Arthropod eye
They are also possessed by Limulus, the horseshoe crab, and there are suggestions that other chelicerates developed their simple eyes by reduction from a compound starting point.
The organ's development through the lineage can be estimated by comparing groups that branched early, such as the velvet worm and horseshoe crab to the advanced eye condition found in insects and other derived arthropods.
Many other organisms, such as vertebrates and Cephalopoda are similarly and analogously dichoptic, which is the common state in animals that are members of the Bilateria and have functionally elaborate eyes.
In some groups of animals whose ancestors originally were dichoptic, the eyes of modern species may be crowded together in the median plane; examples include many of the Archaeognatha.
The result is that the eyes occupy most of the available surface of the head, reducing the area of the frons and the vertex and crowding the ocelli, if any.
Good examples may be seen in the Mantodea and Mantispidae, in which seeing prey from particular ommatidia in both compound eyes at the same time, indicates that it is in the right position to snatch in a close-range ambush.
Their eyes accordingly are placed in a good position for all-round vision, plus particular concentration on the anterior median plane.
Sometimes the needs for visual acuity in different functions conflict, and different parts of the eyes may be adapted to separate functions; for example, the Gyrinidae spend most of their adult lives on the surface of water, and have their two compound eyes split into four halves, two for underwater vision and two for vision in air.
[13][14] In addition, it has been shown in horseshoe crabs and spiders that Pax6 homologs are not expressed in the same way as their counterparts in insects, suggesting that Pax6 may not be required as a top-level eye patterning switch in chelicerates.
[17] Hexapods are currently thought to fall within the Crustacean crown group; while molecular work paved the way for this association, their eye morphology and development is also markedly similar.
The largest clue into their appearance comes from the onychophorans: a stem group lineage that diverged soon before the first true arthropods.
[2] A conflicting view notes, however, that compound eyes appeared in many early arthropods, including the trilobites and eurypterids.
And while many arthropods today have three, four, or even six, the lack of a common pathway suggests that a pair is the most probable ancestral state.
The eye morphology of trilobites is useful for inferring their mode of life, and can function as indicators of the palaeo-environment conditions.
This eye morphology was found in the Cambrian trilobites (the earliest) and survived until the Permian extinction.
Schizochroal eyes developed as an improvement on holochroal; they were more powerful, with overlapping visual fields, and were particularly useful for nocturnal vision and possibly for colour and depth perception.
Early schizochroal eye designs appear haphazard and irregular – possibly constrained by the geometrical complications of packing identical sized lenses on a curved surface.
It also falls near the base of the chelicerates; its eyes are believed to represent the ancestral condition because they have changed so little over evolutionary time.
[24] In addition to these obvious structures, it also has two smaller ocelli situated in the middle-front of its carapace, which may superficially be mistaken for nostrils.
[20] Unlike the compound eyes of trilobites, those of horseshoe crabs are triangular in shape; they also have a generative region at their base, but this elongates with time.
[20] However, members of the chilopod genus Scutigera have a compound eye, which is composed of facets [26] and not, as earlier interpretations had it, of clustered stemmata.
