Thesternum (pl.:sterna) is theventral portion of a segment of anarthropodthorax orabdomen.
Ininsects, the sterna are usually single, largesclerites, and external. However, they can sometimes be divided in two or more, in which case the subunits are calledsternites,[1] and may also be modified on the terminal abdominal segments so as to form part of the functionalgenitalia, in which case they are frequently reduced in size and development, and may become internalized and/or membranous. For a detailed explanation of the terminology, see[2]
Kinorhynchs have tergal and sternal plates too, though seemingly not homologous with those of arthropods.[3]
Ventrites are externally visible sternites. Usually the first sternite is covered up, so that ventrite numbers do not correspond to sternite numbers.
The term is also used in other arthropod groups such ascrustaceans,arachnids andmyriapods. Sternites on thepleon (abdomen) of a crustacean may be referred to aspleonsternites. These are the sites of attachment of thepleopods (swimming legs). Inspiders, the sternum is the ventral part of thecephalothorax.