Subcapitulum
The subcapitulum, also known as infracapitulum, hypognathum or hipognatum, refers to the ventral part of the gnathosoma or the fusion of the palpal coxae and the labrum complex present in some arthropods on which the mouth, pedipalps, mouthparts and pharynx are generally located. It is delimited by the subcapitular apodeme, which separates it from the cheliceral frame.
Acari
The function or morphology of the subcapitulum can vary within acarines. In its central part, the hypostomal gutter in which the pincers are transformed into a serrated pricking system is formed. The anterolateral parts of the subcapitulum might be equipped with cornicula or rutella. At the base of the hypostome, there can be sternal apophyses, that is, an outgrowth or process in the sternum. In Mesostigmata, this happens in the third segment. The frontal edge of the subcapitulum can be provided with paralaciniae. In Oribatida, its basal section forms a mentum. In some acariforms, the dorsal part forms a "neck". In some cheyletoids and myobiids, the subcapitulum merges with the stylophores to form a gnathosomal capsule. In some prostigmates, the subcapitulum or the whole capsule is referred to as rostrum.Other arthropods
The subcapitulum is also present in other arthropod clades. In Ricinulei, an arachnid order related to acarines, the subcapitulum can also be found. In these organisms, the subcapitulum had a subcapitular apodeme and a crescent-shaped mouth, and lacked lateral lips. In comparison with other arachnids, the ricinuleid subcapitulum may result primitive, but it presents a specialization unique of the order; the coxal regions of the pedipalp are enlarged.Another example would be Necrogammarus, an extinct genus of chelicerate eurypterid only known by its subcapitulum and attached pedipalp. In this genus, this structure adopts the form of a discrete plate.