Crayfish are usually colored in earth tones of muted greens and browns. The body has three primary sections: the cephalothorax (the fused head and thorax), which is entirely encased by a single shell; the carapace, a sixsegmented abdomen; and a five-sectioned, fan-shaped tail, the telson. Five pairs of strong, jointed, armored legs (pereiopods) on the cephalothorax are used for walking and digging. The first pair of legs, known as the chelipeds, end in large pincers (chelae), which are used for defense and food gathering. Two pairs of small antennae (the antennae and antennules) are specialized chemical detectors used in foraging and finding a mate. The antennae project on either side of the tip of the rostrum, which is a beak-like projection at the front of the head. A third and longer pair of antennae are tactile, or touch receptors. Two compound eyes provide excellent vision, except in some cave-dwellers that live in perpetual dark and are virtually blind. Below the rostrum are two pairs of mandibles (the jaws) and three pairs of maxillipeds, which are small appendages that direct food to the mouth. The second pair of maxillipeds facilitate gill ventilation by swishing water through the banks of gills located at the base of each pereiopod on the sides of the carapace in the gill chambers. The strong, long, muscular abdomen has ten tiny appendages (the pleopods) which aid in swimming movements. When threatened, the crayfish propels itself backward quickly with strong flips of the telson, located at the tip of the abdomen.