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. It has long been assumed that the plucking mimics the vibrations caused by an ensnared insect. Hence, the Latin name for pirate spiders: Mimetidae (i.e., imitator).

   However, not all entomologists agree that this is what the pirate spiders are doing. “The behaviour of resident spiders towards pirate spiders and their own prey is quite different, as are the vibrations in the web caused by these two sources,” says Carl Kloock of California State University Bakersfield. He has an alternative suggestion. “It seems to me most likely that pirate spiders are mimicking the vibrations of web-invading spiders of the same species, and possibly spiders of different species,” says Kloock. “A spider on its web needs to defend its web – a valuable resource – from other spiders, who may try to take over the web to avoid the cost of building their own web, or simply try to steal prey from the web.”