The phylum Arthropoda is by far the largest in the animal kingdom, accounting for about 90% of all species. The one characterization of arthropods is the external stiff cuticle known as an exoskeleton. The exoskeleton is comprised of proteins and chitin which is sometimes mineralized with calcium carbonate. It is arranged in segments which are then joined by more flexible tissue which allows for movement.
Arthropods are said to be the most successful group of animals on the planet This is due to the fact that there are arthropods inhabiting and even colonizing most types of terrain and subterranean habitats. There are insects abundant in the skies and crustaceans inhabiting the seas. Arthropods vary in size from the smallest of mites to the giant King Crabs of the sea.
The phylum is further divided into four classes, insecta, arachnida, myriapoda and crustacea with the insects forming the majority of species in the phylum Arthropoda.
Insects have three distinct body segments, three pairs of legs and one pair of antennae and often the adult form of many species can have wings, either one or two pair. Arachnids on the other hand only have two distinct body segments and have four pairs of legs and do not have antennae or wings. Myriapoda have many pairs of legs and many body segments, never less than nine. They have one pair of antennae and never have wings. Crustaceans have several to many pairs of legs but not less than four pair, they have two pair of antennae, never have wings and the body segment count is variable but never three.