Apposition eyes are the most common type of eye and are likely the original form of compound eyes. These eyes are found in all groups of arthropods, though they may have developed more than once within this group. Some annelids and bivalves also have apposition eyes. These eyes are also found in Limulus, the horseshoe crab, and there are suggestions that other chelicerates developed their simple eyes by reducing a compound eye starting point. Some caterpillars seem to have evolved compound eyes from simple eyes in the opposite way.

Arthropods originally had compound eyes, but the type and origin of these eyes differ among groups. Some species later developed simple eyes instead. By comparing groups that branched off early, such as velvet worms and horseshoe crabs, to the more advanced eye structures in insects and other evolved arthropods, scientists can estimate how the eye developed over time.

Eyes and functions

Most arthropods have at least one of two types of eyes: large compound eyes on the sides of their heads and smaller simple eyes called ocelli in the center of their heads. When both types are present, they work together because each has special strengths. Some insect larvae, like caterpillars, have a different kind of simple eye called stemmata. These eyes usually form only rough images, but in some species, such as sawfly larvae, they can see fine details, detect light patterns, and become much more sensitive to light at night. Flying insects can still fly well if one type of eye is removed, but using both types improves their vision. Ocelli are better at seeing in low light and react quickly, while compound eyes are better at seeing edges and forming clear images.

• Many insects, like the female Tabanus lineola shown here, have compound eyes that are on opposite sides of their heads.
• The male Tabanus lineola has compound eyes that are joined together, with the top parts of the eyes having larger lenses than the bottom parts.
• In some male mayflies, the eyes are divided into separate parts for different visual tasks.
• River crabs have compound eyes that are on opposite sides of their heads, and the way light reflects off their eyes shows how they use 3D vision.
• Many small crustaceans, like copepods, have a single compound eye in the middle of their head, visible as a dark spot near their antennae.

Most arthropods with compound eyes have two eyes, one on each side of their head, placed symmetrically. This arrangement is called dichoptic. Examples include most insects and larger crustaceans, like crabs. Many other animals, such as vertebrates and cephalopods, also have dichoptic eyes, which is common in animals with complex vision systems. However, some animals have different eye arrangements. In some species, eyes are pushed together in the middle of the head, as seen in some Archaeognatha. In extreme cases, eyes may fuse into one, as in some copepods, like the genus Cyclops. This arrangement is called cycloptic.

Some animals need very sharp vision, which requires many small lenses in their compound eyes. This makes their eyes large, covering most of their head and reducing space for other features. Though still technically dichoptic, in extreme cases, the eyes may meet, forming a cap over the head. This is called holoptic. Examples include some dragonflies and flies, like certain species of Acroceridae and Tabanidae.

Other animals need precise vision for tasks like hunting. For example, mantises and some flies have compound eyes that allow them to see prey from specific lenses in both eyes at the same time, helping them strike accurately. Their eyes are positioned to provide full vision and focus on the front middle area. The lenses are arranged in all directions, creating a dark spot (called a pseudopupil) that shows which lenses are looking at a particular area. This spot appears the same on both eyes when viewed from the front middle area.

Sometimes, different parts of an eye may serve different purposes. For example, water beetles have compound eyes split into four parts: two for seeing underwater and two for seeing above water. In some flies, the top part of the eye has larger lenses than the bottom part. In some mayflies, the top part of the eye is raised, while the bottom part looks like a separate structure.

Compound eyes are not always perfectly symmetrical. For example, honeybees and some flies have slightly different numbers of lenses on each side. This asymmetry has been linked to behavior patterns in ants, such as a tendency to turn in one direction more often.

Genetic controls

In the fruit fly Drosophila melanogaster (a species widely studied for how animals develop), two genes called eyeless (ey) and twin of eyeless (toy) are among the most important for forming insect eyes. These genes help cells grow and multiply during early eye development. If either gene is missing, the eyes do not form properly. The activity of ey and toy causes other genes, sine oculis (so) and eyes absent (eya), to become active. These genes work together to control the activity of other genes that are needed later in eye development. After this, the two types of eyes in D. melanogaster develop differently. The front part of the head is controlled by a gene called orthodenticle (otd), which marks the area from the top-middle of the head to the sides. The ocelli (simple eyes) are located in an area rich in otd. If otd is missing, the ocelli do not form, but the compound eyes remain unaffected. In contrast, a gene called dachshund (dac) is needed for the compound eyes to develop. If dac is missing, the compound eyes are not affected, but the ocelli remain. Different types of opsins (proteins that detect light) are used in the ocelli of compound eyes.

The visual systems of Chelicerata (a group of arthropods closely related to other arthropods) are not as well understood. Studies show that genes similar to those involved in eye development in insects are expressed differently in the eyes of various spider species. However, the reasons for these differences are unclear due to a lack of experiments testing their functions. In horseshoe crabs and spiders, the Pax6 gene (similar to eyeless and twin of eyeless in insects) is not used in the same way as in insects, suggesting that Pax6 may not be the main gene controlling eye development in chelicerates. Most research on eye development in chelicerates comes from the daddy-longlegs Phalangium opilio. Studies on this species show that eyes absent plays a role in forming both types of eyes (a shared function with insects), and dachshund affects the development of side eyes but not the middle eyes (another shared function with insects).

Evolution

Hexapods are now believed to be part of the Crustacean group. Scientists found this connection through molecular studies, and the structure and development of their eyes are also similar. The eyes of hexapods are very different from those of myriapods, which were once thought to be closely related to hexapods.

Both ocelli (simple eyes) and compound eyes were likely present in the last common ancestor of all arthropods. These features might also be unique to other groups, such as annelids. Median ocelli (central simple eyes) are found in chelicerates and mandibulates, while lateral ocelli (side simple eyes) are also present in chelicerates.

No fossils have been found that look like the last common ancestor of arthropods. This means scientists still do not know what the eyes of the first arthropods looked like. The best clue comes from onychophorans, a group that split from the arthropod line before true arthropods appeared. These creatures have eyes connected to the brain through nerves that enter the center of the brain. Only one part of the brain handles vision, similar to the median ocelli of many arthropods. This suggests that onychophoran eyes evolved from simple ocelli. Since no other eye structures are found in onychophorans, it is likely that the earliest arthropods only had median ocelli to detect light and darkness.

Some scientists argue that compound eyes appeared in early arthropods, such as trilobites and eurypterids. This suggests compound eyes may have evolved after onychophorans and arthropods split but before arthropods diversified. This idea is supported if Cambrian organisms with compound eyes, like Radiodontids, are closely related to arthropods. Another possibility is that compound eyes evolved independently multiple times in arthropods.

The earliest arthropods probably had only one pair of ocelli, as Cambrian fossils show this. While many modern arthropods have three, four, or six ocelli, the lack of a shared pathway suggests a single pair was the original state. Crustaceans and insects mostly have three ocelli, indicating their common ancestor had this arrangement.

It is likely that compound eyes evolved from the duplication of ocelli. This led to networks of independent eyes in some arthropods, like the larvae of certain insects. In some insects and myriapods, lateral ocelli may have formed by reducing lateral compound eyes.

Trilobites had three types of eyes: holochroal, schizochroal, and abathochroal. These eye structures help scientists understand how trilobites lived and the environments they inhabited.

Holochroal eyes were the most common and ancient. They had many small lenses (100 to 15,000) covered by a single cornea. These eyes were found in early Cambrian trilobites and lasted until the Permian extinction.

Schizochroal eyes were more complex and found only in the Phacopina suborder of trilobites. These eyes had up to 700 large lenses, each with its own cornea and sclera. They improved vision compared to holochroal eyes, helping with night vision and possibly color and depth perception. Early schizochroal eyes had irregular lens arrangements, but later versions had lenses of varying sizes.

Abathochroal eyes were found only in Eodiscina trilobites. These had up to 70 small lenses, each separated by a cornea and sclera.

Horseshoe crabs are often studied for their eyes because they have large ommatidia (eye parts) with big nerve fibers, making them easy to examine. They are near the base of the chelicerates, and their eyes are thought to represent an ancient design because they have changed little over time. Most other chelicerates have lost their compound eyes, replacing them with simpler eyes. Scorpions have up to five pairs of lateral eyes, while spiders and related groups typically have three pairs.

Horseshoe crabs have two large compound eyes on the sides of their heads. Each has a small simple eye at the back. Two smaller ocelli are on the front of their carapace, and another simple eye is on the underside. These eyes may fuse during development. Additional simple eyes are near the mouth. Simple eyes are likely important during early life stages, while compound eyes and median ocelli become dominant in adulthood. These ocelli are simpler than those of Mandibulata. Unlike trilobite compound eyes, horseshoe crab compound eyes are triangular and have a growing region at their base. The original eye of the larva was a single ommatidium at the triangle’s tip, with more rows added as the crab grew.

Insects are generally considered part of the Crustacea group, which is a single evolutionary group. This is supported by the similar eye development in both groups. While many crustacean and insect larvae have only simple median eyes, like the Bolwig organs in fruit flies or the naupliar eyes in crustaceans, some larvae have simple or compound lateral eyes. Adult compound eyes develop in a different part of the head than larval median eyes. New ommatidia form in semicircular rows at the eye’s back. Early in growth, these are square, but later become hexagonal. The hexagonal pattern becomes visible after molting.

Some crustaceans and insects have stalked eyes on peduncles (stalks). Only certain crustaceans, like crabs, have articulated peduncles that allow their eyes to move out of the way.

Most myriapods have stemmata (single-lensed eyes) that evolved from compound eyes. However, the chilopod genus Scutigera has compound eyes made of facets, not clusters of stemmata as previously thought.