Arthropods contain around 80% of all described animal species, currently there are over 1 million described arthropod species.

They are one of the oldest animal groups with fossils going back 555 million years. One of the best known of all fossils, Trilobites, are Arthropods, they were already a  very  diverse group 530 million years ago.

Arthropoda - Arthropods Arthro - joint, Pod - foot

Arthropods are found everywhere on earth in every kind of habitat, they fly, swim, crawl, burrow, and live as external and internal parasites.

The combined mass of Arthropods is easily greater than that of all other animals combined. The total human mass on earth has been estimated at being similar to the world termite total, the figures for ants are up to twenty times greater (both are insects and so also arthropods).

The combined biomass of a crustacean, Euphausia superba, the Antarctic krill is estimated to be in the region of 500 million tonnes making it by mass, the most abundant single species on earth.

However you look at it, it is difficult not to consider that the earth is the planet of the Arthropods, they are older than nearly other animal groups, more numerous and of much greater biomass. Wherever you go on earth, if there is an animal living there it will most likely be an Arthropod, if it is on land, it will probably be an insect, in the sea a crustacean.


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A fossil Trilobite, Paraceraurus, an early Arthropod from the Ordovician period (480-440 Million years ago) Volchow River area, Russia
picture - Vassil, used under CC3 Attribution ShareAlike Unported License

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Flea - Siphonaptera, Insecta
A common ectoparasite
picture Doc. RNDr. Josef Reischig CSc. used under CC3-Attribution Share Alike Unported license.

Basic Features:

  • Exoskeleton made of chitin, which leads to a discontinuous pattern of growth (the animal grows into it, then it can't get bigger until the next stage)
  • Metamorphosis common
  • Segmented bodies, segments not always evident externally though
  • Jointed appendages used for feeding and locomotion
  • Gas exchange through gills, tracheae or book lungs
  • Nitrogenous excretion through gills or Malpighian tubules
  • Bilateral symmetry (one line of symmetry down the middle - like you)

Kingdom - Animalia
Phylum - Arthropoda

Classes:

  • Arachnida - spiders, scorpions, ticks, mites
  • Chilopoda - centipedes
  • Diplopoda - millipedes
  • Crustacea - crabs, lobsters, shrimps, woodlice, barnacles etc.
  • Insecta - beetles, butterflies, true flies, bugs etc.

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Horseshoe crab, found in the fossil record for about 450 Million years
picture - Didier Descouens, used under CC3 Attribution Share Alike Unported license.

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A centipede commonly found in houses in the tropics
Centipedes are fast moving predators of smaller invertebrates.

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Krill - Euphausia superba
A contender for possibly the most successful animal species on the planet, individually a maximum of 6cm long, but with a standing biomass of as much as 500 million tonnes

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Sally lightfoot crab, Graspus graspus
A modern Arthropod from the Galapagos Islands

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Male peacock spider
A small jumping spider about 5mm long, notable for an elaborate and colourful mating display to the females.

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A Honey Bee, (Hymenoptera) Apis mellifera.
Bees of all kinds are invaluable as pollinators for wild plants and crops, they collect pollen in a "pollen basket" on their hind legs, the bright yellow mass you can see in this picture. In the process of collecting pollen and nectar for their own purposes, they conincidentally spread it from plant to plant pollinating them.


Top banner picture - Alvesgaspar, used under GNU-1.2 license.