Arachnid << uh RAK nihd >> is the name of any member of a class of small, insectlike animals. The best-known arachnids are spiders, ticks, mites, scorpions, and daddy longlegs (also called harvestmen). Arachnids, unlike insects, have no wings. Their bodies are divided into two main parts, the abdomen and the cephalothorax, which consists of the head and the thorax joined together. Insects, however, have three main body parts: the head, the thorax, and the abdomen. Arachnids have four pairs of legs but have no antennae (feelers). Insects have antennae, but only three pairs of legs.
Arachnids have from one to six pairs of simple eyes. Some species are eyeless. Unlike insects, arachnids have no compound eyes (eyes that are made up of many smaller eyes crowded together). Some arachnids breathe like insects by means of air tubes. Others have breathing organs somewhat like lungs, called book lungs. These are small sacs within the abdomen, connected with the outer air by small openings. Within each sac are many layers of tissue resembling the leaves of a book. The air enters the sac through the small openings and furnishes oxygen to the blood flowing through the leaves. Most spiders have air tubes and book lungs.
Some arachnids are harmful to human beings. Certain of them can inflict venomous bites or stings. Others suck the blood of human beings and animals, and may carry serious diseases. Many arachnids are helpful to human beings because they eat harmful insects.
