It is clear that amongst the arthropods as a whole the compound eye has evolved at least twice, and possibly even more times.

Arthropods are the exemplar of the compound eye, most notably amongst the insects. In these animals there are actually two distinct ways in which the light can be collected, by apposition and superposition, and it is likely that these have evolved multiple times. In addition, it is clear that amongst the arthropods as a whole the compound eye has evolved at least twice, and possibly more times than that. In fact, convergent evolution of compound eyes can be recognised at two different levels: within the crustaceans, and between the major arthropod phyla, as decribed below.

Compound eyes in myodocopid ostracodes

The ostracodes are an important group of diminutive crustaceans, with their body enclosed in a bivalved carapace. Many have a simple median eye, but one group (the myodocopids) are active scavengers/carnivores and it appears that they have evolved a compound eye independently. Give that the crustaceans almost universally possess a compound eye this claim of convergence has met with some scepticism, but detailed analysis of the alternative hypothesis, that is that a compound eye has been lost multiple times, seems distinctly less likely.

Compound eyes and ancestral arthropods

At a more fundamental level there is now evidence that a fully functioning compound eye with precise focus and good visual acuity has evolved at least twice in the arthropods. Thus whilst there is little doubt that the ancestral arthropod possessed some sort of eye it is inferred to have been relatively simple and by implication the animal was slow moving. Moving on to consider the major living arthropod groups, the compound eye found in the chelicerates (that is the marine horse-shoe crabs and the terrestrial equivalents like scorpions and spiders (although in the latter there is sometimes the evolution of a camera-like eye, as in the dinopsids)) plus myriapods (i.e. centipedes plus millipedes) is different in a number of respects to the equivalent eye found in the crustaceans plus insects (it is widely agreed that the insects evolved from within the crustaceans). Not only is it very likely that these two groups (chelicerates+myriapods and crustaceans+insects) have arrived independently at a compound eye construction, but intriguingly only the crustacean plus insect type is predisposed to colour vision, as well as the ability to detect polarized light.

Arthropod vision: a case study in convergence

Arthropods provide a whole series of insights into the evolution of vision and the nature of convergence. Here we discuss the emergence of camera-like eyes, corneal nipple arrays, extreme visual accuity, fibre optics and eye loss.

Trilobites

The extinct trilobites were equipped with eyes composed of calcite. One type, the holochroal, is relatively simple, but the more advanced schizochroal eye has been claimed to have lenses with a doublet structure whereby the upper and lower layers have slightly different compositions and so can correct for spherical aberration (which is “convergent” with human technology of lens grinding), and in terms of such a correction has a parallel in the adjustment of refractive indices in the lens of the camera-eyes of cephalopods and vertebrates. There has been some debate with respect to the doublet structure in the trilobites as to whether it is generally original, or the result of chemical alteration after death and burial (diagenesis). A significant convergence between the calcitic compound eye of the trilobite is found with remarkably equivalent structures in the tips of the arms of the brittle-stars (echinoderms), and these latter structures derived from the mesodermal skeleton (stereom) are evidently optical.

Strepsipterans

The eyes of trilobites also have a striking convergence to those found in an interesting group of insects known as the strepsipterans. The insects, of course, do not have any calcite to form the lens, but the widely spaced ommatidia are strikingly reminiscent of the arrangement of the schizochroal eye and the convergence has attracted quite a lot of attention. It is worth adding, perhaps, that it is only the male strepsipteran that has these eyes which it employs on its nuptial flight, in search for the female which is permanently enclosed in a host insect (often a wasp), with only the reproductive organs protruding. Sharp eyes are presumably called for.

Ogre-faced spiders and mysid shrimps

Whilst compound eyes are the norm in the arthropods, and as well as in the insects and trilobites are also found in the crustaceans (which are probably ancestral to the insects) and chelicerates such as the horse-shoe crab. However, especially amongst the spiders the appropriately named ogre-faced spiders (or dinopids) have evolved a camera-like eye. In addition, a mysid shrimp has made a fair stab at transforming its compound eye into a camera-like eye.

Arthropods with extreme visual acuity

Many arthropods evidently possess good visual acuity; honeybees can recognize shapes and wasps other individuals. Particularly impressive in this regard is the remarkable compound eye of the mantid shrimp which obviously requires excellent vision for its hunting with its hammer-like claw (which convergently employs cavitation), and this animal also has extensive colour vision. Another striking example of visual acuity can be found in the jumping spiders (salticids), which are also highly effective predators. In particular, a telephoto component has been recognized. In addition, a number of insects have surprising adaptations to nocturnal activity, including halticinid bees such as Megalopta (that are also instructive with respect to the evolution of eusociality). A number of arthropods have independently evolved visual scanning of objects, notably the salticid spiders and some crustaceans.

Fibre-optics and eye loss in crustaceans

Arthropods have independently evolved a system similar to fibre-optics, notably in the deep-water crustaceans known as euphausiids and a galatheid pelagic crab (Pleuroncodes), in both cases evidently to maximise the amount of light collected. As is often the case in cave dwellers eyes are lost, and so it is with some deep-water crustaceans, but interestingly the eye is then recruited for a tactile/chemosensory role.

Fovea and pupil structures

In many vertebrates part of the eye is richly endowed with light-sensitive cells and so serves as the fovea. Much the same structure has evolved independently in some compound eyes, such as the mantids. These, of course, are well-known for their convergence with similar raptorial limbs in such groups as the mantids, and given their legendary ability to strike forcibly and accurately the visual acuity of their fovea doubtless has an important role. In addition, some insects have an analogue of the pupil.

Corneal nipple arrays

A striking feature of compound eyes, especially in the lepidopterans and trichopterans, are so-called corneal nipples, which form a striking array of projections across the eye surface. Most probably they have evolved more than once, and serve to reduce greatly the reflective glare from the eye surface and so render the eye (and animal) much less conspicuous to potential predators. There is, however, a much wider point of interest in the context of evolutionary convergence because a very similar arrangement of protuberances in aquatic animals helps to render them invisible.

Eye spots on butterfly genitalia

Finally, whilst arthropod eyes are used for vision, and so not surprisingly are normally found on the head, did you know that some butterflies have eye-spots on their genitalia?