Chromolithograph of Clematis, Cicada & beetles by Friedrich Kuhnert that appeared in Meyers Konversationslexikon, 1894 edition. Insects are a class of invertebrates within the arthropod phylum that have a chitinous exoskeleton, a three-part body (head, thorax and abdomen), three pairs of jointed legs, compound eyes and one pair of antennae. They are among the most diverse groups of animals on the planet, including more than a million described species and representing more than half of all known living organisms.