ELI5: Insects and sunlight
Kent - March 15, 2023
How do insects deal with sunlight in their eyes given that they have no eyelids and no moving eye parts?
The human eye uses a large lens which focuses light onto the retina. It’s a very effective way to capture light, and our retinas are quite sensitive over a wide range of brightness. A downside is that very bright lights like the sun get focused onto the delicate retina so intensely, it causes damage.
Insects, on the other hand, have compound eyes that don’t use large lenses. These eyes have an excellent field of view and ability to detect motion, but aren’t so sensitive to light.
Overall, insect eyes don’t focus light as intensely, and focus it onto less sensitive tissue.
They don’t really care because their eyes are sub-divided into hundreds of smaller directional compound eyes. The ones that are directly in line with the sun are blinded, but all the others are fine.
This sub-division gives them fairly poor image resolution because the eyes can’t focus or track, but they have excellent motion detection and field of view.
Take a look at a dragonfly’s head sometime - they’re seeing everything all the time, nearly full spherical coverage.
Blind spiders
There is a cool exception to this – the ogre-faced spider! These nocturnal spiders have two particularly large eyes with excellent night vision. Unfortunately, they have no eyelids or irises to keep out light during the day. Every morning when the sun rises, the light of the dawn burns out the spider’s retinas and blinds it. When the sun sets, the light-sensitive cells regenerate, and it’s ready to hunt once more.
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