Ricinuleids are commonly known as 'hooded tick spiders' because of their hood-like structure that cover their mouthparts. There are approximately 55 described species.

Ricinuleids were thought to be the rarest of the arachnids. Modern explorations into Mexican caves have now shown that they are more widespread than previously thought.

Ricinuleids have other unique features including:

  • copulatory organs on the third pair of legs of the adult male
  • a telescopic pygidium
  • a locking device which couples the prosoma (head region) and abdomen

The Ricinulei is the only arachnid order that was first known from a fossil, and this was originally thought to be a beetle. 


The pygidium is the posterior body region of some insects and invertebrates.