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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... ;)"
What an incredibly hollow article. "We've used some technical majiggers to look at some stuff and wow! Look what we came up with!" It's a good thing there wasn't any actual details in there, it may have been interesting.
Al Qaeda has ninjas!
Hooray!
I've never seen a puzzled dragonfly. Oh. The scientists.
I'd assume that the dragonfly merely tries to keep the thing it's hiding from in the same position on *its* retina. It'd be a fairly simple feedback mechanism, if you did it with analogue electronics.
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King Charles 'beheaded' guests who bored or annoyed him by viewing them at such an angle that his blindspot was over their head. Try it for yourself
Well duh. Didn't they ever catch flies when they were young? The way to do it is to take two fingers and follow the fly with them, maintaining the distance between your hand and the fly. after a while the fly will think your fingers are part of the background and will easily let you catch it.
x <- Moderators, keep staring at this point.
Everone else can look here -> x
Now, if my calculations are correct, I should be able to get away with this:
Imagine a beowulf cluster of F1r5t P05t!
Mwa ha ha!
Oh wait, you mean that I'm too big to be a dragonfly?
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."
I never read the articles and jump straight to the posting as everyone else here. But this seemed interesting so I went and read it. waste of 20 seconds of my time. There are no details! Why not wait until there is something to see before posting these stories?
The Picard maneuver. Although I doubt dragonflies can punch it up to warp speed yet.
Yet another example of the universal truth: Everything I need to know, I learned from "The Karate Kid."
-Isaac
I am not a lawyer, and this is not legal advice. For Entertainment Purposes Only.
So that's why my monitor keeps disappearing if I look at it for more than a few mi... oh...
So basically your brain tunes them out, since they're stationary as soon as they come into your field of view, making you tune them out like one might tune out the frames of your glasses, sunglasses, etc.
moox. for a new generation.
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.
If something was chasing after me, and I had wings, I'd fly away.
Doesn't it make you feel good to know that our freedoms are protected by politicans, lawyers and journalists.
Since there aren't any deatils.
1. What is are the distances involved?
2. Best guess, they're using a single lens camera. I believe dragonflies eat flies. If this is so and the fact that flies have compound eyes, does this test really hold true for their natural prey or just for 'human-style' eyes?
3. I'm not 100% sure myself that dragonflies have compound eyes, but if they do then I would expect that their eyes are accurate enough to see the retina of it's prey (or whatever) and keep itself in the same position relative to those movments.
BTW these are just the quick thoughts of an amateur scientist with 20 years of software trouble-shooting expereience. The points I've made seem logical to me but I've come to the conclusion that logic really doesn't work that well in the waking world.
The GEEK shall inherit the earth...
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.)
Thank you. The National Post link was sufficiently detailed to be meaningful.
I'm sure i'm probably too late... someone's probably already applied for a patent on dragonfly flight.... which means I'd be infringing if i walk a certain way, i guess....
Stupid people make stupid things profitable.
and here is a brief writeup from Nature.
If it were stationary, the dragonfly would just need to fly straight toward it.
Patrick Doyle
I mod down every jackass who puts his moderation policy in his sig. Oh, wait a sec....
I've seen something like this before. Once I was outside cleaning up after a party. I went to pick up a vase of flowers and I noticed a few tiny fruit flies(?) that were hovering near the flowers. The funny thing was that when I picked up the flowers, these flies would maintain the exact same relative position to the flowers. Even if I rotated the vase around its axis.
It was like taking the flies for a walk on an imaginary but invisible leash.
I guess that the flies had an instinct that to remain still, they must reduce the error in *their* retina between the current background image and the stored background image. I am guessing that dragonflies have evolved to do the same thing but with a greater degree of freedom. i.e. a chosen target rather than the whole background.
The article suggests that armies could learn something useful about camouflage from dragonflies. What are they suggesting, that soldiers flit about the battlefield to keep in the same position in the enemies' retinas all the time - how would they manage this? Jet packs? Pogosticks? Even a helicopter which has maneuverability analogous to a dragonfly could never do this. The scale for humans is just all wrong - a dragonfly only has to dart short distances, because the relative scale is quite small, but a human can quite quickly track his/her visual field across many miles of sky / terrain simply by a turn of the head. Are they suggesting that helicopters or soldiers would instantaneously flit miles through space to come to a complete stop at exactly the right position at exactly the right moment with exactly the right orientation to maintain the same retinal pattern? Somehow this strikes me as unlikely. Furthermore, this trick of "becoming invisible" does not apply to the mammalian optic system - its fine for insects & reptiles because their optical systems rely on picking up changes in the visual field so still objects become in effect invisible. Evolutionarily this makes sense, because it allows them to maintain relatively useful vision without devoting a lot of sophisticated neurons which they simply can't afford - its like a cheap hack, but it works. Mammals have much more developped brains, and much more developped visual systems, and can afford the luxury of actually seeing things that don't move. What's more, is that this is fine in a hunter-prey scenario where there is only one individual you need to hide from. You could never accomplish this trick when you need to hide from several different observers. Another example of inane scientific journalism.
There are a thousand forms of subversion, but few can equal the convenience and immediacy of a cream pie -Noel Godin
Frogs like the same conditions as dragonflies. Frogs are sensitve to motion. Lots of frogs in the pond near my house, and when I was bored I could "herd" the frogs along the shorefront by tossing pinecones or sticks. After awhile I could get a dozen of them all sitting together, waiting for me to throw something else.
/fast/. But it didn't seem to be a problem for dragonflies.
I've seen dragonflies land on frogs. that's no small achievement for any other insect, because frogs are
I posit the movement patterns of a dragonfly correlate directly with the limits of a frogs eye to percieve motion. The frog and dragonfly have lived in close proximity to each other for eons. If you see a frog, you will see dragonflies too.
Imagine a dragonfly flying in a circle around its prey. It yaws appropriately so that it always faces its prey, and so it maintains the prey's image at the same position on its retinae. Instead of appearing to stay at a fixed point to the prey, however, the dragonfly revolves around it a full 360 degrees.
Isn't that called "circle-strafing"?
The dragonfly isn't using camouflage to avoid getting attacked by a predator, it's using camouflage to avoid being detected as it goes after its prey... so the prey doesn't fly away.
The scientists aren't claiming to have discovered the well known fact that stationary objects in a visual field are difficult to see. The surprising discovery is that a primitive insect like a dragon fly is able to take advantage of that knowledge to hide from its prey.
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There is also a question of spatial resolution of the prey's visual system to consider, as well. A more coarse-grained vision (i.e. less photoreceptors/mm^2) would be easier to fool than a setup like our own -- which has densely packed receptors in the most sensitive central portion of the retina, called the 'fovea' -- because it would allow for more movement before the target (i.e. the dragonfly) activated more receptor units. Evolutionarily speaking, the fovea is quite new, compared to that old-school stuff you see in invertebrates. The downside is that our acuity is much lower everywhere but the fovea, so we now need a pretty intensive control system to steer the fovea where we need it to look. (But good for me, since studying this keeps me off the streets.)
Put these two limiting factors on the prey's visual system, and the dragonfly's feat is somewhat less daunting. But still pretty spiffy.
Theory and practice are the same in theory, but different in practice.