especially in dim light, and makes it possible for nocturnal species to use starlight to navigate at night. The chelicerae are at the front and underneath the carapace. They are pincer-like and have three segments and sharp "teeth". The brain of a scorpion is in the back of the cephalothorax, just above the esophagus. As in other arachnids, the nervous system is highly concentrated in the cephalothorax, but has a long ventral nerve cord with segmented ganglia which may be a primitive trait. The pedipalp is a segmented, clawed appendage used for prey immobilization, defense and sensory purposes. The segments of the pedipalp (from closest to the body outward) are coxa, trochanter, femur, patella, tibia (including the fixed claw and the manus) and tarsus (moveable claw). A scorpion has darkened or granular raised linear ridges, called "keels" or "carinae" on the pedipalp segments and on other parts of the body; these are useful as taxonomic characters. Unlike those of some other arachnids, the legs have not been modified for other purposes, though they may occasionally be used for digging, and females may use them to catch emerging young. The legs are covered in proprioceptors, bristles and sensory setae. Depending on the species, the legs may have spines and spurs. Mesosoma The mesosoma or preabdomen is the broad part of the opisthosoma. In the early stages of embryonic development the mesosoma consist of eight segments, but the first segment disappear before birth, so the mesosoma in scorpions actually consist of segments 2-8. These anterior seven somites (segments) of the opisthosoma are each covered dorsally by a sclerotized plate called the tergite. Ventrally, somites 3 to 7 are armored with matching plates called sternites. The ventral side of somite 1 has a pair of genital opercula covering the gonopore. Sternite 2 forms the basal plate bearing the pectines, which function as sensory organs. The next four somites, 3 to 6, all bear pairs of spiracles. They serve as openings for the scorpion's respiratory organs, known as book lungs. The spiracle openings may be slits, circular, elliptical or oval according to the species. There are thus four pairs of book lungs; each consists of some 140 to 150 thin lamellae filled with air inside a pulmonary chamber, connected on the ventral side to an atrial chamber which opens into a spiracle. Bristles hold the lamellae apart. A muscle opens the spiracle and widens