The Spider Web That Gets Stronger When It Touches Insects – The Atlantic
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.