Arachnida is a class in the subphylum Chelicerata. For more taxonomic information, see "Taxonomy" in the left-had navigation menu. Arachnids include many common and well-known animals, such as spiders, scorpions, mites, and ticks. Arachnids ARE NOT INSECTS. Some people refer to "spiders" as "bugs," which is inaccurate in several respects.
What makes an arachnid an arachnid?
There are several characters that distinguish animals in the order Arachnida from animals in the order Insecta. For one thing, Arachnids do not have antennae. Some arachnids have locomotory appendages adapted as feelers, or a little like a blind individual's cane, such as the order Uropygi, also known as Thelyphonida, commonly known as a vinegaroon, but these are not known as true antennae.
Another useful character in determining an arachnid is the presence of eight legs. All arachnid adults grow up to have four pairs locomotory appendages. Their mouthparts, the palps and chelicerae, are technically appendages that evolved into mouthparts slowly over time.
Arachnids have one or two tagma, or body segments, but never three. Spiders have two: a cephalothorax and an abodmen. The cephalothorax, as its name implies, is a combination head and thorax.
No arachnids have wings, but don't think this limits them to living close to the ground. Newly born baby spiders sometimes practice "ballooning," the act of releasing a length of silk and letting the wind carry the tiny spiders anywhere from a few feet to hundreds of miles. Some species of spiders live high up in trees, catching flying insects. Mites, which are also arachnids, are so tiny, they are usually microscopic, and can travel through the air like dust.
Why should we care about arachnids?
Some arachnids are important vectors of disease: ticks can transmit diseases like Lyme Disease and Rocky Mountain Spotted Fever. House mites, which you're most likely inhaling right now, are one of many causes of indoor allergies. Many species of spiders create webs to trap flying insects, which include disease vectors like mosquitoes. My favorite reason why arachnids are important is simply because they're really cool. Spiders are my favorite animal of all time. What's not to love about eight eyes, eight legs, silk, fangs, and venom?