Arthropod

ArthropodaarthropodsEuarthropodabugeuarthropodeuarthropodsarthropod venomsBugsarthropod-borneArthropoda Phylum
An arthropod (, 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.wikipedia
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Invertebrate

invertebratesmacroinvertebratemacroinvertebrates
ποδός)) is an invertebrate animal having an exoskeleton, a segmented body, and paired jointed appendages.
Familiar examples of invertebrates include arthropods (insects, arachnids, crustaceans, and myriapods), mollusks (chitons, snails, bivalves, squids, and octopuses), annelids (earthworms and leeches), and cnidarians (hydras, jellyfishes, sea anemones, and corals).

Arachnid

Arachnidaarachnidsarachnoid
Arthropods form the phylum Euarthropoda, which includes insects, arachnids, myriapods, and crustaceans.
Arachnida is a class of joint-legged invertebrate animals (arthropods), in the subphylum Chelicerata.

Animal

Animaliaanimalsmetazoa
ποδός)) is an invertebrate animal having an exoskeleton, a segmented body, and paired jointed appendages.
The Bilateria include the protostomes—in which many groups of invertebrates are found, such as nematodes, arthropods, and molluscs—and the deuterostomes, containing both the echinoderms as well as the chordates, the latter containing the vertebrates.

Chitin

chitinouschitinous polymer matrixchitohexaose
Arthropods are characterized by their jointed limbs and cuticle made of chitin, often mineralised with calcium carbonate.
It is a primary component of cell walls in fungi, the exoskeletons of arthropods, such as crustaceans and insects, the radulae of molluscs, cephalopod beaks, and the scales of fish and lissamphibians.

Segmentation (biology)

segmentssegmentsegmentation
ποδός)) is an invertebrate animal having an exoskeleton, a segmented body, and paired jointed appendages.
This article focuses on the segmentation of animal body plans, specifically using the examples of the taxa Arthropoda, Chordata, and Annelida.

Spider

Araneaespidersegg sac
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. The three-part appearance of many insect bodies and the two-part appearance of spiders is a result of this grouping; in fact there are no external signs of segmentation in mites. The four major groups of arthropods – Chelicerata (includes spiders and scorpions), Crustacea (shrimps, lobsters, crabs, etc.), Tracheata (arthropods that breathe via channels into their bodies; includes insects and myriapods), and the extinct trilobites – have heads formed of various combinations of segments, with appendages that are missing or specialized in different ways.
Spiders (order Araneae) are air-breathing arthropods that have eight legs and chelicerae with fangs able to inject venom.

Hemolymph

haemolymphcirculatory systemheamolymph
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.
It is the major tissue type of the open circulatory system characteristic of arthropods (e.g. arachnids, crustaceans and insects).

Japanese spider crab

Macrocheiradeep sea crabsgiant crabs
Arthropods range in size from the microscopic crustacean Stygotantulus up to the Japanese spider crab. The lightest insects weigh less than 25 micrograms (millionths of a gram), while the heaviest weigh over 70 g. Some living crustaceans are much larger; for example, the legs of the Japanese spider crab may span up to 4 m, with the heaviest of all living arthropods being the American lobster, topping out at over 20 kg (44 lbs).
It has the largest leg span of any arthropod.

Blood

human bloodhematologicaloxygen consumption
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.
Arthropods, using hemolymph, have hemocytes as part of their immune system.

Compound eye

compound eyeseyeseye
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.
A compound eye is a visual organ found in arthropods such as insects and crustaceans.

Ecdysozoa

ecdysozoanecdysozoans
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.
Ecdysozoa is a group of protostome animals, including Arthropoda (insects, chelicerata, crustaceans, and myriapods), Nematoda, and several smaller phyla.

Brain

brain functionmammalian braincerebral
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.
This category includes tardigrades, arthropods, molluscs, and numerous types of worms.

Mite

mitesMetaseiulus occidentalismite hockey
The three-part appearance of many insect bodies and the two-part appearance of spiders is a result of this grouping; in fact there are no external signs of segmentation in mites.
Mites are small arthropods belonging to the class Arachnida and the subclass Acari.

Opiliones

harvestmenharvestmanopilionid
Calcification of the endosternite, an internal structure used for muscle attachments, also occur in some opiliones.
English speakers may colloquially refer to species of Opiliones as "daddy longlegs" or "granddaddy longlegs", but this name is also used for two other distantly related groups of arthropods, the crane flies of the family Tipulidae, and the cellar spiders of the family Pholcidae, most likely because of their similar appearance.

Telson

pleotelsonpretelsonsting
Arthropods also have two body elements that are not part of this serially repeated pattern of segments, an acron at the front, ahead of the mouth, and a telson at the rear, behind the anus.
The telson is the posterior-most division of the body of an arthropod.

American lobster

lobsterHomarus americanuslobsters
The lightest insects weigh less than 25 micrograms (millionths of a gram), while the heaviest weigh over 70 g. Some living crustaceans are much larger; for example, the legs of the Japanese spider crab may span up to 4 m, with the heaviest of all living arthropods being the American lobster, topping out at over 20 kg (44 lbs).
It can reach a body length of 64 cm, and a mass of over 20 kg, making it not only the heaviest crustacean in the world, but also the heaviest of all living arthropod species.

Moulting

moultmoltmolting
The rigid cuticle inhibits growth, so arthropods replace it periodically by moulting.
In some groups, other body parts may be shed, for example, wings in some insects or the entire exoskeleton in arthropods.

Antenna (biology)

antennaeantennaantennal
In some segments of all known arthropods the appendages have been modified, for example to form gills, mouth-parts, antennae for collecting information, or claws for grasping; arthropods are "like Swiss Army knives, each equipped with a unique set of specialized tools."
Antennae (singular: antenna), sometimes referred to as "feelers", are paired appendages used for sensing in arthropods.

Marrella

MarrellidaeMarella
In addition, some extinct arthropods, such as Marrella, belong to none of these groups, as their heads are formed by their own particular combinations of segments and specialized appendages.
Marrella splendens is an extinct arthropod known from the middle Cambrian Burgess Shale of British Columbia.

Ventral nerve cord

nerve cordnerve cords ladder nerve system
Their nervous system is "ladder-like", with paired ventral nerve cords running through all segments and forming paired ganglia in each segment.
The ventral nerve cord (VNC) makes up a part of the central nervous system of some phyla of the bilaterians, particularly within the nematodes, annelids and the arthropods.

Arthropod head problem

intercalaryIntercalary segmenthead
Working out the evolutionary stages by which all these different combinations could have appeared is so difficult that it has long been known as "the arthropod head problem".
The (pan)arthropod head problem is a long-standing zoological dispute concerning the segmental composition of the heads of the various arthropod groups, and how they are evolutionarily related to each other.

Atelocerata

Tracheata
The four major groups of arthropods – Chelicerata (includes spiders and scorpions), Crustacea (shrimps, lobsters, crabs, etc.), Tracheata (arthropods that breathe via channels into their bodies; includes insects and myriapods), and the extinct trilobites – have heads formed of various combinations of segments, with appendages that are missing or specialized in different ways.
Atelocerata is a proposed clade of arthropods that includes Hexapoda (insects and a few related taxa) and Myriapoda (millipedes, centipedes, and similar taxa), but excludes Crustacea (such as shrimp and lobsters) and Chelicerata (such as spiders and horseshoe crabs).

Lobster

lobstersNephropidaeClawed lobster
The four major groups of arthropods – Chelicerata (includes spiders and scorpions), Crustacea (shrimps, lobsters, crabs, etc.), Tracheata (arthropods that breathe via channels into their bodies; includes insects and myriapods), and the extinct trilobites – have heads formed of various combinations of segments, with appendages that are missing or specialized in different ways.
Like most arthropods, lobsters must moult to grow, which leaves them vulnerable.

Arthropod cuticle

cuticleepicuticleexocuticle
Arthropods are characterized by their jointed limbs and cuticle made of chitin, often mineralised with calcium carbonate.
The cuticle forms the major part of the integument of the Arthropoda.

Nervous system

neuralnervousneurogenic
Their nervous system is "ladder-like", with paired ventral nerve cords running through all segments and forming paired ganglia in each segment.
Protostomes, the more diverse group, include arthropods, molluscs, and numerous types of worms.