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What Is an Arachnid?

Most people think of spiders when they see the word "arachnid," but arachnids includes other creatures that we don't usually associate with the term as well. Scorpions, mites and ticks are also part of this class, and you might be surprised to know that the "daddy long-legs" spiders, though belonging to the class arachnida, are in their own category, separate from other spiders. All arachnids are also arthropods, the phylum that includes other animals with exterior skeletons such as insects and crustaceans.

Appendages
- Most members of the class arachnida have eight legs that are paired symmetrically and jointed. The set closest to the front of the arachnid, called chelicerae, are often fang or pincer-like and equipped with venom for paralyzing prey. The next set of limbs, called pedipalps, sometimes also form pincers or are used as feelers or for grasping rather than for locomotion, as in tarantulas. Limbs positioned farther toward the posterior of the arachnid are used exclusively for movement.
Hunting Behavior
- All arachnids prey on living organisms, particularly smaller arthropods, though some, such as mites and ticks, survive solely on the blood and tissue from larger organisms. Some arachnids spin webs of silk to trap prey and wrap the victims to secure them. Often they wait motionless for prey to pass by; however, some arachnids are able to pursue prey and hunt freely. Arachnids rely on sensing vibrations to aid them in finding and killing prey. A few types of mite feed on plant matter, rather than animal matter.
Venom
- Many arachnids have a venom used to kill their prey; this venom is a neurotoxin, which paralyzes the victim and then breaks down the tissue into a liquid that the hunter can consume. All spiders have venom, excluding the daddy long-legs variety, as they are not true spiders. Scorpions also produce a venom in stingers located on their tails, rather than in fangs. Generally the neurotoxins arachnids are capable of injecting is not potent enough to cause damage to a person. However out of the 70,000 species of arachnids, about 30 types of spider and a handful of scorpions have venom that is dangerous to humans. Ticks and mites do not produce a venom, though some can carry diseases harmful to humans.
Notable Species of Arachnid
- A rare species of arachnid from the order Ricinuleida is Ricinoides, an African species that has an exoskeleton ranging from 5 to 10 mm thick. Whip-scorpions are different from other arachnids in that they lack eyes and are thin-skinned; one species of whip-scorpion, Palpigradi Leptokoenenia scurra, is strange in that it is semi-aquatic. Some of the smallest type of arachnid are ticks that are tiny enough to live in a human hair follicle.

Animals & Science