Arachnids have two main body segments (the cephalotorax and the abdomen) and four pairs of jointed legs. By contrast, insects have three main body segments and three pairs of legs—making them easily distinguishable from arachnids. Arachnids also differ from insects in that they lack wings and antennae. It should be noted that in some groups of arachnids such as mites and hooded tickspiders, the larval stages have only three pairs of legs and fourth leg pair appears after they develop into nymphs. Arachnids have an exoskeleton that must be shed periodically in order for the animal to grow. Arachnids also have an internal structure called an endosternite that is composed of a cartilage-like material and provides a structure for muscle attachment.
In addition to their four pairs of legs, arachnids also have two additional pairs of apendages that they use for a variety of purposes such as feeding, defense, locomotion, reproduction or sensory perception. These pairs of appendages include the chelicerae and the pedipalps.
Most species of arachnids are terrestrial although some groups (especially ticks and mites) live in aquatic freshwater or marine environments. Arachnids have numerous adaptations for a terrestrial lifestyle. Their respiratory system is advanced although it varies among the different arachnid groups. Generally, it consists of tracheae, book lung and vascular lamellae that enable efficient gas exchange. Arachnids reproduce via internal fertilzation (another adaptation to life on land) and have very efficient excretory systems that enable them to conserve water.
Arachnids have various types of blood depending on their particular method of respiration. Some arachnids have blood that contains heamocyanin (similar in function to the heamoglobin molecule of vertebrates, but copper-based instead of iron-based). Arachnids have a stomach and numerous diverticula that enable them to absorb nutrients from their food. A nitrognenous waste (called guanine) is excreted from the anus at the back of the abdomen.
- Araneaa (true spiders) - There are about 40,000 species of true spiders, making this the most species-rich of all arachnid groups. Spiders are known for their ability to produce silk from spinneret glands located at the base of their abdomen.
- Opiliones (harvestmen or daddy-long-legs) - There are about 6,300 species of harvestmen (also known as daddy-long-legs). Members of this group have very long legs and their abdomen and cephalothorax are almost completely fused.
- Acarina (ticks and mites) - There are about 30,000 species of ticks and mites. Most members of this group are very small, although a few species can grow to as much as 20mm in length.
- Scorpiones (scorpions) - There are about 2000 species of scorpions. members of this group are easily recognized by their segmented tail that bears a venom-filled telson (sting) at the end.
Arachnids belong to the group of arthropods known as the Chelicerata. The in addition to arachnids (spiders, ticks mites scorpions and harvestmen) the Chelicerata also includes horseshoe crabs and sea spiders.