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#1
Old 05-17-2005, 12:32 PM
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Animals with Multiple eyes: How Do they Work?

Spiders (tarantuals) come with 6 eyes 9I think). Does anybody know how they are wired into the animal's brain? I assume that the smaller eyes have special functions, and the large eyes are used for longer distances.
Anyway, as for spiders-how far can they see? When they bit huge things like humans, what are they trying to do? obviously, they cannot hope to eat a prey animal as large as us, so why do they waste venom on lareg things?
The large spiders 9which attack and eat birds, mice, small mammals)-do they commonly eat this type of prey? or do they prefer smaller insects-I imagine a mouse is powerful enough (even in its death throes) to do a large spider irreparable damage.
#2
Old 05-17-2005, 12:41 PM
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They generally bite big animals like us to protect themselves. For example, if I'm wandering through a bitey spider's territory with a machete to clear away the underbrush, that spider might see me as a threat to it or its babies.
#3
Old 05-17-2005, 02:18 PM
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Just like animals with two eyes, only more so.
#4
Old 05-17-2005, 09:07 PM
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Quote:
Originally Posted by ralph124c
Spiders (tarantuals) come with 6 eyes 9I think). Does anybody know how they are wired into the animal's brain?
Someone does, certainly. Unfortunately it isnít me.

Seriously though, you need to realise that spiders donít have eyes like we do, nor does their vision compare to ours. Their eyes lack lenses and so they canít be focussed, the ayes are also fixed and canít swivel around as ours do. They really only provide basic information on movement, light/dark and so forth. With such basic visual information processing and limited control of the eye itself it isnít really all that difficult to wire multiple eyes to the brain.



Quote:
Anyway, as for spiders-how far can they see?
Itís a bit hard to answer that. They can Ďseeí for hundreds of millions of miles, just like humans can. A spider can certainly see the light of the sun for example. Itís doubtful they can see more distant stars.

Thatís the problem with asking how far something can see. It doesnít really mean much. Spiders donít have great resolution at range and are extremely short-sighted. As a result they probably couldnít see an elephant from more than a few metres away except as vague movement, but then there are people like that too.


Quote:
When they bit huge things like humans, what are they trying to do? obviously, they cannot hope to eat a prey animal as large as us, so why do they waste venom on lareg things?
As others have said, itís primarily a defensive move. Grab a spider or crush it and it will bite just to let you know itís there.

Quote:
The large spiders 9which attack and eat birds, mice, small mammals)-do they commonly eat this type of prey? or do they prefer smaller insects-I imagine a mouse is powerful enough (even in its death throes) to do a large spider irreparable damage.
A mouse isnít much stronger than an insect of equivalent weight and I suspect that a bird would actually be weaker. But to answer the question, yes, those spiders that eat mammals and birds usually do so routinely. Generally these are large tarantula type spiders that are themselves robust animals with large fangs and potent venom. They are often in the same weight range as their prey. There are also a few species of lighter spiders that catch birds in their webs. These use the same tactic as spiders use to catch insects, with the web effectively rendering the prey helpless an leading it to exhaust itself enough that the spider can finish binding it fairly safely.
#5
Old 05-18-2005, 12:12 AM
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The adult fly has 2 compound eyes consisting of 750-800 ommatidia or unit eyes apiece, each containing 8 photoreceptors in a 6+2 trapezoidal orientation (the 6 are on the outside and the 2 are on the inside). Their wiring is a pretty complicated thing. The outer 6 photoreceptors from each ommatidium project and make a primary synpase in the lamina of the optic lobe of the brain in a retinotopic fashion (in the same orientation that they are found in the eye).

The primary synapse, though, is slightly reorganized. Each of the 6 photoreceptors of each ommatidium projects in a slightly different orientation. It works out that each photoreceptor from each ommatidium projects in a similar orientation than a photoreceptor from 5 neighboring ommatidium. So there is a reshuffling; instead of the 6 ommatidial photoreceptors all synapsing on a single cassette of neurons in the lamina, 6 individual photoreceptors from 6 neighboring unit eyes project to a single lamina cassette.

The two inner photoreceptors of each ommatidium project to a deeper brain level, the medulla, and are mostly responsible for color detection.

Anyway, flies only have 2 main eyes, but they have 3 accessory light organs called ocelli on the far anterior surface of the head. These are organized in a triangle, such that one ocellus is more ventral than the other two. Each ocellus has one photoreceptor, and they are derived from the same larval tissue as the eye. These are horizon-sensors. The fly likes to orient itself so the top two ocelli are level above the horizon while the bottom one detects less light from below the horizon. These project to separate ganglia in the brain. There are also other photoresponsive tissues in the fly, including Hofbauer-Buchner eyelets, a few cells expressing a blue photopigment, and some dorsal neurons in the brain that appear to be involved in circadian rhythm.

You have to remember that stereoscopic vision is something that is not possessed by most animals. For instance, rodents or horses have very little if any overlap of their visual fields. I would assume that spiders and other creatures with many eyes (including things like scallops which have hundreds of eyes on the rims of their shells) use eyes more for primitive things like motion and light/dark detection. The resolution of these eyes cannot be all that great either, and are not used for things like tracking prey, but rather much more primitive tasks like evasion, circadian rhythms, and basic environment evaluation.
#6
Old 05-19-2005, 04:53 AM
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Quote:
Originally Posted by edwino
The resolution of these eyes cannot be all that great either, and are not used for things like tracking prey, but rather much more primitive tasks like evasion, circadian rhythms, and basic environment evaluation.

There are certainly numerous spider species that use their eyes for tracking prey. Although most spiders build webs there areplenty of species of wolf-spider, jumping spiders and so forth that run down their prey or ambush it. To do so they rely on their eyes. They may not have great resolution at range but these spiders certainly have good enough vision to grab a running insect without difficulty.
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