simple eyes | 2016-05-11
continuing with eye month, let’s take a look at simple eyes.
simple eyes are any light receptor that contains only one lens. the name is slightly counter-intuitive because simple eyes can be quite complex, in and of themselves. for example, human eyes are simple eyes because they contain only one lens by which they focus. this is in contrast to complex eyes like the ones you can picture on a fly that have a tonne of lens which each focus light to different retina or retinula cells.
many insects and sea creatures have very simple eyes which they use to see only motion or colour differences instead of the multi-functional purpose of a human eye.
for example, a common bee has two complex eyes on the sides of its head and three simple eyes (or ocelli) in the center of its head which it uses to detect movement but nothing else. they look like small black bubbles.
some creatures, like jellyfish, flatworms, and starfish have pigment spot ocelli which simply sit at certain spots upon them to help them tell the difference between facing up and facing down by detecting the light from the top of the water.
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| flatworm vision |
the box jellyfish, one of the water’s deadliest creatures, has a whopping 24 eyes around it’s 1/2 inch frame. it contains packages of different eyes in a structure called a rhopalia. 16 of the eyes are simple photo receptors while the other 8 are slightly move complex photo-focusing receptors. 4 of those always point up, due to a ‘heavy’ crystal on the bottom of each rophalium, to help the box jelly see the contrasting colour of the mangroves it sees as safe homes and feeding ground.
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| box jelly |
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| jelly vision |
most mammals, birds, and reptiles have simple eyes with refractive corneas which means that their eyes deconstruct light in a more complex way than the air does, splitting light into different colours. thus, these creature perceive a much higher level of detail using their simple eyes than, say, a bee does with its simple eyes.
tune in next week when we’ll talk about some of the coolest eyes on particular creatures.
also, these bay scallops have a lot of tiny blue eyes… neat!:
so far on simple eyes
(images from National Geographic or Wikipedia)
