mber of all ecological guilds in land and fresh-water environments.[8] The lightest insects weigh less than 25 micrograms (millionths of a gram),[10] while the heaviest weigh over 70 grams (2.5 oz).[11]

    8 References
        8.1 Bibliography
    9 External links

Description

Arthropods are invertebrates with segmented bodies and jointed limbs.[3] The limbs form part of an exoskeleton, which is mainly made of α-chitin, a derivative of glucose.[4] One other group of animals, the tetrapods, has jointed limbs, but tetrapods are vertebrates and therefore have endoskeletons.[5] Arthropods bony back evolved so that it became segmented letting it move about more freely and not reducing its protection.
Diversity

One estimate indicates that arthropods have 1,170,000 described species, and account for over 80% of all known living animal species.[6] Another study estimates that there are between 5 to 10 million extant arthropod species, both described and yet to be described.[7] Estimating the total number of living species is extremely difficult because it often depends on a series of assumptions in order to scale up from counts at specific locations to estimates for 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. Arthropods are one of the most ubiquitous groups of multicellular organisms found in the biosphere.[8]

They are important members of marine, freshwater, land and air ecosystems, and are 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.[9] One arthropod sub-group, insects, is the most species-rich member of all ecological guilds in land and fresh-water environments.[8] The lightest insects weigh less than 25 micrograms (millionths of a gram),[10] while the heaviest weigh over 70 grams (2.5 oz).[11] Some living crustaceans are much larger; for example, the legs of the Japanese spider crab may span up to 4 metres (13 ft).[10]
Segmentation
Head
_______________________
Thorax
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Abdomen
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Segments and tagmata of an arthropod[9]
    = Body
    = Coxa (base)
    = Gill branch
// = Gill filaments