Armoured spiders - yes, these are a thing

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. 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. 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 muscle structure inside the armoured spiders appears completely different... 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.

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. "It is a fundamental change of the whole body construction," explains Kropf.

So it seems, to accommodate their armour and the protection it provides, tetrablemmids have had to radically redesign their own body plan and even their development.
http://www.bbc.com/earth/story/20161102-there-are-spiders-that-encase-their-bodies-in-thick-armour