I.

Introduction

Arachnid, term for animals in the class including the scorpions, spiders, daddy longlegs, mites, and ticks, and certain other eight-legged land invertebrates. Fossils suggest that arachnids were among the first animals to live on land, perhaps in the early Devonian Period, nearly 400 million years ago. About 60,000 species are known, although many, especially mites, remain undiscovered or undescribed. Arachnids are found throughout the world in nearly every habitat, but they reach their greatest size and diversity in warm arid and tropical regions.