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Portal:Arthropods

The arthropods portal

An arthropod (/ˈɑːrθrəpɒd/, from Greek ἄρθρον arthron, "joint" and πούς pous, "foot" (gen. ποδός)) is an invertebrate animal having an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Euarthropoda, which includes insects, arachnids, myriapods, and crustaceans. The term Arthropoda as originally proposed refers to a proposed grouping of Euarthropods and the phylum Onychophora.

Arthropods are characterized by their jointed limbs and cuticle made of chitin, often mineralised with calcium carbonate. The arthropod body plan consists of segments, each with a pair of appendages. The rigid cuticle inhibits growth, so arthropods replace it periodically by moulting. Arthropods are bilaterally symmetrical and their body possesses an external skeleton. Some species have wings.

Their versatility has enabled arthropods to become the most species-rich members of all ecological guilds in most environments. They have over a million described species, making up more than 80 percent of all described living animal species, some of which, unlike most other animals, are very successful in dry environments. Arthropods range in size from the microscopic crustacean Stygotantulus up to the Japanese spider crab.

Arthropods' primary internal cavity is a haemocoel, which accommodates their internal organs, and through which their haemolymph – analogue of blood – circulates; they have open circulatory systems. Like their exteriors, the internal organs of arthropods are generally built of repeated segments. Their nervous system is "ladder-like", with paired ventral nerve cords running through all segments and forming paired ganglia in each segment. Their heads are formed by fusion of varying numbers of segments, and their brains are formed by fusion of the ganglia of these segments and encircle the esophagus. The respiratory and excretory systems of arthropods vary, depending as much on their environment as on the subphylum to which they belong.

Their vision relies on various combinations of compound eyes and pigment-pit ocelli: in most species the ocelli can only detect the direction from which light is coming, and the compound eyes are the main source of information, but the main eyes of spiders are ocelli that can form images and, in a few cases, can swivel to track prey. Arthropods also have a wide range of chemical and mechanical sensors, mostly based on modifications of the many bristles known as setae that project through their cuticles.

Arthropods' methods of reproduction and development are diverse; all terrestrial species use internal fertilization, but this is often by indirect transfer of the sperm via an appendage or the ground, rather than by direct injection. Aquatic species use either internal or external fertilization. Almost all arthropods lay eggs, but scorpions give birth to live young after the eggs have hatched inside the mother. Arthropod hatchlings vary from miniature adults to grubs and caterpillars that lack jointed limbs and eventually undergo a total metamorphosis to produce the adult form. The level of maternal care for hatchlings varies from nonexistent to the prolonged care provided by scorpions.

The evolutionary ancestry of arthropods dates back to the Cambrian period. The group is generally regarded as monophyletic, and many analyses support the placement of arthropods with cycloneuralians (or their constituent clades) in a superphylum Ecdysozoa. Overall, however, the basal relationships of animals are not yet well resolved. Likewise, the relationships between various arthropod groups are still actively debated.

Arthropods contribute to the human food supply both directly as food, and more importantly indirectly as pollinators of crops. Some species are known to spread severe disease to humans, livestock, and crops.

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A small black spider with an angular red marking along its round abdomen.
The katipo, Latrodectus katipo, is an endangered species of spider native to New Zealand. A member of the genus Latrodectus, it is related to the Australian redback spider, and the North American black widow spiders. The species is venomous to humans, capable of delivering a dangerous spider bite. Katipo is a Māori name and means "night-stinger". It is a small to medium-sized spider with the female having a distinctive black body with a white bordered red stripe on its back. North of 39° 15' S females do not have a red stripe and are all black. The male is much smaller than the female and quite different in appearance, being white with black stripes and red diamond shaped markings. Katipo have a narrow habitat, being only found living in sand dunes close to the seashore. They range throughout most of coastal New Zealand, but are not found at the southernmost regions. Spinning an irregular tangled web amongst dune plants or other debris, they feed mainly on ground dwelling insects.

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Fossil of a giant ant, with a rufous hummingbird, showing the similar size of the two.

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A colourful mantis shrimp is seen head-on. Its antennae are spread sideways, and its clubbed claws are held ready to strike.
Cscr-featured.svg Credit: Jens Petersen

Mantis shrimp (peacock mantis shrimp – Odontodactylus scyllarus – pictured) are marine crustaceans of the order Stomatopoda. They take their name from the physical resemblance to praying mantises and shrimp.

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indicates an extinct taxon.

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Biology • Animals  • Arthropods • Crustaceans • Insects • Marine life  

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