Hexapoda

Definition

A subphylum of arthropods comprising all six-legged terrestrial arthropods, including the vast majority of extant arthropod species. Hexapoda unites the class Insecta (true insects) with the smaller class Entognatha, which contains three wingless groups—Collembola (springtails), Protura (coneheads), and Diplura—that were historically classified as insects but are now recognized as a separate paraphyletic or monophyletic assemblage depending on phylogenetic interpretation. The defining synapomorphy is the body plan of three tagmata (head, thorax, abdomen) with three pairs of thoracic legs, though some entognathans show secondary reductions. Hexapods dominate terrestrial ecosystems as pollinators, herbivores, detritivores, predators, and disease vectors, and their evolutionary divergence from other arthropod lineages (particularly Crustacea, with which they form Pancrustacea/Tetraconata) is a major topic in arthropod phylogenetics.

Etymology

From Greek hexa- (six) + pous (foot), referring to the three pairs of legs characteristic of the group.

Example

While all insects are hexapods, not all hexapods are insects: springtails (Collembola) possess a ventral springing organ (furcula) and internal mouthparts, distinguishing them from true insects despite their six-legged form.

Synonyms

• hexapods

Related Terms

• Insecta
• Entognatha
• Collembola
• Protura
• Diplura
• Pancrustacea
• Tetraconata
• Arthropoda
• Myriapoda
• Crustacea
• Chelicerata
• tagma
• Thorax
• pleuron

Usage Notes

In modern systematics, Hexapoda is treated as a subphylum, though some older literature may use it as a superclass or informal grouping. The internal relationships remain debated: molecular studies often place Entognatha as paraphyletic with respect to Insecta, or render Hexapoda itself paraphyletic if certain crustacean groups are nested within. The term 'hexapod' in common parlance usually means any six-legged arthropod, but technically refers only to members of this subphylum—some other arthropods (e.g., certain larval Crustacea) also have six legs but are not hexapods. Contrast with Myriapoda (centipedes, millipedes), which have numerous leg pairs and a different tagmosis.