While they belong to the group that includes all the orb-weaving spiders, pirate spiders cannot weave webs of their own

Build a web, catch a fly, wrap the fly in silk, then devour at leisure. This hunting strategy has proven so effective that orb-weaving spiders are one of the most successful groups of animals. They are found in almost every corner of the world and there are more than 3,000 species.

Making a web is a fairly sophisticated ploy. As well as multiple forms of silk and glue, the spider needs to perform a sequence of precise manoeuvres.
But why bother building your own web, when you can just invade somebody else's and devour the architect?
A clandestine group of spiders known as "pirates" have adopted this nefarious method of nabbing their prey. Their hunting strategies are among the most remarkable in the animal kingdom.
Pirate spiders are members of the spider group that includes all the "orb weavers" – those that make the prototypical, circular webs we are all familiar with – but they do not make webs.
In fact, they have lost the ability. They can still produce silk, which they use to build egg sacs and wrap prey. But they are anatomically incapable of spinning a web. The number of silk "spigots" on their spinnerets is dramatically small compared to their relatives.
Instead, they invade the webs of other spiders, in a bid to lure and then kill the hapless architect. Gently, they pluck the strings of the web, enticing the host to approach.
Once the host spider has ventured close enough, the pirate makes its move.
First, it encloses its duped prey within its two enormous front legs. These are fringed with massive spines, called "macrosetae", which they use to trap the host within a prison-like basket.
Then, the final move: the pirate bites its prey and uses its fangs to inject a powerful venom that instantly immobilises it.
It is a powerfully effective hunting technique.

"It can be riveting to watch a pirate stealthily wandering while waving its long, first pair of legs to narrow in on the location of the other spider," says Mark Townley of the University of New Hampshire. "Despite many hours spent feeding pirates for our studies on spinnerets, I never became jaded by the sight of them searching for and attacking prey. It was always a marvel to watch. They can wield that first pair of legs so delicately that I've seen them touch prey spiders so lightly without them reacting in any way, not seeming to even notice."
But we do not yet fully understand how the pirate's strategy works.
In particular, it is not clear why the pirate spiders pluck the strings of the host spider's web.