The pygidium (plural pygidia) is the posterior body part or shield of

crustacean Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. The crustacean group ...

s and some other

arthropod Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and Arth ...

s, such as

insect Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body ( head, thorax and abdomen), three pairs ...

s and the extinct

trilobite Trilobites (; meaning "three lobes") are extinct marine arthropods that form the class Trilobita. Trilobites form one of the earliest-known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the At ...

s. In groups other than insects, it contains the

anus The anus (Latin, 'ring' or 'circle') is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, the residual semi-solid waste that remains after food digestion, which, d ...

and, in females, the

ovipositor The ovipositor is a tube-like organ used by some animals, especially insects, for the laying of eggs. In insects, an ovipositor consists of a maximum of three pairs of appendages. The details and morphology of the ovipositor vary, but typical ...

. It is composed of fused body segments, sometimes with a tail, and separated from

thoracic The thorax or chest is a part of the anatomy of humans, mammals, and other tetrapod animals located between the neck and the abdomen. In insects, crustaceans, and the extinct trilobites, the thorax is one of the three main divisions of the crea ...

segments by an articulation.Shultz, J.W. (1990). Evolutionary Morphology And Phylogeny of Arachnida. Cladistics 6: 1–38.

Chelicerates

arachnid Arachnida () is a class of joint-legged invertebrate animals (arthropods), in the subphylum Chelicerata. Arachnida includes, among others, spiders, scorpions, ticks, mites, pseudoscorpions, harvestmen, camel spiders, whip spiders and vinegaroon ...

s, the pygidium is formed by reduction of the last three opisthosomal segments to rings where there is no distinction between tergites and sternites. A pygidium is present in

Palpigradi Palpigrades, commonly known as microwhip scorpions, are arachnids belonging to the order Palpigradi. Description Palpigrades belong to the arachnid class. They are the sister group to Solifugae, no more than in length, and averaging . They have ...

Amblypygi Amblypygi is an ancient order of arachnid chelicerate arthropods also known as African cave-dwelling spiders, whip spiders and tailless whip scorpions (not to be confused with whip scorpions or vinegaroons that belong to the related order Thelyph ...

Thelyphonida Thelyphonida is an arachnid order comprising invertebrates commonly known as whip scorpions or vinegaroons (also spelled vinegarroons and vinegarones). They are often called uropygids in the scientific community based on an alternative name for ...

Schizomida Schizomida (common name shorttailed whipscorpion) is an order of arachnids, generally less than in length. The order is not yet widely studied. About 300 species of schizomids have been described worldwide, most belonging to the Hubbardiidae fa ...

Ricinulei Ricinulei is a small order of arachnids. Like most arachnids, they are predatory, eating small arthropods. They occur today in west-central Africa (''Ricinoides'') and the Neotropics (''Cryptocellus'' and ''Pseudocellus'') as far north as Texas. ...

and in the extinct order

Trigonotarbida The order Trigonotarbida is a group of extinct arachnids whose fossil record extends from the late Silurian to the early Permian ( Pridoli to Sakmarian).Dunlop, J. A., Penney, D. & Jekel, D. 2020A summary list of fossil spiders and their relativ ...

. It is also present in early fossil representatives of

horseshoe crabs Horseshoe crabs are marine and brackish water arthropods of the family Limulidae and the only living members of the order Xiphosura. Despite their name, they are not true crabs or crustaceans: they are chelicerates, most closely related to arac ...

Trilobites

In trilobites, the pygidium can range from extremely small (much smaller than the head, or cephalon) to larger than the cephalon. They can be smooth, as in order

Asaphida Asaphida is a large, morphologically diverse order of trilobites found in marine strata dated from the Middle Cambrian until their extinction during the Silurian. Asaphida contains six superfamilies (Anomocaroidea, Asaphoidea, Cyclopygoidea, ...

, or spiny, as in order

Lichida Lichida is an order of typically spiny trilobite that lived from the Furongian to the Devonian period. These trilobites usually have 8–13 thoracic The thorax or chest is a part of the anatomy of humans, mammals, and other tetrapod animals l ...

. They can be classified into four categories according to their relative size in comparison to the cephalon. * Micropygous – the pygidium is considerably smaller than the cephalon. * Subisopygous – the pygidium is slightly smaller than the cephalon. * Isopygous – the cephalon and the pygidium are more or less of equal size. * Macropygous – the pygidium is larger than the cephalon. They can further be subdivided in their morphological similarity to the

thorax The thorax or chest is a part of the anatomy of humans, mammals, and other tetrapod animals located between the neck and the abdomen. In insects, crustaceans, and the extinct trilobites, the thorax is one of the three main divisions of the cre ...

. Pygidia that are similar in shape and form to the thoracic segments are termed homonomous, while pygidia that vary significantly from the shape and form of the thoracic segments (like by the presence or absence of spines) are heteronomous. Trilobite Pygidia types

Insects

In insects, the pygidium is the dorsal

tergite A ''tergum'' (Latin for "the back"; plural ''terga'', associated adjective tergal) is the dorsal ('upper') portion of an arthropod segment other than the head. The anterior edge is called the 'base' and posterior edge is called the 'apex' or 'mar ...

of the last external abdominal segment.G. Gordh, D. Headrick. 2003. ''A Dictionary of Entomology''. CABI Publishing. p. 757.

Other uses

''Pygidium'' is also a superseded

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

fish Fish are aquatic, craniate, gill-bearing animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish as well as various extinct related groups. Approximately 95% of li ...

of the

family Family (from la, familia) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its ...

Trichomycteridae Trichomycteridae is a family of catfishes commonly known as pencil catfishes or parasitic catfishes. This family includes the candiru fish (''Vandellia cirrhosa''), feared by some people for its alleged habit of entering into the urethra of hum ...

. Most species of this genus have been reassigned to the genus ''

Trichomycterus ''Trichomycterus'' is a genus of fish in the family Trichomycteridae, the largest genus of its family with over 170 species currently described. This genus is native to freshwater habitats in Central and South America. These fish are generally sm ...

''.

References

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External links