An arthropod is an invertebrate animal having an exoskeleton (external skeleton), a segmented body, and jointed appendages. Arthropods are members of the Phylum Arthropoda (from Greek ἄρθρον arthron, "joint", and ποδός podos "foot", which together mean "jointed feet"), and include the insects , arachnids, crustaceans, and others. Arthropods are characterized by their jointed limbs and cuticles, which are mainly made of α-chitin; the cuticles of crustaceans are also biomineralized with calcium carbonate. The rigid cuticle inhibits growth, so arthropods replace it periodically by molting. The arthropod body plan consists of repeated segments, each with a pair of appendages. It is so versatile that they have been compared to Swiss Army knives, and it has enabled them 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% of all described living species, and are one of only two animal groups that are really successful in dry environments – the other being the amniotes. They range in size from microscopic plankton up to forms a few meters long.
Arthropods' primary internal cavity is a hemocoel, which accommodates their internal organs and through which their 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 setae (bristles) 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, except for scorpions, who 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 zero to the prolonged care provided by scorpions.
The versatility of the arthropod modular body plan has made it difficult for zoologists and paleontologists to classify them and work out their evolutionary ancestry, which dates back to the Cambrian period. From the late 1950s to late 1970s, it was thought that arthropods were polyphyletic, that is, there was no single arthropod ancestor. Now they are generally regarded as monophyletic. Traditionally the closest evolutionary relatives of arthropods were considered to be annelid worms, as both groups have segmented bodies. It is now generally accepted that arthropods belong to the superphylum Ecdysozoa ("animals that molt"), while annelids belong to another superphylum, Lophotrochozoa. The relationships between various arthropod groups are still actively debated.
Although arthropods contribute to human food supply both directly as food and more importantly as pollinators of crops, they also spread some of the most severe diseases and do considerable damage to livestock and crops.
All items (1365)
- Aceria fraxinivorus
- Aceria macrorhynchus
- Aceria pseudoplatani
- Acleris sparsana
- Acorn Barnacles
- Acrocercops brongniardella
- Adela rufimitrella
- Adonis Blue
- Aethes beatricella
- Aethes cnicana
- Agapeta hamana
- File:Agapeta hamana.JPG
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- Agonopterix heracliana
- Agriphila geniculea
- Agrypnus murinus
- Alabonia geoffrella
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- Alder Fly
- Alder Kitten
- Alder Moth
- Allantus cinctus
- Allogamus auricollis
- Amauromyza labitatarum
- Amblyteles armatorius
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- Ancylis achatana
- Andricus fecundator
- Andricus kollari
- Andricus quercuscalicis
- Angle Shades
- Angle-striped Sallow
- Annulet
- Anomalous
- Antispila metalella
- Antispila treitschkiella
- File:Apamea lithoxylaea ~ Light Arches.jpg
- File:Apamea monoglypha ~ Dark Arches.jpg
- File:Apamea ophiogramma ~ Double Lobed.jpg
- File:Apamea sublustris ~ Reddish Light Arches.jpg
- Aphodius ater
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- Apple Ermine
- Apple Fruit Moth
- Apple Leaf Miner
- Apple Leaf Skeletonizer
- File:Aquilegia leaf.jpg
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- Archer's Dart
- Archips podana
- Argent & Sable
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- Argyresthia bonnetella
- Argyresthia brockeella
- Argyresthia curvella
- Argyresthia dilectella
- Argyresthia goedartella
- Argyresthia pygmaeella
- Argyresthia retinella
- Argyresthia trifasciata
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- Ash Bud Moth
- Ashworth's Rustic
- Aspilapteryx tringipennella
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- August Thorn
- Australian Cockroach
- Autumn Green Carpet
- Autumnal Moth
- Autumnal Rustic
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- Azalea Leaf Miner
- Azure Damselfly
- Azure Hawker
- Balsam Carpet
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- Banded Demoiselle
- Barberry Carpet
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- Barred Carpet
- Barred Chestnut
- Barred Hook-tip
- Barred Red
- Barred Rivulet
- Barred Sallow
- Barred Straw
- Barred Tooth-striped
- Barred Umber
- Barred Yellow
- Bathyporeia pelagica
- Batia lunaris
- Beaded Chestnut
- Beautiful Carpet
- Beautiful Demoiselle
- Beautiful Golden Y
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- Beautiful Hook-tip
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- Beautiful Plume
- Beautiful Snout
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- Bedstraw Hawkmoth
- Bee Beetle
- Bee Moth
- Beech-green Carpet
- Belted Beauty
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- Birch Mocha
- Birch Shieldbug
- Bird Cherry Aphid
- Bird's Wing
- Bird-cherry Ermine
- Black Arches
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- Black Darter
- Black Garden Ant
- Black Hairstreak
- Black Mountain Moth
- Black Oil Beetle
- Black Rustic
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- Black V Moth
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- Black-banded
- Black-Bean Aphid
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- Black-tailed Skimmer
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- Blair's Shoulder-knot
- Blair's Wainscot
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- Blomer's Rivulet
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- Blood-vein
- Bloody-nosed Beetle
- Blossom Underwing
- Blotched Emerald
- Bloxworth Snout
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- Blue-bordered Carpet
- Blue-tailed Damselfly
- Bluebottle
- Bond's Wainscot
- Bordered Beauty
- Bordered Grey
- Bordered Pug
- Bordered Sallow
- Bordered Straw
- Bordered White
- Bramble Shoot Moth
- Brick
- Bright Wave
- Bright-line Brown-eye
- Brighton Wainscot
- Brilliant Emerald
- Brimstone Moth
- Brindled Beauty
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- Brindled Ochre
- Brindled Pug
- Brindled White-spot
- Broad-barred White
- Broad-bodied Chaser
- Broad-bordered Bee Hawkmoth
- Broad-bordered Yellow Underwing
- Broad-clawed Porcelain Crab
- Broken-barred Carpet
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- Broom Moth
- Brown Argus
- Brown Hairstreak
- Brown Hawker
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- Brown Rustic
- Brown Scallop
- Brown Silver-line
- Brown-spot Pinion
- Brown-tail
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- Brussels Lace
- Bucculatrix nigricomella
- Bucculatrix thoracella
- Bucculatrix ulmella
- Buff Arches
- Buff Ermine
- Buff footman