Close-up of a wasp showing three single ocelli and a compound eye. Image taken from an article on ocelli at www.wikipedia.org
Sticking with the carapace, you can be see that scorpions have multiple eyes; a fairly noticeable pair in the middle of the dorsal surface known as the median eyes and anything from 1 to 6 pairs on the antero-lateral surface (Just a posh way of saying 'slightly to the side and at the front') known as the lateral eyes. On a basic level, arthropod eyes can fall into two groups: poorly sensitive, simple eyes (made uper from a single OCELLUM) and more sensitive, compound eyes (clusters of OMMATIDIA- this may be sounding too confusing already, but ocelli and ommatidia are discussed below). In the former, sensitivity is geared towards detecting areas of strongly contrasting light intensity, whereas the latter can detect movement and light direction. In scorpions, the median eyes have always been simple, and although today they are more sensitive than the lateral pairs, these were once large and compound. Even though scorpions only possess simple eyes, studies have shown that they are remarkably sensitive, however, despite this fact, it is thought that scorpions rely primarily on their trichobothria and pectens for finding food.
Cross section of a single ocellum. Taken from Brusca and Brusca (1990)
OCELLI: Simple eyes can be found in a whole host of invertebrates and differences in their structure can be found. However, in scorpions they comprise a cuticular lens over a layer of cells known as the vitreous body. This in turn covers the retina (light sensitive cells... or to be a bit posh, photoreceptive cells).Cross section of a single ommatidium. Taken from Brusca and Brusca (1990)
OMMATIDIA: These are more comlex structures than ocelli. Like ocelli these have a cuticular lens (called a cornea just to add to the confusion) and a layer of epidermally derived cells (not a vitreous body this time but corneagen cells!) under which is a crystalline cone. Separating this part of the ommatidium from the photoreceptive cells (not a retina but retinula) is a crystalline stalk. In modern scorpions both median and lateral eyes are simple. Although the lateral eyes evolved from the earlier compound eyes (see below), they are not isolated ommatidia, but true ocelli, their different origin being identified by the fact that they lack a vitreous body.
Prosoma of Proscorpius osbornii showing the primitive condition for scorpion eyes. Reconstruction based upon KjellesvigWaering's 1986 monograph
EYE EVOLUTION As with early scorpions, eurypterids are known to have possessed simple, median eyes and pairs of compound eyes. Whilst all of these are found on the dorsal surface in the Stylonurina and Eurypteracea; in the Mixopteracea and Pterygotacea the compound eyes are found antero-laterally on the carapace, as might be expected if scorpions evolved from a small mixopterid (see general evolution). However, unlike eurypterids, early scorpions had anteriorally placed median eyes and these only later moved to a more central location, a movement that can still be seen in scorpion embryology (See the 2005 paper by Farley in the reference section ). In many Palaeozoic scorpions, the median eyes were located on prominent eye nodes. In the Palaeoscorpionina and Mesoscorpionina (don't worry about the names, a full description will be provided in the General evolution and Systematics sections, suffice to say both groups are relatively derived), the median eyes/ eye node was so far forward it actually formed an anterior median process (see carapace evolution).
The posterior movement of the median eyes, and reduction of the lateral compound eyes into 10 or fewer isolated ocelli is a trend which begins with the Palaeosterni (this includes fossils such as Eoctonus, Buthiscorpius and Anthracoscorpius) and as such, is only present in neoscorpions (according to Jeram, 1994) With the Neoscorpionina being found from the Carboniferous, the trends in eye evolution seen within the scorpion lineage happened long after their terretrialization and may therefore be linked to a change in predatory behaviour towards a more modern approach. Today, the primitive number of lateral eyes is 3, a number shared with the fossil orthostern Palaeopisthacanthus. Interestingly, the lateral eyes of scorpions are not the same as the secondary eyes of spiders. Although both groups have lost their compound eyes, in spiders the secondary eyes have become inverted (the light sensitive cells facing away from the lens) whereas in scorpions the lateral eyes remain direct and it is thought that they evolved through the fusion of ommatidial rhabdomes into a single retina and the similar fusion of the lenses.
Loss of Eyes
The loss of/ extreme reduction of eyes in scorpions that live in caves (ie. Liocheles polisorum) is well known. Although researchers such as Kjellesvig-Waering have used this as a family level criterium, in modern varieties it is only important on a species level (just compare L. polisorum with other species of Liocheles). This may also be true of fossil forms, but because lateral eyes are hard to preserve, conclusive eye loss in fossil scorpions has only been shown in a few Mesophonids and their habitats remain obscure.