SPIDERS, SCORPIONS, MITES, AND TICKS
Number of families: 570 families
Arachnids (uh-RAK-nihds) are related to sea spiders and horseshoe crabs. Among the many members of this group are ticks and mites, scorpions, spiders, and even the common har-vestman, also known as daddy longlegs—all with their own distinct appearance. Despite these differences, many adult arachnids have two distinct body regions: the front portion, a sort of head area combined with a thorax, or midsection, contains the mouthparts and six sets of paired, leglike limbs, and the second portion, the abdomen, has a stomach, reproductive opening, and lunglike structures or breathing tubes. While arachnids all have these two body parts, some have narrow waists, and others have thick waists. In some arachnids, such as mites and harvestmen, the two body parts are closely joined together to form a single region; the two parts cannot easily be seen as different from each other. The front region of arachnids is covered by a carapace (KARE-a-pays), a smooth, shieldlike plate, which in some arachnids, such as harvestmen and sun spiders, is divided into three parts. In ticks, mites, and most spiders the abdomen is usually smooth, without any segments, but in all other arachnids the abdomen is plainly divided into segments.
Arachnids have mouthparts that look like small pinchers. They are used to capture and chew prey, or food animals. The mouth-parts are sometimes used as fangs to inject venom, or poison, and digestive chemicals into the wounds of their prey. Arachnids also have a small set of leglike structures, called pedipalps, phylum class subclass order monotypic order suborder family attached to either side of the mouth; in some arachnids these are used to cut and crush food, and, in others, they serve as antennae. Adult arachnids all have four pairs of legs. The legs often have bristles (brih-SUHLS), or short, stiff hairs, that can sense vibrations (vie-BRAY-shuns). The first pair is sometimes not used for walking at all and is used instead as feelers, or antennae. The other three pairs of legs are used for walking and digging, as well as for capturing prey. Some arachnids have fingerlike projections on their abdomens, used to produce silk; in spiders these structures are called spinnerets. Some arachnids have additional sensory equipment on their abdomens. Whip scorpions have a bristly and sensitive whiplike tail on the tip of the abdomen, and scorpions have comblike structures underneath the abdomen, used to detect vibrations.
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