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Camouflage in Motion
Adrian writes "Remember Jurassic Park, where Goldbloom stood really still and the T-Rex couldn't see him? Well, there might be a better way. Scientists have found that dragonflies can dissappear by keeping their image on your retina in the same place, even if you move. How they manage it still has them puzzled... ;)"
Actually the book "The Lost World", which was written by Michael Crichton who wrote "Jurassic Park", shows an opposite behavior of the T-Rex.
The following lines from the book says:
Sarah Harding said, "Why did Dodgson just stand there like that? That's not the way to act around predators. You get caught around lions, you make a lot of noise, wave your hands, throw things at them. Try to scare them off. You don't just stand there."
.....
"Roxton," Levine said, "believed that tyrannosaurs had a visual system like an amphibian: like a frog. A frog sees motion but doesn't see stillness. But it is quite impossible that a predator such as a tyrannosaur would have a visual system that worked that way. Quite impossible. Because the most common defense of prey animals is to freeze. A deer or something like that, it senses danger, and it freezes. A predator has to be able to see them anyway. And of course a tyrannosaur could."
Discovery.com provides a few more details but since the scientists themselves are still baffled, I don't think we will find any lengthy explanations of the phenomenom except perhaps by reading the article in Nature itself which is not available except by subscription.
The thing new in the Discovery article I found significant was that they performed the movements with "millimetric" precision.
I wonder if the dragonfly's 3 foot long ancestors were also capable of such precision, or whether the need to remain so precise led to their reduced current size.
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Boy, that MSNBC article was bad. They even mispelled the researcher's name. It is "Akiko Mizutani" not "Aikiko Mizutani".
Here is some better coverage of the story. discovery, NationalPost, and Ananova.
And here is a nice page from the Insect Vision, Navigation and "Cognition" Laboratory at ANU, but it doesn't cover the dragonfly work.
Generally speaking, the dragonfly moves in such a way that if you draw a line from the dragonfly to the prey at each increment of some time step, the lines will (nearly, because it's not perfect) cross at one point. Thus, to the prey, it appears that the dragonfly is a stationary object located at the point where the lines cross.
It relies on a lack of depth perception, obviously. As a guess, perhaps the dragonfly is able to accomplish this by using the same visual cues it evokes in its prey - if the dragonfly moves in the right way, then its prey will appear to be a stationary object (from the dragonfly's perspective) as well.
However, this doesn't account for situations where the dragonfly emulates an object that is behind it (i.e., the lines cross at a point on the far side of the dragonfly) or an object at a large distance (where the dragonfly directly shadows the prey, copying its every move).
If you are still confused, think of it this way: You're playing your favorite first-person shooter, and you want to hide behind a tree/pillar/rock so that an approaching target can't see you. You can move around the tree so that it always forms an intervening object. If you draw a line between yourself and your target at each moment in time, they all intersect at the tree. If your target happened to have really crappy eyesight (compound eyes, perhaps) then you could just remove the tree, and at every moment in time they'd see you there along the same line of sight where the tree would have been, so the target perceives you as being located where the tree would have been and moving along as if you were a part of the landscape. (The advantage, though, is that you can move around and close in on your prey, while your prey remains unaware of the soon-to-occur frag.)
So that's why my monitor keeps disappearing if I look at it for more than a few mi... oh...
If you take your finger and hold your eyeball in place things will fade to black (so long as you don't move your head and close the other eye.) I don't think this trick would work against us since we can and do move our eyeballs independently of our body. Fireflies only have to pull this trick on the flies they eat...
The eyemovements we make to be able to sit practically motionless before our monitors is called saccade. (Or "Freedom Eyes" in New American.)