While doing field work on plant defenses in Japan, biologist Kazuo Yamazaki kept finding plants that appeared to be covered in spiderwebs. Halfway around the world, botanist Simcha Lev-Yadun found the same thing while doing research in Israel, Greece, and Estonia.
On its own, that isn’t out of the ordinary. While you or I might not like a spider calling our bodies home, it’s a great arrangement for plants because spiders eat various plant-eating insects. It’s like having a round-the-clock, live-in (or live-on) bodyguard. Spiders are such effective protectors that some plants lure and reward them for their service with extrafloral nectar (which isn’t in the flowers and isn’t used to attract pollinators).
When Yamazaki and Lev-Yadun inspected the plants more closely, though, they found that they weren’t looking at spiderwebs after all. What they’d been seeing were patches of trichomes, hair-like growths on the plants’ outer layers. The plants had fooled them, and maybe, they thought, that was the point. If a plant can’t attract a spider bodyguard, the next best thing might be to fake it. By mimicking the signs that a spider is nearby, a web being an obvious signal, the plants could scare off insect pests without having to invest in rewards or attractants for actual spiders.
From their own field work and searching through plant guide books, the pair found half a dozen plants with trichomes on their stems and leaves that seemed to mimic spiderwebs in both appearance and structure. Trichomes come in a variety of shapes, sizes and densities, from thick downy coverings of long “hairs” to sparse lines of bristles that point in the same direction. That this handful of plants all have dense, thread-like white trichomes that strongly resemble webs makes Yamazaki and Lev-Yadun think that they may be using mimickry to deter bugs that would feed on them.
Aside from fooling insects into thinking a spider was around, the researchers say the faux-webs might also attract birds, lizards, and parasitic wasps that prey on spiders and use their webs as hunting cues, but that would also eat any insect that were around.
Of course, trichomes have other functions. In deserts, for example, dense trichomes protect new plant growths from drying out in the sun. That some trichomes look like spiderwebs might just be a coincidence. To test their mimicry idea, Yamazaki and Lev-Yadun say they can try a few different experiments, like testing how much insects are deterred from varieties of the same plant that have different types of trichomes, or adding and removing web-like trichomes from plants and comparing insect activity. With an optical model of what plants look like through the eyes of insect herbivores, they could also test whether the trichomes look as much like webs to bugs as they do to us.