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 eyes are strikingly different from the
myriapods, which were traditionally considered to be a sister group to the Hexapoda. Both ocelli and compound eyes were probably present in the last common arthropod ancestor, and may be apomorphic with ocelli in other phyla, Median ocelli are present in
chelicerates and
mandibulates; lateral ocelli are also present in chelicerates.
Origin No fossil organisms have been identified as similar to the last common ancestor of arthropods; hence the eyes possessed by the first arthropod remains a matter of conjecture. The largest clue into their appearance comes from the
onychophorans: a stem group lineage that diverged soon before the first true arthropods. The eyes of these creatures are attached to the brain using nerves which enter into the centre of the brain, and there is only one area of the brain devoted to vision. This is similar to the wiring of the median ocelli (small simple eyes) possessed by many arthropods; the eyes also follow a similar pathway through the early development of organisms. This suggests that onychophoran eyes are derived from simple ocelli, and the absence of other eye structures implies that the ancestral arthropod lacked compound eyes, and only used median ocelli to sense light and dark. It is deemed probable that the compound eye arose as a result of the 'duplication' of individual ocelli. The more complex
schizochroal eye was found only in one sub-order of trilobite, the
Phacopina (Ordovician-Silurian). There is no exact counterpart to the
schizochroal eye in modern animals, but a somewhat similar eye structure is found in adult male insects in the order
Strepsiptera.
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. Schizochroal eyes have up to 700 large lenses (large compared to
holochroal lenses). Each lens has a cornea, and each has an individual
sclera that separates it from the surrounding lenses. The multiple lenses for the eye were each constructed from a single
calcite crystal. Early
schizochroal eye designs appear haphazard and irregular – possibly constrained by the geometrical complications of packing identical sized lenses on a curved surface. Later
schizochroal eyes had size graduated lens. The
horseshoe crab has traditionally been used in investigations into the eye, because it has relatively large ommatidia with large nerve fibres (making them easy to experiment on). 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. Most other living chelicerates have lost their lateral compound eyes, evolving simple eyes in their place that vary in number. Up to five pairs of lateral eyes occur in scorpions, whereas three pairs of lateral eyes are typical for
Tetrapulmonata (e.g.,
spiders;
Amblypygi). Horseshoe crabs have two large compound eyes on the sides of its head. An additional simple eye is positioned at the rear of each of these structures. and not, as earlier interpretations had it, of clustered stemmata. that were thought to grow in rows, inserted between existing rows of ocelli. ==See also==