? Hexapods | ||||||||||
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| Scientific classification | ||||||||||
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| Classes & Orders | ||||||||||
| Class Insecta (insects) Unplaced orders: Order Diplura Order Collembola (springtails) Order Protura |
The subphylum Hexapoda constitutes the largest (in terms of number of species) grouping of arthropods and includes the insects as well as a few much smaller groups of wingless arthropods closely related to insects: Collembola, Protura, and Diplura. The Collembola (or springtails) are very abundant in terrestrial environments. The term, hexapoda, alludes to having "six feet", the most distinctive feature of the group: a consolidated thorax with three pairs of legs. Most other arthropods have more than three pairs of legs.
The mouth lies between the second and third segments and is covered by a projection from the first, called the labrum (upper lip). In true insects (class Insecta herein) the mouthparts are exposed or ectognathous, while in other groups they are enveloped or endognathous. Similar appendages are found on the heads of Myriapoda and Crustacea, although these have secondary antennae.
The thorax is composed of three segments, each of which bears a single pair of legs. These give the group its name, from the Greek "six feet". As is typical of arthropods adapted to life on land, each leg has only a single walking branch composed of five segments, without the gill branches found in some other arthropods. In most insects the second and third thoracic segments also support wings. It has been suggested that these may be homologous to the gill branches of crustaceans, or they may have developed from extensions of the segments themselves.
The abdomen consists of eleven segments in all true insects, but in Protura it has twelve, and in Collembola only four to six. The appendages on the abdomen are extremely reduced, restricted to the external genitalia and sometimes a pair of sensory cerci on the last segment.