The answer depends on who you ask.

The term "arachnid" likely brings to mind thoughts of spiders: from creepy, crawling creatures that quietly spin webs in the corner of a room, to large, hairy beasts with fangs – like the mythical Shelob or Aragog – that hunt you in the dark. But just how big (and ferocious) can arachnids be? What is the largest one that has ever lived?

Answering this question is deceptively simple. Arachnids are eight-legged arthropods, which means that this group includes not only spiders, but also scorpions and mites, as well as other, much smaller and rarer groups. Of these, spiders are the most diverse group of arachnids (the 50,000th species was recently discovered, and the number is expected to grow). The problem remains – the question of what exactly can be considered an arachnid is still current.
Photograph by Hemis / Alamy