Sun spiders are related to other spiders, but differ from them in having very well developed mouth parts. There are over 600 known species of sun spider. They live in the hot, dry, parts of the topics, though some are found living in humid forest environments. The biggest of them rival the bird eating spiders in size. For the most part they are nocturnal but there are a few members of the order which love the sun and are very active in the burning tropical rays. Sun spiders can run very fast on their long hairy legs. They never drink water, since the fluid they derive from their prey is all they need to survive. Sun spiders are not poisonous but are capable of inflicting extremely painful bites.