An arthropod (from Greek arthro-, joint + podos, foot) is an invertebrate animal having an exoskeleton (external skeleton), a segmented body, and jointed appendages. Arthropods form the phylum Arthropoda, and include the insects, arachnids, myriapods, and crustaceans. Arthropods are characterized by their jointed limbs and cuticle made of α-chitin, often mineralised with calcium carbonate. The arthropod body plan consists of segments, each with a pair of appendages. They have over a million described species, making up more than 80% of all described living animal species, some of which, unlike most animals, are very successful in dry environments. Arthropods also have a wide range of chemical and mechanical sensors, mostly based on modifications of the many setae (bristles) that project through their cuticles