The pygidium (plural pygidia) is the posterior body part or shield of crustaceans and some other arthropods, such as insects and the extinct trilobites. 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 segments by an articulation.Shultz, J.W. (1990). Evolutionary Morphology And Phylogeny of Arachnida. Cladistics 6: 1–38.

Chelicerates

In arachnids, 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, Amblypygi, Thelyphonida, Schizomida, Ricinulei and in the extinct order Trigonotarbida. It is also present in early fossil representatives of horseshoe crabs.

Trilobites

In trilobites, the pygidium can range from extremely small (much smaller than the head, or

cephalon Cephalon, Inc. was an American biopharmaceutical company co-founded in 1987 by pharmacologist Frank Baldino, Jr., neuroscientist Michael Lewis, and organic chemist James C. Kauer—all three former scientists with the DuPont Company. Baldino s ...

) 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. 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 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 of fish of the family Trichomycteridae. Most species of this genus have been reassigned to the genus '' Trichomycterus''.

References

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External links