Arachnids are arthropods, comprising some 70,000 named species, and include spiders, scorpions, harvestmen, ticks and mites. Nearly all are terrestrial. There is no separate head and the body is organized into two parts called the prosoma, covered by a carapace-like shield, and the opisthosoma, or abdomen, which may be segmented. There are 6 pair of appendages. The first pair, the chelicerae, serve in feeding and defense. The next pair, the pedipalps have been adapted for feeding, locomotion, and reproductive functions. The other 4 pairs serve as limbs.