Arthropods Arthropods inhabit all three terrestrial, aquatic, and avian habitats. More than three-quarters of all known living and fossil creatures are members of this group, and there are more than a million species in total.  The actual number of known arthropod species could be in the tens of millions, due to the large number of unidentified species, particularly in tropical rainforests.  Arthropods may be as large as tiny insects and crustaceans or as little as the Japanese spider crab, which can reach a length of over 30 cm and has claws that can extend to an incredible 3.5 m.  In general, arthropods have a similar body design.  Chitin and proteins make up the bulk of an arthropod's tough cuticle.  These develop an exoskeleton that might or could not have calcium carbonate added to them for stiffness.  They fuse their body segments in different ways to create integrated portions, like as the head and abdomen, which gives them segmented bodies.