How do spider eyes work?

Submitted by NoMoreMonkeyBrain t3_115j2j5 in askscience

Are any/all of them compound eyes? Do each pair synch up to make a separate image? Do all their eyes synch up to make a single image? Are different eyes specialized for different shapes/colors/motion?

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PeteyMax t1_j93sebp wrote

Most spiders have two main eyes similar to ours, each having a lens and a retina with multiple light sensing organs for detecting images. Unlike our eyes, however, it is the retina rather than the eye that moves in order to both focus and direct the view. Since the field of view tends to be quite narrow and spiders cannot move their heads, in addition to the two main eyes, spiders also have between four and six other simple eyes scattered around their heads. These eyes can detect motion so that the spider can orient itself towards the source of interest.

Here is a video showing a translucent spider where you can see the motion of its retina:

https://www.youtube.com/watch?v=gvN_ex95IcE

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Allfunandgaymes t1_j94a6oz wrote

It isn't the same for all spiders. Some - like the jumping spiders - have extremely acute vision with front-facing primary eyes for images and auxillary side eyes to detect motion. Other spiders - such as tarantulas and many web weavers - are nearly blind, as they detect prey mainly through vibrations and do not require refined vision.

Mechanically, spider eyes are not like compound eyes of insects that transmit hundreds to thousands of micro-images to an immobile inner eye. They are made of single immobile lenses, with mobile retinas behind them that can shift focus to separate lens ports. This is readily apparent in certain species of spider with partially translucent exoskeletons and eye lenses, such as the magnolia green jumping spider. The two front eye lenses remain immobile, but you can see a small black retina shifting behind them. Because their lenses are immobile, the spider's field of view is very narrow, so they must turn their bodies to look around. Not all spiders are capable of such focused sight - the aforementioned tarantulas basically have a teeny tiny eyeport on the top of their bodies with rudimentary eye spots that can only detect light and some motion, but can't produce a focused image.

Generally, spiders that evolved to actively hunt down prey evolved the more sophisticated, forward-facing primary eyes, and those that adopted a more passive method of obtaining prey - like spinning a web and waiting for something to get caught - did not. The diversity of body morphology among spiders is truly mind-blowing.

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