Insects (Class Insecta) are a major group of arthropods and the most diverse group of animals on the Earth, with over a million described species—more than half of all known living organisms with estimates of un-described species as high as 30 million, thus potentially representing over 90% of the differing life forms on the planet.Insects may be found in nearly all environments on the planet, although only a small number of species occur in the oceans, a habitat dominated by another arthropod group, the crustaceans.