What happens when an insect touches a spider’s web?
Most web-spinning spiders line their silken threads with droplets of glue, which snag blundering insects. But one group—the cribellate spiders—does something different. Their threads are surrounded by clouds of even more silk—extremely fine filaments, each a hundred times thinner than regular spider silk. These nanofibers give the silk a fuzzy, woolly texture, and since they have no glue, they’re completely dry. And yet they’re clearly sticky. Insects that stumble into the webs of cribellate spiders don’t stumble out again.
Raya Bott and colleagues at Aachen University in Germany have now shown that cribellate silk adheres to insects in a previously unknown and unsettlingly macabre way. When an insect touches the strands, waxy chemicals in its outer surface get sucked into the woolly nanofibers and reinforce them, turning the tangled mass of delicate threads into a solid, sturdy rope. The victim literally becomes a part of the web, inadvertently strengthening the instrument of its own capture.
Cribellate spiders are among the most ancient of spiders, and they use their silk in a variety of startling ways. The ogre-faced or net-casting spiders hold their webs in their front legs and drop them onto passing insects. The uloborid spiders have lost the venom that most spiders use, and instead crush their prey to death by wrapping them in excessive amounts of silk; one species was documented using 140 meters to envelop a single insect. And some uloborids—the triangle spiders—spin triangular bungee webs. They hold one corner in tension; when an insect lands, the spider lets go and the entire web collapses onto the target.
Despite these different tactics, all of these species rely on the sticky nature of their dry silk. Scientists used to think that the nanofibers in cribellate silk make such close contact with surfaces that they stick using the forces that hold molecules together over very small distances. But that couldn’t be the whole story, because cribellate silk adheres far more strongly to insects than it does to artificial surfaces.
Bott found a clue to cribellate silk’s powers by breaking out a powerful microscope. She noticed that whenever the silk had touched an insect, she couldn’t make out the individual nanofibers any more. It was as if they had fused together. She even filmed the process, showing that a wave of fusion begins at the point of contact, and then travels up the silk.
Looking more closely, she saw that the fibers were still there. They had just become embedded in some kind of fluid—think spaghetti strands drenched in a thick marinara sauce. And when she analyzed the chemicals in the fluid, she realized that it was a match for the waxes found in insect shells. It seemed like the silk absorbs these waxes right off the insects, just as cotton balls will soak up water. In the process, the silk reinforces itself.
Bott confirmed her idea by showing that cribellate silk sticks eight times more strongly to normal insect shells than to those that were chemically treated to remove their waxes. Similarly, it sticks more strongly to artificial surfaces that have been coated in those same waxes. “Insects typically use the wax to reduce evaporation, but the spider misuses that protective layer,” says Anna-Christin Joel, who led the study. And all of this happens automatically.
“They make a strong case,” says Todd Blackledge from the University of Akron, who studies the evolution of spider silk. “It makes sense that silk would have features that make it function best on natural insect surfaces rather than synthetic surfaces commonly tested in the laboratory.”
Insects, in turn, evolved countermeasures. They couldn’t get rid of their waxes entirely or they would lose too much water, but they could make the wax so viscous that it wouldn’t soak into the silk, or simply cover it with a protective shield. Joel suspects that these adaptations drove spiders to evolve new ways of trapping their prey, which might explain why some of them started adding glue to their silk.
But ironically, the gluey threads stick less well to insects with unprotected waxy shells. Joel thinks that insects can defend against either the dry cribellate silk or the wet glue-coated kind, but not both. And conversely, both kinds of silk only work on some kinds of prey, which is why the ancient cribellate spiders weren’t totally displaced by their glue-using descendants. Thanks to their self-reinforcing silk, they’ve stuck around.