Arthropods are famous for their compound eyes, but some groups have had a fair crack at evolving the optically superior camera eye…

Arthropods possess a variety of eyes, but undoubtedly the most characteristic are the compound eyes, which even within this phylum are convergent. As the optical acuity of camera eyes is considerably superior to that of compound eyes and as camera eyes seem to be relatively easy to evolve (having appeared at least six times independently), there has been considerable discussion as to why such an eye has never arisen in the arthropods. Or maybe it has? In point of fact, several groups have had a fair crack at arriving at this optical solution…

Ogre-faced spider eyes

Most striking among arthropod eyes are those of the aptly named ogre-faced spiders (e.g. Deinopis). These curious creatures suspend a net of silk between their long front legs, which is then cast over their prey – a rather unusual hunting technique. Because they hunt at night, their eyes need to be highly sensitive. Indeed, these spiders have a gigantic set of posterior median eyes (which are, in fact, among the largest single lens eyes found in arthropods) with a huge aperture that are excellent at gathering light. Their lens, which is similar to a fish lens, suppresses spherical aberrations and provides excellent spatial resolution. Interestingly, adult Deinopis males, which do not construct a net, have much smaller posterior median eyes than females.

Strepsipteran eyes

Another example of a move towards something like a camera eye is found in a group of insects known as the Strepsiptera (or twisted-winged insects).These insects have a curious life cycle. While the female is parasitic for its entire life, embedded in a host insect with only the reproductive organs sticking out, the male is free-living for merely an hour or so when it seeks out the female. It is during this nuptial flight that the male strepsipteran requires sharp eyes, as it must locate the reproductive organs of the female protruding from the host. Here the compound eye has undergone an extraordinary transformation. It now bears a series of relatively enormous lenses, surrounded by a hard cuticle that typically bears rather prominent spines. Unlike the usual compound eye of insects, where typically each lens serves as only a point source, in the strepsipterans each lens can provide a separate image. How these are integrated by the brain is not known, but it is possible that this type of eye acts more as a general light collector rather than providing high acuity. It is likely that the strepsipterans originated from nocturnal ancestors and this might explain the transition from the classic compound eye.

Trilobite eyes

The extinct trilobites, which flourished in the Palaeozoic oceans, were equipped with compound eyes composed of calcite.Several eye types have been described for this group. Holochroal eyes were relatively simple, consisting of hundreds of minute lenses, whereas schizochroal eyes were more advanced. They had fewer, larger lenses often clearly separated from each other, an arrangement strikingly reminiscent of that found in the eyes of modern strepsipterans. Schizochroal eyes were limited to the bottom-dwelling phacopids and have been claimed to contain lenses with a doublet structure. Here, slightly different compositions of the upper and lower layers allow correction for spherical aberrations (similar to the human technology of grinding a lens to correct for just such aberration). There has, however, been some debate as to whether this doublet structure is genuinely original or the result of chemical alteration after death and burial within sediment (diagenesis). Another intriguing convergence on the calcitic compound eye of trilobites involves the brittle star Ophiocoma wendtii. Like in all echinoderms, its exoskeleton is composed of calcareous plates, but remarkably those located on the upper arms form an array of calcitic microlenses with exceptional optical performance. These lenses compensate for spherical aberration, and the amount of light they receive seems to be controlled by adjacent chromatophores. The whole system is thought to function like a compound eye, possibly enabling its bearers to detect predators and seek out hiding places.

Mysid shrimp eyes

Amongst the other arthropods that have constructed a camera-like eye we find a crustacean, the mysid shrimp Dioptromysis paucispinosa. With a body length of merely 5 mm, this tropical shallow-water species is one of the smallest mysid shrimps and, accordingly, its eyes are tiny (approx. 0.4 mm). They are compound eyes of the refracting superposition type – but with a twist. In the rear of each eye sits a huge lens, which projects an upright image onto a specialised retina consisting of densely packed, narrow photoreceptors. In the resulting acute zone, resolution is enhanced by about six times (but at the expense of sensitivity), which probably aids the detection of small prey. Thus, this shrimp basically possesses a simple camera eye within a compound eye and has aptly been described as a “shrimp carrying a pair of binoculars” (Nilsson and Modlin 1994, Journal of Experimental Biology, vol. 189, pp. 213-236). Oddly enough, the acute zone points backwards, but the eyes are located on stalks, which can be rotated to a considerable degree. A possible advantage of this arrangement could be that the forward visual field is not impaired by a region of inferior sensitivity, possibly crucial for the detection of predators.