Insect morphology least three different positions: prognathous, opisthognathous, and hypognathous. In species with prognathous articulation, the head is positioned vertically aligned with the body, such as species of Formicidae; while in a hypognathous type, the head is aligned horizontally adjacent to the body. An opisthognathous head is positioned diagonally, such as species of Blattodea and some Coleoptera. The mouthparts vary greatly between insects of different orders, but the two main functional groups are mandibulate and haustellate. Haustellate mouthparts are used for sucking liquids and can be further classified by the presence of stylets, which include piercing-sucking, sponging, and siphoning. The stylets are
Insect term also form the usual word for "insect" in Welsh (, from ' "to cut" and " mil", "animal"), Serbo-Croatian ("zareznik", from "rezati", "to cut"), Russian ( "nasekomoje", from "seč'/-sekat"', "to cut"), etc. The precise definition of the taxon Insecta and the equivalent English name "insect" varies; three alternative definitions are shown in the table. In the broadest circumscription, Insecta "sensu lato" consists of all hexapods. Traditionally, insects defined in this way were divided into "Apterygota" (the first five groups in the table) – the wingless insects – and Pterygota – the winged insects. However, modern phylogenetic studies have shown