The estimable Matthew Cobb called my attention to this post on The Featured Creature, which he found via the Facebook page of “Spider” Dave Penney, a freelance scientist (!) with his own publishing company.
Have a look at this moth. If you saw it in the wild, would you have any idea what the wing pattern means?
You can see other photos by John Horstman at his Flicker photostream.
Well, it almost certainly evolved to make the moth mimic a spider, presumably a predator. As the website says, somewhat breathlessly:
Now this is an example of mimicry at its finest! This newly discovered species (2005) of moth dubbed the Lygodium Spider Moth (Siamusotima aranea) is so named for its preference of feeding on Lygodium species, an invasive Old World climbing fern, and has markings on its wings that make it look just like a spider with orange, spindly legs! This moth mimics a spider so well that I couldn’t even tell what it was at first when I saw the picture from far away!
Why would a moth evolve to mimic a spider? Earlier research on insects (see below) suggests that when prey like this sense an approaching spider, they display their wings (as in the photo below), and that deludes the spider into thinking it’s encountered another spider of its own species. That would be an aggressive conspecific encounter—one from which spiders often flee. A moth with this pattern, then, might escape predation by actually driving away the predator, or at least distracting it for long enough to allow the moth to fly away unharmed.
This evolutionary scenario is speculative, for as far as I know it hasn’t been tested (or even observed) in the moth shown below, but I don’t see any other explanation for the wing pattern.
Note that the patterns on the moth give it eight legs, just like a spider!
National Geographic has another case of a moth mimicking a jumping spider (there’s a video at the link, too).
A paper in Science in 1987 by Mather and Roitberg (no free link, but reference below) shows something similar: patterns on a “true fruit fly” (the tephritid snowberry fly, Rhagoletis zephyria). For a long time these wing patterns were thought to be simply species-recognition marks, until clever zoologists realized that they looked like something else.
This is another rare case of a prey actually mimicking its predator—in this case a jumping spider (the authors used the zebra spider Salticus scenicus for their tests). When the fly senses something approaching, it spreads its wings a bit and wobbles from side to side, much like the gait of a jumping spider. This makes an approaching jumping spider think that the fly is actually a conspecific spider. And the spider, sensing an aggressive encounter on tap, usually flees. The displaying fly is saved.
Experiments show that spider fled from displaying flies at rates similar to which they fled from other spiders. Further experiments that obliterated the flies’ wing patterns with ink showed that this made them more susceptible to predation (predators didn’t flee as often, but pounced), although there were no controls for the effect of ink-painting on the flies’ well being and vigor.
Here’s what the predator and the tephritid fly look like:
Why this ubiquitous dimorphism? It resembles some types of Batesian mimicry in butterflies, in which edible females evolve to resemble both males and females (who look alike) of a toxic or distasteful model species, but the edible males don’t show mimicry, resembling the ancestor. In both cases, I suspect, the explanation is the same: there would be an advantage to the males evolving mimicry, too, but an even larger disadvantage to changing their pattern if females have a fixed genetic preference for the ancestral type of male.
I’m not sure whether this explanation is correct, and don’t even know if it’s been tested, but it is at least conceptually testable and makes some sense in light of what we know about sexual dimorphism (females have strong preferences for certain types of males).
The mimicry does show, though, that there were visually-hunting predators around then, for why else would these phasmids evolve mimicry? Wedmann et al. posit that the predators may have included birds, primates, and bats, all of them known from the same fossil deposits.
Mather, M. H., and B. D. Roitberg. 1987. A sheep in wolf’s clothing: Tephritid flies mimic spider predators. Science 236:308-10.
Wedmann, S., S. Bradler, and J. Rust, The first fossil leaf insect: 47 million years of specialized cryptic morphology and behavior PNAS 2007 104 (2) 565-569; published ahead of print December 29, 2006, doi:10.1073/pnas.0606937104