Arachnids are the group of arthropods that include spiders, ticks, scorpions and mites. They have several defining characteristics.

• The head and thorax are fused together into one body part called a cephalothorax.
• They have four pairs of legs attached to the cephalothorax.
• They have two other sets of jointed appendages.
  • The first pair is called the chelicerae. These appendages are used to pierce their pray and suck out its body fluids.
  • The pedipalps form a second pair. They are used as sensory organs.

Spiders are the best-known arachnids. All spiders produce a form of liquid silk, but not all spiders spin webs. Spiders have tiny slits in their exoskeletons called book lungs to allow air into their bodies.