Wednesday, April 18, 2012

Eyespots and Innate Avoidance of Them





            Eyespots are defined as light-sensitive, pigmented spots on the bodies of organisms. Often found in invertebrates, these rudimentary “eyes” allow the organism to sense their environment and are thought to be the ancestor of the mammalian eye. A false eyespot (also known as an eyespot, too) is a rounded, eyelike marking on the bodies of organisms to mimic an eye.
False eyespots are often found on insects, such as butterflies and moths, but not just on their adult forms, but their larvae, too. Both serve the function to discourage a hungry predator by fooling that predator into thinking the insect is actually another predator. This is a unique form of camouflage, called mimicry, because the prey is not just trying to blend in, but trying to scare off the predator. This form of mimicry is known as Batesian mimicry, in which an edible species evolves to physically resemble an inedible one (whether it be a predator or something colored, i.e. king snake mimicking a coral snake). These predators include birds, snakes, lizards, and some mammals that are usually small and are prey for other, eye-containing predators. Biologists are quite sure that these eyespots are used to discourage predation, because they’re often only disturbed when the larva is disturbed or detected by a potential predator. And, logically, the larvae that can scare off a predator will be able to live and potentially make it to adulthood and reproduce. Those without the eyespots were more likely not to scare off their attackers, and their genes were potentially lost, thus, selection acted in favor of eyespots because it increased fitness of the insects. Oftentimes, on the larva, the eyespots are on the tail for better maneuvering if they are discovered by a predator on the leaves.
There has been an additional theory proposed, however, that the predators’ avoidance of the eyespots are innate – hard-wired, genetic-based behavior the result of natural selection the same as the evolution of eyespots – rather than learned by each individual predator via trial and error. Janzen et al. say this is the case because if these mid-level predators didn’t immediately avoid eyes of their predators (i.e. owls, snakes, large animals, etc.), that mid-level predator was dead; no chance to pass on genes. So, the ones who survived must have some innate response of flight when that stimulus appears. Thus, the caterpillars and other insects may have evolved in great numbers in response to this realization about their predators.
So the question is, if this is co-evolution, is the relationship between caterpillars with eyespots and birds (for example) still evolving because much of insect class has eyespots and it has worked for millions of years.

Bibliography:
http://whyevolutionistrue.wordpress.com/2010/06/30/why-eyespots/
 

4 comments:

  1. Cool explanation! In elementary school, I had learned that the purpose of the eye spots on the butt of caterpillars and such was to trick birds into thinking that was the head and eat that part because it was less important and they could live without it. But really this explanation makes more sense. I've been believing a lie this whole time! haha

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    1. Interestingly, I learned a similar explanation as well about how it was a deception method for caterpillars.

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