Hexapoda (Insects)

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The name Hexapoda denotes the presence of six legs (three pairs) in these animals as differentiated from the number of pairs present in other arthropods. Hexapods are characterized by the presence of a head, thorax, and abdomen, constituting three tagma. The thorax bears the wings as well as six legs in three pairs. Many of the common insects we encounter on a daily basis—including ants, cockroaches, butterflies, and flies—are examples of Hexapoda.

Amongst the hexapods, the insects (Figure) are the largest class in terms of species diversity as well as biomass in terrestrial habitats. Typically, the head bears one pair of sensory antennae, mandibles as mouthparts, a pair of compound eyes, and some ocelli (simple eyes) along with numerous sensory hairs. The thorax bears three pairs of legs (one pair per segment) and two pairs of wings, with one pair each on the second and third thoracic segments. The abdomen usually has eleven segments and bears reproductive apertures. Hexapoda includes insects that are winged (like fruit flies) and wingless (like fleas).

Figure 1. In this basic anatomy of a hexapod insect, note that insects have a developed digestive system (yellow), a respiratory system (blue), a circulatory system (red), and a nervous system (red).

Insects are the dominant terrestrial arthropod and have many adaptations. Insects are present in almost every ecological niche. They can live independently or in social colonies. Insects undergo metamorphosis. There are two types of metamorphosis, incomplete and complete.

In incomplete metamorphosis, there are three stages (larva, nymph, adult) and insects look like miniature adults when they hatch. Grasshoppers and cockroaches are two insects that undergo incomplete metamorphosis.

During the process of complete metamorphosis, the insect changes form entirely. There are four stages in complete metamorphosis (egg, larva, pupa, adult) and two completely different life forms. The juvenile and adult life form are different, and this prevents them from competing for the same resources.

An insect's mouth parts are adaptations related to its specialized diet. Go online to read more about these adaptations Feeding I: Structure and Function Mouthparts.

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