Legs, the Pedipalp and their Evolution

If we turn the prosoma over so we are looking at the underside (ventral side), we can now see the sternum, pectines, legs and jaws. Being chelicerates, scorpions have eight pairs of walking legs and another pair of legs modified for manipulating food (the pedipalps). Each of the legs seen in scorpions are made of 7 podomeres (leg segments). Within the Arthropoda, these have been given various names and there is some confusion over their homology (a posh way of saying we don't really know which segments in the legs of crabs are the same in scorpions or moths etc.). Working our way to the tip of a scorpion leg, the segments are the coxa, trochanter, femur, patella, tibia, basitarsus and tarsus. The pedipalps of scorpions have reduced segmentation (6 segments including the moveable finger) and these are known as the coxa, trochanter, femur, patella and manus (occasionally known as the tibia). As with the chelicerae, the moveable finger is known as the tarsus. Whilst the limbs can be important in understanding scorpion evolution, equally important are the sense structures on them and these will be looked at in the next section.

LEG EVOLUTION The legs of terrestrial arthropods are characterised by a differentiation in podomere length and an oval/compressed cross section in order to support the animals weight on land. We should be careful in using this feature as a sole indication of habitat though, as in very large aquatic eurypterids such as Tarsopterella this can also be seen - because they are so big they needed greater support!. It is generally thought that terrestrialisation of the scorpion lineage took place somewhere in the Palaeoscorpiones. If this is true then you might expect the aquatic protoscorpions to demonstrate the primitive condition. Sure enough, the four genera belonging to the Protoscorpiones (Allopalaeophonus, Palaeoscorpius, Palaeophonus and Dolichophonus) all have podomeres of roughly equal length (albeit decreasing distally) and tubular cross sections. Although Stockwell (In a very important, widely read but unpublished PhD thesis!) claimed the proscorpioids also showed this feature, Jeram (1997) codes these fossils as possessing the more typical, modern condition.

PEDIPALP EVOLUTION Chelate (pincer-like) pedipalps are found in ALL scorpions, but if scorpions evolved from mixopterid eurypterids (which also have chelate pedipalps) this is unsuprising. Scorpion pedipalps come in 2 main forms, GRACILE (with fingers longer than the palm) and ROBUST (with fingers shorter than the palm). The different pedipalp morphs are related to the mode of feeding in scorpions (Gracile morphs being found in much more toxic varieties and Robust form being used to crush prey in less toxic scorpions). It is of little importance in the systematics of fossil scorpions, but in recent scorpions it can be quite important as the Buthidae are all Gracile scorpions.

Photo of the extant, gracile scorpion Lychas suctilus taken from the 'Scorpion Files'.

Image of the Robust clawed Tarsoporosus yustizi taken from the 'Scorpion Files'.