What the 'False Eyes' of Butterflies Are Really For

Picture this: it's a beautiful spring day and the graceful fluttering of a butterfly catches your eye. The delicate insect alights on a nearby flower and, for a moment, it's wings remain unfurled. Suddenly you're face-to-face with a hideous monster, complete with 18 eyes and a crooked, segmented nose. For some time, this is what most people thought the strange "eye spot" patterning on some butterflies' wings were for. Now, however, researchers are arguing that they have a far better use than simply frightening gullible humans.

"Eyespots are conspicuous, they draw your attention and are thought to be used by many animal species to avoid death or attack, by either startling or confusing the predator," Katy Prudic, a researcher from Oregon State University explained in a statement.

Eye spots in reptiles and birds have one very obvious use. Mimicking large and fearsome visages, this patterning is meant to trick a predator into thinking they are looking at something far more intimidating than it actually is. Even for a wild cat like the serval, eye-like patterning on the back of its ears suggest the animal sees you even when its head is turned - making competitors think twice about sneak attacks.

However, in the case of insects this 'mimicry' strategy doesn't make as much sense, as their primary predators are often other insects - creatures too small to be deceived by the patterning in its whole and frightening glory.

So what's the point? A study recently published in the Proceedings of the Royal Society B detail how one species of butterfly actually changes its eye-spots with the seasons in an aim to misdirect the attacks of certain predators when they are around.

Past studies of the squinting bush brown butterfly (Bycyclus anyana) have revealed that the insects usually produce about five generations of offspring a year over both dry and wet seasons. Through what is called "phenotypic plasticity," how inherited genes are expressed can change depending on seasonal conditions, in turn influencing what the butterflies look like. (Scroll to read on...)

These wet-season eyespots draw a great deal of attention, and would likely make them easier targets for predatory birds in flight. However, it's not birds that the insects worry about most during this time. In warm weather, predatory insects, particularly praying mantids, are abundant - aggressively stalking bush browns in their eastern African habitats.

Mantids' with their massive compound eyes and cat-like reflexes are difficult to avoid. That's why Prudic and her colleague believe that the butterflies use their eyespots as a distraction.

In several simulated hunts, an attacking mantis often aimed for the misleading eyespots, rather than a butterfly's vulnerable body or head. In this way, even with damaged wings, the bush brown can escape for a chance to reproduce one last time. Bush browns without vibrant eye-spots, however, usually always fell victim to a killing strike. (Scroll to read on...)

[Credit: Oregon State University]

Conversely, cold dry seasons mean that most mantids are long dead and the surrounding vegetation has turned a dull brown color. In these times, the butterflies want to be as inconspicuous as possible, blending in with the brown bushes that earned them their name so that predator birds cannot see them.

"Having the right type of eyespot in the right season allowed the butterflies to live long enough to lay eggs and have more offspring in the next generation," Prudic explained. "With the wrong eyespot at the wrong time, they were quickly annihilated."

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