| about EYES |
| Eye Type |
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| Invertebrates have two main eye types; simple & compound. Some are totally blind and don't have eyes at all. Simple eyes are known as ocelli (this is plural, singular is ocellus). When we didn't know what they were we called them "jewels" as some wasps had very large purple ones and seemed to be an adornment. While not much is known about them, it is believed that when an animal has both compound eyes plus ocelli, the ocelli are used to help them fly and know their relation to the horizon. This theory seems to hold as most inverts that have compound eyes and don't fly have reduced ocelli. This is most obvious in ants where the alate (flying) ants have obvious ocelli, but the workers & soldiers have very reduced/invisible ones. It also seems ocelli aren't very good; think of living life with a toilet window stuck to your face. You see shapes, but no clear definition of detail. Most insects have 3 ocelli, however most Lepidoptera (moths & butterflies) only have 2. It is very difficult to see them with Lepidoptera as scales & antennae cover them (they are just above the compound eye, and very small). They are most visible with pale pyralids. There are inverts that have only simple eyes, spiders & centipedes, eg, and many larvae (caterpillars, etc). Spiders are well known for their ocelli, and clearly don't use them for flying ;-) They usually have 8 but can have 4 or 6. The interesting thing with spiders is the number & placement of their eyes is diagnostic (ie helps determine the specific species). Jumping spiders are well known for their 2 big anterior eyes, and wolf spiders for their rearward facing eyes. Sometimes they don't need all their eyes, and over time some have become reduced. When an invert dies, it's ocelli go a milky white colour. Compound eyes are of course much more complex, and can be thought of as being structures containing many ocelli; giving much better vision. Still not as good as ours, more like looking at old pixelated photo's. You can tell the difference between different people, but that's about it. Regardless, inverts are very successful with their impaired eye sight and rely on their other senses to help them survive. | |||||||||
| Insects in General |
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| Retired Professor of Anatomy, Ian Gibbins, has kindly shared his thoughts with us to help people learn: "Despite extensive differences in size, shape, and functional organization, all insect compound eyes share common ancestry and consist of the same repetitive structure, the unit eye or ommatidium... The insect retina can be divided into large territories, zones, bands, or stripes with different morphological or functional properties... Very often, dorsal and ventral regions of the retina show important differences in morphology, physiology, and Rhodopsin (visual pigment) expression..." (Trends Genet. 2015 Jun; 31(6): 316-328.) It turns out that in many insects, including dragonflies, crickets, butterflies, some bees and more, there are differences in the sensitivity of the upper and lower parts of the eyes: the upper parts are more sensitive to polarisation, which provides navigational info (the polarization of skylight follows the sun across the sky); upper parts often have yellow pigment in their lenses to filter out blue skylight => much higher contrast for moving objects agains a blue sky (like the filters you can use for black-and-white photography), and so on. As I suspected, some of the genetic control of this differentiation is known, but not a lot. But one thing does seem pretty well established => the underlying pattern is highly conserved and all the variants we see really are variations on a theme, controlled by subtle yet powerful molecular genetic mechanisms :) Ian Gibbins |
| Mayflies |
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| Retired Professor of Anatomy, Ian Gibbins, has kindly shared his thoughts with us to help people learn: Somewhere I vaguely remember reading about the function of those split eyes. Many insects have compound eyes that are effectively split into different zones with different optical properties / light sensitivities, usually for seeing different things above and below them. You can often see the difference in dragonfly eyes, but its really extreme in mayflies. Meanwhile, it's worth remembering that many creatures have split visual systems eg the various eyes in spiders, with the mediam anterior two usually specialsed for colour and detail, the rest more for movement detection... which is actually not very different from our own eyes... The human retina is effectively two concentric light detectors => the inner part is high resolution, colour; the outer part is mostly monochromatic, low light movement detection. Ian Gibbins |
| Whirligig Beetles |
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| Retired Professor of Anatomy, Ian Gibbins, has kindly shared his thoughts with us to help people learn: Whirligig beetles have split eyes: the upper part for seeing in air, the lower part for seeing in water. Turns out that the visual parts fo the brains are also mostly duplicated => the visual info is processed selectively from each input and then combined somehow. Their larvae, which are under-water ambush predators, have a different visual processing system again! Ian Gibbins |
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