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... ;)"

Comments

Re:How they manage it still has them puzzled...

[barakn]

It doesn't seem that simple to me. 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. This very unstealthy maneuver shows that trying "to keep the thing it's hiding from in the same position on *its* retina" is insufficient.

In the simplest case, with the prey not moving, all the knowledge the dragonfly needs is the position of the prey. The solution is to fly straight at the prey. It never seems to move from its position on the background but appears larger and larger as it moves in for the kill.

Cases with the prey moving are more difficult to visualize. You can simplify it by assuming that they are confined to a 2D plane and then drawing their positions on a sheet of paper (or a computer screen). Imagine two diifferent scenarios:

Case #1. The dragonfly is on a straight line and about half way between the prey and a bush. The dragonfly is superimposed on the bush, from the prey's point of view. The prey is flying perpendicular to this straight line. In order to stay on a straight line between prey and bush, the dragonfly must also move.

Case #2. Same situation, except that now the dragonfly is practically touching the bush. The prey moves but the dragonfly hardly needs to move at all to appear to remain at the same spot on the bush.

It should thus be obvious that the distance of the dragonfly to the background object is an important variable. Perhaps it somehow memorizes what object is exactly 180 degrees away from the prey, and then it keeps an eye on both at the same time and flies so as to maintain their positions 180 degrees apart on its retinae (both objects might drift across the retinae, so long as they are exactly opposite each other).

Re:A few things guessed...

[KillerBob]

Dragonflies catch many insects, not just flies. You're right that most bugs have compound eyes, though, so it is an interesting question.

A point, though... they wouldn't have to be thinking "human-style" eyes. It's probably more of a defensive measure than an offensive measure, though... as birds have normal eyes, not compound eyes. Their eyes are also, largely, on the sides of their heads, and wouldn't give them great depth perception.

Still... a very interesting idea.