Eyes and their Evolution

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.

The Carapace and its Evolution

This is the dorsal covering of the prosoma on which the eyes are found. Its shape can be variable although it is always longer than wide. As with other parts of the exoskeleton this can be ornamented. Common ornamentations are granules (whether covering the whole surface or just the margins) and carina. Whilst granules are usually round bumps on the surface, carina are long angular ridges, usually running from anterior to posterior and there may also be a thin median depression (sulcus) in the same orientation.

Reconstruction of the prosoma of Allopalaeophonus caledonicus. Adapted from Kjellesvig-Waering's 1986 monograph

CARAPACE EVOLUTION Ornamentation is of little importance in the higher classification of scorpions. In modern scorpions the shape of the carapace is likewize too variable to be of higher systematic importance, the same, however, can’t be said of fossil scorpions. One of the defining features of the Protoscorpiones (don't worry about names like this, they will be discussed in the sections on general evolution and systematics!) is an anterior margin of the carapace forming two bulbous areas. This unusual carapce is a derived feature of this group and not an ancestral condition, as neither eurypterids, nor any later scorpions possess it. To date, any functional significance of this shape is unknown.

Prosoma of Stoermeroscorpio delicatus. Reconstruction found in Kjellesvig-Waering's 1986 monograph

In conjunction with the extremely anterior position of the median eyes in palaeoscorpions (Proscorpius and Stoermeroscorpio), an anterior median node evolved. This is not seen in the more derived group of palaeoscorpions known as the archaeoctinoids, but if these specimens represent only 1 genus, it is possible that this feature was later lost. Again, the function behind this structure is unclear, however in some derived, Triassic, mesoscorpions such as Mesophonus this protrusion is also found.

Moving onto Modern forms, the median sulcus (which has been lost several times) can be quite diagnostic, with an inverse Y suture having evolved in the Hemiscorpidae, but lacking in the Bothruridae and many Scorpionidae, beyond this it is of little use in suprafamily relationships.

Liocheles waigiensis Showing the inverse-Y suture on the carapace. Although formerly Ischnuridae the Hemiscorpiidae has a complicated history (see the 'Scorpion Files' for an overview). Image courtesy of the 'Scorpion Files'- http://www.ub.ntnu.no/scorpion-files/

The Basics

Scientists love to use long and complicated names to describe various things. Whilst this can lead to different scientists understanding one another, it also makes relatively easy concepts incomprehensible to the lay person! I've provided some of the basics to scorpion morphology here, but another excellent website is http://www.ub.ntnu.no/scorpion-files/

Scorpions are ARTHROPODS, this means that they do not have bones, but instead have a segmented shell covering their jointed body and limbs. The Arthropoda includes spiders, ants, crabs and millipedes, each of which belong to a diffent group within the Arthropoda. The group of arthropods to which the scorpions and spiders belong is the CHELICERATA, this basically means that the body can be split into 2 major parts and instead of antennae (like all other arthropods), the 1st pair of limbs are modified into pincer-like jaws (chelae/ chelicerae).

The 2 main parts of a chelicerate are the PROSOMA (Head) and OPISTHOSOMA (Rest of the animal). However, if you look at a scorpion, you can see that the opisthosoma can in turn be split into a body (MESOsoma) and tail (METAsoma).

PROSOMA: This contains the chelicerae and the limbs that have been modified into pincers to help with catching food (PEDIPALPS). On the top (dorsal) side is the CARAPACE and EYES. on the bottom (ventral) side are all the LEGS and STERNUM.

MESOSOMA: This is made up from 7 segments. The BOOK LUNGS and GENITAL OPERCULUM and PECTENS are located on the mesosoma.

METASOMA: This has 5 tubular segments and the sting ACULEUS (just for confusion, this is also known as the telson). The anus is located on the 5th segment.

Introduction

This Blog Site is going to serve as a temporary home for a website on fossil scorpions and scorpion evolution. I have decided to begin this as I have noticed that very little can be found on the net regarding the evolution of scorpions and their kin. Although this site will touch on modern scorpions, the 'Scorpion Files' already provide an excellent resource (http://www.ub.ntnu.no/scorpion-files/).

Now, before I get onto the main purpose of this site, I think it would help to have someone try and explain the terms used when describing scorpion morphology. Don't get me wrong, a great many sites give a good overview of this, but none seem to bring the detail upto a level for someone to be able to understand technichal papers! After that I'll discuss how scorpions evolved through geological time and how each group may be related (so called systematics).

It may take me a while to put this together, but please keep checking back!