section_path: Description/Diversity
section_name: Diversity
paragraph_text: Diversity
  
  Arthropoda is the largest animal phylum with the estimates of the number of arthropod species varying from 1,170,000 to 5 to 10 million and accounting for over 80 percent of all known living animal species. One arthropod sub-group, the insects, includes more described species than any other taxonomic class. The total number of species remains difficult to determine. This is due to the census modeling assumptions projected onto other regions in order to scale up from counts at specific locations applied to the whole world. A study in 1992 estimated that there were 500,000 species of animals and plants in Costa Rica alone, of which 365,000 were arthropods.
  
  They are important members of marine, freshwater, land and air ecosystems and one of only two major animal groups that have adapted to life in dry environments; the other is amniotes, whose living members are reptiles, birds and mammals. Both the smallest and largest arthropods are crustaceans. The smallest belong to the class Tantulocarida, some of which are less than 100 micrometres (0.0039 in) long. The largest are species in the class Malacostraca, with the legs of the Japanese spider crab potentially spanning up to 4 metres (13 ft) and the American lobster reaching weights over 20 kg (44 lbs).