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Morphology

Almost all adult arachnids have eight legs, unlike adult insects which all have six legs. However, arachnids also have two further pairs of appendages that have become adapted for feeding, defense, and sensory perception. The first pair, the chelicerae, serve in feeding and defense. The next pair, the pedipalps, have been adapted for feeding, locomotion, and/or reproductive functions. In scorpions, pseudoscorpions, and ricinuleids the pedipalps end in a pair of pinchers, while in whip scorpions, Schizomida, Amblypygi, and most harvestmen, they are raptorial and used for prey capture. In Solifugae, the palps are quite leg-like, so that these animals appear to have ten legs. The larvae of mites and Ricinulei have only six legs; a fourth pair usually appears when they moult into nymphs. However, mites are variable: as well as eight, there are adult mites with six or, like in Eriophyoidea, even four legs. While the adult males in some members of Podapolipidae have six legs, the adult females have only a single pair.

Arachnids are further distinguished from insects by the fact they do not have antennae or wings. Their body is organized into two tagmata, called the prosoma and opisthosoma, also referred to as the cephalothorax and abdomen. However, there are questions about the validity of the latter terms. While the term cephalothorax implies a fused cephalon (head) and thorax, there is currently neither fossil nor embryological evidence that arachnids ever had a separate thorax-like division. Likewise, the 'abdomen' of many arachnids contains organs atypical of an abdomen, such as a heart and respiratory organs.

The cephalothorax is usually covered by a single, unsegmented carapace. The abdomen is segmented in the more primitive forms, but varying degrees of fusion between the segments occur in many groups. It is typically divided into a preabdomen and postabdomen, although this is only clearly visible in scorpions, and in some orders, such as the mites, the abdominal sections are completely fused. A telson is present in scorpions, where it has been modified to a stinger, and into a flagellum in the Palpigradi, Schizomida (very short) and whip scorpions. At the base of the flagellum in the two latter groups there are glands which produce acetic acid as a chemical defense. Except for a pair of pectines in scorpions, and the spinnerets in spiders, the abdomen has no appendages.

Like all arthropods, arachnids have an exoskeleton, and they also have an internal structure of cartilage -like tissue, called the endosternite, to which certain muscle groups are attached. The endosternite is even calcified in some Opiliones.