The tail and sting of a South African burrowing scorpion (Scorpionidae: Opistophthalmus sp.) © Jan A. Neethling 2012
Scorpions have been around for more than 420 million years. Originally aquatic, they were some of the first creatures to venture onto land and have since taken up a completely terrestrial lifestyle. They occur on every continent except Antarctica, and are most prevalent in arid regions, though many species do occur in the tropics. They are amongst the most recognizable arachnids and are characterized by an elongate body, pincer-like grasping appendages (called pedipalps), four pairs of walking legs and a tail that terminates in a sting (telson).
Image: Fossil of Eurypterus remipes, a sea scorpion that lived 420 million years ago. © Millard H. Sharp 2017
How far back can you trace your family tree? A couple of generations to maybe a couple of hundred years? How about 460 million years? Because that is when the granddaddy of arachnids swam in the earth’s oceans, making the arachnid family tree truly ancient. And with more than 100 000 identified species living today, their family tree is not only ancient, but gigantic.
Certainly the most recognizable of all arachnids, spiders are ubiquitous little predators on every continent except Antarctica. Their lineage stretches back 380 million years, with some of the oldest known fossils such as Attercopus fimbriunguis dating from the Devonian period and still having segmented abdomens.
Pre-Devonian in origin, the Pseudoscorpiones are one of the oldest extant lineages and over the past 392 Ma have diverged into more than 3400 known species in 26 families. Most are less than five millimetres in length, though they range from less than one millimetre in some Chthoniidae to just over ten millimetres in females of Garypus titanius Beier, 1961. They superficially resemble true scorpions, but lack the elongated tail and sting.
The 11 orders of Arachnids
The class Arachnida contains some of the most fascinating and enigmatic arthropods on earth, many of which are completely unknown to the general public. To remedy this, this article provides information covering each of the 11 different groups or orders of arachnids.