Eyes: Eye Type:
Wasp with 3 Purple Jewels
Eyes: Eye Type:
Same fly's Compound Eye
Fly with 3 Ocelli: Black & White Giant Fly (Formosia speciosa)
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.