Arthropod

Arthropods are <a href="/facts/Invertebrate/3CyqCVnN">invertebrates</a> in the <a href="/facts/Phylum/ELT3Jyax">phylum</a> Arthropoda. They possess an <a href="/facts/Arthropod_exoskeleton/pabIdrnJ">exoskeleton</a> with a <a href="/facts/Cuticle/LWaIDBIG">cuticle</a> made of <a href="/facts/Chitin/Fo1f8b86">chitin</a>, often <a href="/facts/Mineralization_(biology)/m5mGPaTM">mineralised</a> with <a href="/facts/Calcium_carbonate/UQ1wLFna">calcium carbonate</a>, a body with differentiated (<a href="/facts/Metamerism_(biology)/8FGR2KpP">metameric</a>) <a href="/facts/Segmentation_(biology)/S4Cw0UcN">segments</a>, and paired jointed <a href="/facts/Appendage/lFv11d3f">appendages</a>. In order to keep growing, they must go through stages of <a href="/facts/Moulting/2wdfTSL5">moulting</a>, a process by which they shed their exoskeleton to reveal a new one. They form an extremely diverse group of up to ten million species.
<a href="/facts/Haemolymph/6tHgrY6c">Haemolymph</a> is the analogue of <a href="/facts/Blood/03zEulkj">blood</a> for most arthropods. An arthropod has an <a href="/facts/Open_circulatory_system/WXwudzzc">open circulatory system</a>, with a body cavity called a <a href="/facts/Haemocoel/WXwudzzc">haemocoel</a> through which haemolymph circulates to the interior <a href="/facts/Organ_(anatomy)/FUYsLaxz">organs</a>. Like their exteriors, the internal organs of arthropods are generally built of repeated segments. They have ladder-like <a href="/facts/Nervous_system/CDxRKU2U">nervous systems</a>, with paired <a href="/facts/Anatomical_terms_of_location/IpJ1fq2f">ventral</a> <a href="/facts/Ventral_nerve_cord/s7Q7toBB">nerve cords</a> running through all segments and forming paired <a href="/facts/Ganglia/Q5rx1UlU">ganglia</a> in each segment. Their heads are formed by fusion of varying numbers of segments, and their <a href="/facts/Brain/EKla9NkJ">brains</a> are formed by fusion of the ganglia of these segments and encircle the <a href="/facts/Esophagus/XWa7WoWn">esophagus</a>. The <a href="/facts/Respiratory_system/IG8v5JEr">respiratory</a> and <a href="/facts/Excretion/mspsTfnv">excretory</a> systems of arthropods vary, depending as much on their environment as on the <a href="/facts/Subphylum/TQopDJJ5">subphylum</a> to which they belong.
Arthropods use combinations of <a href="/facts/Compound_eye/yoRE9HWY">compound eyes</a> and <a href="/facts/Simple_eye_in_invertebrates/zV5g5Eqo">pigment-pit</a> <a href="/facts/Ocelli/zV5g5Eqo">ocelli</a> for vision. In most species, the ocelli can only detect the direction from which light is coming, and <a href="/facts/Arthropod_eye/J9S3QKdT">the compound eyes</a> are the main source of information, but the main eyes of <a href="/facts/Spider/aqpU7eg3">spiders</a> 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 <a href="/facts/Seta/jUmts6C2">setae</a> that project through their cuticles. Similarly, their reproduction and development are varied; all <a href="/facts/Terrestrial_animal/AhP2mont">terrestrial</a> species use <a href="/facts/Internal_fertilization/CJtuSyeG">internal fertilization</a>, but this is sometimes by indirect transfer of the <a href="/facts/Sperm/RCxpXygQ">sperm</a> via an appendage or the ground, rather than by direct injection. Aquatic species use either internal or <a href="/facts/External_fertilization/PB4mUeJx">external fertilization</a>. Almost all arthropods lay eggs, with many species giving birth to live young after the eggs have hatched inside the mother; but a few are genuinely <a href="/facts/Viviparity/EEMn4rVx">viviparous</a>, such as <a href="/facts/Aphid/kbvClSLl">aphids</a>. Arthropod hatchlings vary from miniature adults to grubs and <a href="/facts/Caterpillar/PMgzj7F7">caterpillars</a> that lack jointed limbs and eventually undergo a total <a href="/facts/Metamorphosis/nKDTSjsK">metamorphosis</a> to produce the adult form. The level of maternal care for hatchlings varies from nonexistent to the prolonged care provided by <a href="/facts/Eusociality/RrNadVME">social insects</a>.
The evolutionary ancestry of arthropods dates back to the <a href="/facts/Cambrian/jIfzKtod">Cambrian</a> period. The group is generally regarded as <a href="/facts/Monophyletic/pD8Yrrn6">monophyletic</a>, and many analyses support the placement of arthropods with <a href="/facts/Cycloneuralia/fgHpR0gn">cycloneuralians</a> (or their constituent clades) in a superphylum <a href="/facts/Ecdysozoa/L3hxqnxA">Ecdysozoa</a>. Overall, however, the <a href="/facts/Basal_(evolution)/JQPQXpYg">basal</a> relationships of animals are not yet well resolved. Likewise, the relationships between various arthropod groups are still actively debated. Today, arthropods contribute to the human food supply both directly as food, and more importantly, indirectly as <a href="/facts/Pollination/u383g7XJ">pollinators</a> of crops. Some species are known to spread severe disease to humans, livestock, and crops.

Arthropod open-in-new

Arthropod