The ocelli or simple eyes are a type of optical structure composed of a single lens which is present in many types of animals, especially insects. They are called simple eyes to distinguish them from compound eyes that have several lenses or receptor structures (ommatidia), in some insects tens of thousands.
Human and large animal eyes have a single lens, so they could be classified as simple eyes, although the term is usually reserved for simple eyes. simple eyes of arthropodsespecially insects and spiders.
There are other invertebrate animals, such as cnidarians (jellyfish, anemones, polyps), starfish or flatworms (flatworms), which have photoreceptor pigmentary structures that react to light stimuli and are also known as ocelli, more precisely eyespots or eyespotsbut they are not considered true eyes, since they do not have a lens system, retina or cornea.
Simple eyes in insects
Typical simple eyes are the simple eyes of arthropods, especially insects. Most insects have two large compound eyes located on their heads and a variable number of small simple eyes, called dorsal ocellisituated on the top of the head between the compound eyes.
In addition, the larvae of some insects also have ocelli on the side of their heads that are known as lateral ocelli or stemmatavery different in structure and function from the simple dorsal eyes of adults.
dorsal ocelli
The dorsal ocelli of insects are light-sensitive organs located on the dorsal or front part of their heads. They appear in very many species in variable numbers, generally more prominent in flying insects, for example in the order Hymenoptera (bees, wasps, ants), order Orthoptera (grasshoppers, crickets, locusts) or in the order Odonata (dragonflies, damselflies).
In the flying insects it is common for them to appear simple eye tripletstwo lateral ones that go to the left and right respectively, and a central one that goes to the front.
In terrestrial insects with ocelli there are usually only two, with the central ocellus absent. The ocelli of terrestrial insects are usually more to the side of the head, so they are sometimes called lateral ocelli, but they are dorsal type ocelli and should not be confused with the lateral ocelli or stemmata of the larvae that we will see later.
Anatomically, the dorsal ocelli consist of a lens or cornea and a layer of rod-type photoreceptor cells. The cornea is strongly curved in some insects (bees, dragonflies), but it can also be practically flat, for example in cockroaches.
The lenses of the dorsal ocelli do not usually have enough refractive power to form an image, but curiously they are very sensitive to light, more so than compound eyes, which would be very useful during flight.
In fact, one of the most widespread theories of the function of the dorsal ocelli in flying insects is that measure brightness changes in the environment and thus assist in stabilizing the flight; they could also perceive movement, which has been demonstrated in dragonflies. The median ocellus of dragonflies appears to respond to upward motion rods rather than downward motion, although in experiments this was only seen when light in the ultraviolet range was included in the stimuli.
Lateral ocelli or stemmata
Stemmata are a type of simple eyes that appear in insect larvae, many of which have neither compound eyes nor simple dorsal-type eyes until they reach their final stage of growth.
In the larvae of some groups of insects, especially Lepidoptera (butterflies) and Tentredinids, the stemmata appear on the sides of the head and are the embryonic and immature forms of compound eyes.
Stemmata also occur in the adults of many orders of arthropods that do not develop compound eyes even in adults; for example in fleas and springtails.
simple eyes in spiders
Some arthropods do not have compound eyes and instead have several pairs of large simple eyes. The clearest case is that of arachnids. Each pair of simple spider eyes is specialized for one or more focus areas, and they are usually classified into primary and secondary pairs.
Only the primary eyes have mobile retinas. The secondary eyes have reflective systems so that the photoreceptor cells receive direct and reflected light, thus increasing their sensitivity to low light conditions. The pair of primary eyes is usually located in front and in some species they are especially prominent and even have telescopic components, for example in nocturnal hunting spiders and jumping spiders.