[Q] Compound eyes

[paigetheoracle]

Why do insects have compound eyes? Is it down to their size and how it affects the way they see the world, in the same way that gravity has less effect on small creatures as compared to large animals? Is it anything to do seeing in the air as opposed to looking through liquid (Do small fresh or seawater creatures with exoskeletons have this kind of eye too and if not, why not, if related or roughly the same size?).:(

[Qfwfq]

I've read that a common fly can see simultaneously in almost all directions around it; that's part of the reason why they're so hard to strike.

 

Just think, how great it would be to have a similar vision system; you wouldn't need to swivel your head around on your neck every time a hot chick walked past and you want to get another good look, you wouldn't need to crane your neck up to see the highest bit of sky or tree branches, nor bend it down to find out what your foot knocked into! It's lucky for those critters that can't swing their head around and whose eyes aren't on tentacles, good thing their eyes cover such a wide range of directions.

[Vox]

I've read that a common fly can see simultaneously in almost all directions around it; that's part of the reason why they're so hard to strike.

 

Just think, how great it would be to have a similar vision system; you wouldn't need to swivel your head around on your neck every time a hot chick walked past and you want to get another good look, you wouldn't need to crane your neck up to see the highest bit of sky or tree branches, nor bend it down to find out what your foot knocked into! It's lucky for those critters that can't swing their head around and whose eyes aren't on tentacles, good thing their eyes cover such a wide range of directions.

 

"To see with a resolution comparable to our simple eyes, humans would require compound eyes which would each reach the size of their head."

 

Eye - Wikipedia, the free encyclopedia

 

Would you accept the tradeoff ? ;)

[Qfwfq]

Would you accept the tradeoff ?

I don't know I've never tried it...;) I've just kept craning my neck as usual according to where I need to look and sometimes even turning my whole self around.

 

If you look at a fly's head you can see they have to just put up with having huge eyes in proportion to their head. I guess they reckon it's worth the bother since they hardly have a neck.

Edited August 11, 2010 by Qfwfq
typo

[modest]

...Do small fresh or seawater creatures with exoskeletons have this kind of eye too...

 

Yes, as far as shell fish. Compound eyes are common in arthropods. For example: crab and crayfish. Fish are vertebrates and therefore don't have compound eyes.

 

~modest

[Ken]

The first photoreceptors were essentially a flat arrangement of cells that were chemically alterable by the presence or absence of light. They have been variously called light spots or light sensitive patches. The detection/non-detection of light is best understood as a digital coding signalling light on or light off.

 

The amount of useful information increases as the number of detectors increases IF there is some mechanism that differentiates between active and inactive receptors. To detect movement from right to left would require at least two receptors in horizontal alignment and some means of determining which was stimulated first. You can increase the number of receptors in a light spot but you don't get much increase in information beyond better movement detection and the gain in information is off-set by the reduced surface area for other functions.

 

There are only two ways that you can increase the number of receptors without expanding the surface spread of the sensory organ - build up or build in. What you get is either _____n____ or ------u-----, i.e., receptors covering the surface of a bulge or receptors covering the walls of an indentation. The first arrangement, with receptors on the outer surface of a bulge, is termed a Compound Eye while the arrangement of receptors on the surface of an indentation is called a Simple Eye.

 

You have to be careful with these terms since the most successful eyes are highly complex Simple Eyes while the Compound Eyes never achieved similar levels of data detection or analysis.

 

The Compound Eye is a relatively efficient motion detector, with more precise detection directly related to the number of individual receptors (Ommatidia). It's efficient because each ommatidium has a direct "line" to the (rudimentary) nervous system of the organism. Essentially the ommatidia work as a binary-coded system, either On or Off. Lacking much in the way of signal analysis or interpretation to slow things down, the incoming signal produces an extremely rapid response sequence. This works well whether the organism is a predator or prey.

 

The alternate evolutionary branch was to line an indentation with receptors. Under some conditions this was successful in survival and higher survivability was related to the depth of the indentation. Some of that success was due to the optics of the situation where the wall of the indentation, or pit, served to eliminate extraneous movement in much the same way that blinders reduce visual distraction for horses.

 

The challenge for a pit visual system is by providing more detailed information about the external movement, that information is wasted without some increased neural analysis of the data. Two features were selected by the environment for increased survivability - deeper indentations and more complex neural systems.

 

The last step in the pit-type eye came with a small Cephalopod, the Nautilus, which has been viable for millions of years. The pit walls curved in forming the equivalent of a pin-hole camera. The tiny aperture created an optical system without a lens. The result of that adaptation is the incredibly high resolving power of some Simple Eyes, with Avian (bird) eyes having amazing power to resolve minute visual differences.

 

So, to answer the primary question - insects and arthropods have compound eyes because environmental pressures served to select those adaptations which eventually led to that form. The Simple (in name only) Eye followed an alternative evolutionary path, one which led to adequate motion detection and highly enhanced detail detection.

[Tormod]

Brilliant and very informative answer, Ken! Thanks.