Spiders can tackle all manner of prey, from insects to fish to birds. But some of them specialise in killing their own kind. Palpimanus gibbulus and Palpimanus orientalis are two such spider-slayers, and they use special adaptations to tackle their dangerous prey, including ninja-like stealth, blinding-fast strikes, unbreakable grips, and heavy body armour.
Stano Pekár from Masaryk University in the Czech Republic confirmed that these two species (hereafter known as Palpimanus) are indeed specialist spider-hunters. They pounce upon a wide variety of other species and attack spiders more often than insects like flies or crickets. Using high-speed video cameras and staged battles, Pekár revealed their killer technique.
When a Palpimanus spots a potential target, it raises its front pair of legs and approaches very slowly. Once it gets half a body length away, it freezes and waits. Pekar says, “Palpimanus moves so gently and slowly that the prey spider might not be able to recognise it as a predator.” Indeed, many spiders use one pair of eyes as motion detectors, picking up anything that moves above a certain speed. It’s possible that Palpimanus moves below that threshold. Its prey might not actually see it as a threat, until it’s too late.
When Palpimanus gets within striking distance, it attacks as soon as its prey makes a move. It lunges forward and places its legs on the victim’s body, drawing it close and pinning it down for a lethal bite. All of this happens in just a fifth of a second.
Palpimanus can grapple with spiders twice its size, for it has particularly butch forelegs. These powerful limbs undoubtedly help it to hold its struggling prey down, but the spider-slayers don’t just rely on brute force. When Pekár looked at the forelegs under the microscope, he saw that each one ends in a pad of fine hairs. In an area the size of this comma, these pads have around 1,500 hairs, and each of these ends in even tinier hairs.
When Palpimanus presses its legs against another surface, the hairs make such close contact that they stick. They bind to the surface using the same forces that keep molecules together. Other spiders use similar forces to hold themselves to smooth surface while they climb. Palpimanus uses them to gain an unbreakable hold on its prey. This grip is important – when Pekár covered the pads with paraffin, Palpimanus’s success rate plummeted.
Palpimanus’s front feet are so adhesive than if it grabs another spider by the leg, the victim will often break off its own limb to escape. When it does, the amputated leg stays attached to the sticky pads of the attacker (as in the video above).
But it takes more than sticky feet to hunt spiders. These are dangerous prey that can fight back with toxic bites of their own. Palpimanus isn’t immune to the venom of other spiders; instead, it protects itself with heavy armour. The outer layer (cuticle) of its front half is twice as thick as that of its prey, and the cuticle on its back half is five times as thick. With such extreme protection, it can shrug off the fangs of its prey. Only the very largest species managed to pierce its hide.
Palpimanus is far from the only group of spider-eating spiders. Perhaps the most famous is a type of jumping spider known as Portia. It hunts web-spinning spiders and it lures them in by twanging the web with its legs, mimicking the vibrations of struggling prey or the courtship signals of a mate. They even limit their trickery to windy conditions that disrupt their prey’s ability to discern genuine vibrations from fake ones. Portia has none of the physical adaptations that Palpimanus has. It’s less a powerful wrestler, and more a crafty assassin.
Reference: Pekár, Šobotník & Lubin. 2011. Armoured spiderman: morphological and behavioural adaptations of a specialised araneophagous predator (Araneae: Palpimanidae). Naturwissenschaften http://dx.doi.org/10.1007/s00114-011-0804-1