Scorpions (Class Arachnida, Order Scorpiones)

The scorpions are an ancient group of arachnids that date back at least as far as the Silurian, over 400 million years ago. They probably began as aquatic arthropods, as evidenced by the lack of tarsal claws on the earliest fossil scorpion such as Palaeophonus. Some of the earliest forms may have had gills, although this is not firmly established. While scorpions had certainly invaded the land by the Devonian, at least some were probably terrestrial by the Silurian (ca. 420 million years ago). By the Carboniferous (350 million years ago) many were almost indistinguishable from modern scorpions. At least some (including the earliest) appear to have had compound eyes, but these have been lost except for a small cluster of “simple eyes” at the front corners of the carapace. Scorpions are now considered to be closely related either to the harvestmen (Opiliones) or to the extinct Eurypterida (often placed with the horseshoe crabs in the Class Merostomata). The exact relationships are still...