide, but their cuticle can be 14-16 micrometres thick in places. If scaled up, these beasts would have armour thicker than a World War Two Panzer Tiger tank.
Of course, the spiders do not need to deflect shells or bullets. 
Instead, they face a smaller but no less formidable foe.
The tropical and subtropical forests of South East Asia, where most living tetrablemmids are found, are prowled by aggressive predatory wasps that specifically hunt spiders.
The tetrablemmids do not have such a weak spot 
These voracious insects swoop down onto their prey to deliver a paralysing sting that leaves a spider immobilised. Then the wasp drags its victim off to its nest where it lays a single egg on the helpless arachnid's abdomen. When the larva hatches, it eats the spider alive.
Spider-hunting wasps will tackle prey several times their own size. Some species are capable of taking on tarantulas.
This is where tetrablemmids' armour may come into its own, according to Kropf.
"These wasps always sting spiders in the soft [region] between the plates covering the top and bottom of the front section of the body," he says. 
This is where the spider's brain is located, so the arachnid is immediately immobilised.
However, with their hardened plates fused together, the tetrablemmids do not have such a weak spot. The wasps probably find them very hard to attack.
If the tetrablemmids' armour is so beneficial, why has it only evolved in one small group of spiders? 
The spiders' armour may also protect them against other hunters. Most tetrablemmids are found living in the leaf litter on the forest floor, although some are found living under bark and a few even inhabit dark caves. The soil on the forest floor is the domain of predatory mites that feast on any arthropods in their way.
"There was a study conducted in the 1980s that offered soft-bodied and sclerotised arthropods to these mites," says Kropf. "They always preferred the soft prey.
"When we were collecting Tetrablemmidae in Thailand we would find these mites everywhere. It is possible that these armoured bodies may give the spiders more general protection against all kinds of predators, not just predatory wasps."
But in a sense, this turns the original question on its head. If the tetrablemmids' armour is so beneficial, why has it only evolved in one small group of spiders?
One possibility is that it is too bulky to be practical for most spiders. Like Medieval knights, the tetrablemmids may find their movements are far more restricted than their less heavily-armoured cousins.
In line with this, only a few armoured spiders spin webs. Others simply produce a messy tangle amongst the leaf litter on the forest floors. It is tempting to conclude that the extra armour plating makes it harder for tetrablemmids to manoeuvre their abdomens to produce delicate silk structures.
Some of the larger members of this family – which can grow up to 11-12mm (0.4in) long – do produce huge webs though. In pristine patches of forest in Thailand where they live, these wonderful webs can have a diameter of more than 0.5m (1.6ft).
The muscle structure inside the armoured spiders appears completely different There is another reason to suspect that the heavy armour does not interfere significantly with the tetrablemmids' movement.
Spiders move their legs using a combination of muscles and fluid pressure inside their bodies. By squeezing the plates on the top and bottom of their cephalothorax, they can produce the pressure needed to push fluid into the legs, causing them to extend, much like a garden hose when it fills with water.
Muscles in each leg oppose this pressure, allowing the spiders to gain fine control over the movements of their legs.
However, squeezing the body in this way can also cause the base of the leg, where it attaches to the body, to move around. Most spiders have lateral muscles that control this movement.
When Kropf looked at the legs of armoured spiders, he found some differences.
"Those muscles are simply not there," he says.
The muscle structure inside the armoured spiders appears completely different. Rather than the bundles of striated muscles, the armour plates provide attachment points for smooth muscles, which are normally found in blood vessels and the stomach.
I wondered how these spiders stretched their legs 
"Smooth muscles need much less energy than striated muscles," says Kropf.
So rather than slowing the spiders down, the extra armour may actually help them to save energy and move more efficiently. In other words, it is still a mystery exactly why so few spiders have extra armour.
Kropf has also spotted something else.
Without the flexible membrane between the upper and lower plates of the cephalothorax, the tetrablemmids are totally unable to squeeze them together to create the fluid pressure they need to move their legs. This should leave them hopelessly immobile.
"In armoured spiders, the whole prosoma [another name for cephalothorax] is a stiff capsule," says Kropf.  "This seemed like a problem to me when I started studying these spiders. I wondered how these spiders stretched their legs.
The adult spiders seem to be remarkably long-lived for their size 
"What I found out is that they use their abdomen as a pressure pump. This explains why the abdomen is so heavily armoured. All these strong sclerotised plates serve as attachments for strong muscles in the abdomen."
Essentially, the abdomen, with its thick plates on the top and bottom along with the strips of hard cuticle along the sides, acts like a bellows that the spiders can squeeze and expand at will to pump fluid around their bodies.     — BBC

There are spiders that encase their bodies in thick armour

The tetrablemmids are unique among spiders in the steps they take to protect themselves from attack, and they have been around for 100 million years

In the steamy tropical forests of the Late Cretaceous, a male spider stirs beneath the scaly bark of a prehistoric conifer. He scuttles confidently along the tree trunk despite his tiny size he is just a few millimetres long. Perhaps his unusually heavy suit of organic armour explains his self-assurance. Yet it offers him no protection when, without warning, he is engulfed by a dribble of resinous sap running down the trunk. Within minutes the spider is dead. The sap dries around him and becomes hard amber. About 99 million years later, this tiny spider has been unearthed, still encased in his tomb of amber, in modern-day Myanmar, also known as Burma. His species, Electroblemma bifida, is the newest addition to a remarkable and strange family of spiders known as the tetrablemmids. Each of the 161 members of this group boasts several toughened layers of exoskeleton covering its body, including distinctive overlapping plates running down its bulbous abdomen. All spiders have hard plates covering the upper and lower parts of the front of the body, says Paul Selden, director of the Paleontological Institute at the University of Kansas in Lawrence, who was part of the team that discovered E. bifida. But few have hard plates on the abdomen. Exactly why this small group of spiders have encased themselves in such formidable armour while the majority of other arachnids have not is an enduring puzzle among arachnologists. However, new research is starting to provide clues. The armour may offer more than mere protection. Like all arthropods, spiders bodies are contained within a hard sclerotised exoskeleton. In most spiders, the toughest part of the exoskeleton is the carapace, which sits on top of the front section of the body; the cephalothorax. The carapace helps to protect the spiders delicate eyes, mouth and other sensory organs. They really look like little tanks On the underside of the cephalothorax is another toughened plate called the sternum. The carapace and sternum are joined by thinner, less tough material, through which the legs connect to the body. Behind the cephalothorax lies the abdomen. In most spiders this is covered in a much thinner and more flexible cuticle material made of chitin, which allows the abdomen to expand and contract during feeding. Armoured spiders differ from this standard plan in two key ways. First, the carapace on their cephalothorax is usually far thicker and is fused to the sternum plate, leaving no vulnerable join that a sharp point could creep between. These voracious insects swoop down onto their prey to deliver a paralysing sting Second, they have tough armoured plates covering their abdomen, each hardened with extra sclerotin. Beneath this sclerotin, the cuticle membrane is also thickened about three times thicker than the cuticle of soft-bodied spiders. It is like having a layer of chainmail on underneath a suit of armour. They really look like little tanks, said Christian Kropf, an arachnologist at the Natural History Museum of Bern, Switzerland, and the worlds leading authority on tetrablemmids. [The armour] is astonishingly thick when you compare it to spiders of the same body size. Many of these tiny spiders measure just 2mm long and less than 0.5mm wide, but their cuticle can be 14-16 micrometres thick in places. If scaled up, these beasts would have armour thicker than a World War Two Panzer Tiger tank. Of course, the spiders do not need to deflect shells or bullets. Instead, they face a smaller but no less formidable foe. The tropical and subtropical forests of South East Asia, where most living tetrablemmids are found, are prowled by aggressive predatory wasps that specifically hunt spiders. The tetrablemmids do not have such a weak spot These voracious insects swoop down onto their prey to deliver a paralysing sting that leaves a spider immobilised. Then the wasp drags its victim off to its nest where it lays a single egg on the helpless arachnids abdomen. When the larva hatches, it eats the spider alive. Spider-hunting wasps will tackle prey several times their own size. Some species are capable of taking on tarantulas. This is where tetrablemmids armour may come into its own, according to Kropf. These wasps always sting spiders in the soft [region] between the plates covering the top and bottom of the front section of the body, he says. This is where the spiders brain is located, so the arachnid is immediately immobilised. However, with their hardened plates fused together, the tetrablemmids do not have such a weak spot. The wasps probably find them very hard to attack. If the tetrablemmids armour is so beneficial, why has it only evolved in one small group of spiders? The spiders armour may also protect them against other hunters. Most tetrablemmids are found living in the leaf litter on the forest floor, although some are found living under bark and a few even inhabit dark caves. The soil on the forest floor is the domain of predatory mites that feast on any arthropods in their way. There was a study conducted in the 1980s that offered soft-bodied and sclerotised arthropods to these mites, says Kropf. They always preferred the soft prey. When we were collecting Tetrablemmidae in Thailand we would find these mites everywhere. It is possible that these armoured bodies may give the spiders more general protection against all kinds of predators, not just predatory wasps. But in a sense, this turns the original question on its head. If the tetrablemmids armour is so beneficial, why has it only evolved in one small group of spiders? One possibility is that it is too bulky to be practical for most spiders. Like Medieval knights, the tetrablemmids may find their movements are far more restricted than their less heavily-armoured cousins. In line with this, only a few armoured spiders spin webs. Others simply produce a messy tangle amongst the leaf litter on the forest floors. It is tempting to conclude that the extra armour plating makes it harder for tetrablemmids to manoeuvre their abdomens to produce delicate silk structures. Some of the larger members of this family which can grow up to 11-12mm (0.4in) long do produce huge webs though. In pristine patches of forest in Thailand where they live, these wonderful webs can have a diameter of more than 0.5m (1.6ft). The muscle structure inside the armoured spiders appears completely different There is another reason to suspect that the heavy armour does not interfere significantly with the tetrablemmids movement. Spiders move their legs using a combination of muscles and fluid pressure inside their bodies. By squeezing the plates on the top and bottom of their cephalothorax, they can produce the pressure needed to push fluid into the legs, causing them to extend, much like a garden hose when it fills with water. Muscles in each leg oppose this pressure, allowing the spiders to gain fine control over the movements of their legs. However, squeezing the body in this way can also cause the base of the leg, where it attaches to the body, to move around. Most spiders have lateral muscles that control this movement. When Kropf looked at the legs of armoured spiders, he found some differences. Those muscles are simply not there, he says. The muscle structure inside the armoured spiders appears completely different. Rather than the bundles of striated muscles, the armour plates provide attachment points for smooth muscles, which are normally found in blood vessels and the stomach. I wondered how these spiders stretched their legs Smooth muscles need much less energy than striated muscles, says Kropf. So rather than slowing the spiders down, the extra armour may actually help them to save energy and move more efficiently. In other words, it is still a mystery exactly why so few spiders have extra armour. Kropf has also spotted something else. Without the flexible membrane between the upper and lower plates of the cephalothorax, the tetrablemmids are totally unable to squeeze them together to create the fluid pressure they need to move their legs. This should leave them hopelessly immobile. In armoured spiders, the whole prosoma [another name for cephalothorax] is a stiff capsule, says Kropf. This seemed like a problem to me when I started studying these spiders. I wondered how these spiders stretched their legs. The adult spiders seem to be remarkably long-lived for their size What I found out is that they use their abdomen as a pressure pump. This explains why the abdomen is so heavily armoured. All these strong sclerotised plates serve as attachments for strong muscles in the abdomen. Essentially, the abdomen, with its thick plates on the top and bottom along with the strips of hard cuticle along the sides, acts like a bellows that the spiders can squeeze and expand at will to pump fluid around their bodies. BBC