Here's a good question from my 16 year old son. He was feeding Ink a crab and wanted to know what the deal was with the blue spots, why they change color so quickly compared to the rest of him. I thought maybe it was a defensive behavior, like the fish that have fake eyes away from the vital organs so a predator would attack a less vital spot of it's body???

Told him I'd post this and see what the answers are!!!

Carol

Most eyespot colouration in the animal kingdom serves a dual purpose, both as defensive (as you stated) and as aggressive (the eye spots flare when the octopus inflates itself to challenge another octo or a invader, making it look larger and much more impressive)...caterpillars use this strategy a lot also...
Good question! Bimacs are very fascinating animals...
Greg

Best bet is so that they suddenly look like the face of a big predator...

The caterpillar is a good example like Greg said...

Also owl butterflys, paradoxical frogs and many others...

http://pooh.unl.edu/~scotth/samantha/moths-butterflies.html?page=caligo