Re: What are eye-spots/ocelli for?

Hello Lin,

I assume that you're referring to the eyespots on the tail feathers of a 
peacock and other birds.  In addition to being decorative, these eyespots 
can indicate the health of the bird - a peacock with a large tail is 
probably in good health, and would make a suitable mate for a peahen.  In 
fact, the elaborate tail feathers of a peacock probably evolved through 
sexual selection, in which female mate choice compels males to compete to be 
chosen.  Male-male competition can take the form of ritualized combat, as 
with elephant seals or rams, or physical displays such as the sword of male 
swordtails or the tail feathers of peacocks and male lyrebirds.

Many moths and butterflies have large eyespots on their wings.  These spots 
may attract a predator's attention to non-essential parts of the moth's 
body.  A moth can survive if its wing is torn, but probably can't survive if 
its head or thorax is attacked.  Eyespots on wings also give the illusion of 
larger size, which may deter a potential predator.

While eyespots are non-functional (i.e., they don't function at all as 
eyes), an ocellus is different.  An ocellus is a collection of light-
detecting cells and can usually detect variations in light intensity.  
Ocelli, or simple eyes, cannot form images, but they can be very sensitive 
to changes between light and dark.  Even the most "simple" of invertebrate 
animals have ocelli, so they are very common.  A quick flip through any 
invertebrate zoology textbook will show you many examples of animals with 
ocelli.

I hope this answers your question.

Allison J. Gong
Mad Scientist