Hexapoda

Hexapoda is a large group of arthropods distinguished by having six legs and a body divided into three tagmata: head, thorax, and abdomen. Members typically possess an external exoskeleton made of chitin, a segmented form, and one pair of antennae. Most hexapods respire through a network of tracheal tubes and exchange gases directly with body tissues. The group includes the true insects and several primitive, wingless relatives.

Taxonomy and diversity: In modern classifications, Hexapoda comprises Insecta and the Entognatha (Collembola, Diplura, Protura) as primitive, wingless hexapods; together they form a clade within Pancrustacea. Insects are by far the largest hexapod group, with about a million described species, occupying a wide range of habitats and lifestyles.

Life cycles and development: Insects exhibit various developmental strategies. Some undergo ametabolous development with juveniles resembling adults; most insects undergo metamorphosis, including incomplete metamorphosis (hemimetabolism) and complete metamorphosis (holometabolism). Entognathans tend to be wingless and have mouthparts that develop internally (entognathy), distinguishing them from true insects.

Ecology and evolution: Hexapods inhabit nearly every terrestrial and freshwater habitat. They play essential ecological roles as pollinators, herbivores, predators, detritivores, and prey, and have major economic importance as both pests and beneficial organisms. The fossil record indicates hexapods originated well before flowering plants, with diversification accelerating through the Mesozoic and continuing to the present.