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.

Hexapod morphology

Hexapods have bodies divided into an anterior head, thorax, and posterior abdomen. The head is composed of a presegmental acron that usually bears eyes, followed by six segments, all closely fused together, with the following appendages:

Segment I. None

Segment II. Antennae (sensory), absent in Protura

Segment III. None

Segment IV. Mandibles (crushing jaws)

Segment V. Maxillae (chewing jaws)

Segment VI. Labium (lower lip)

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.

Hexapod relationships

The myriapods have traditionally been considered the closest relatives of the hexapods, based on morphological similarity. These were then considered subclasses of a subphylum called Uniramia or Atelocerata. New work, however, has called this into question, and it appears their closest relatives may be the crustaceans instead. The non-insect hexapods have variously been considered a single evolutionarily line, typically treated as Class Endognatha, or several lines with different relationships with the Class Insecta. In particular, the Diplura may be more closely related to the Insecta than the Collembola or the Protura. There is also some evidence suggesting that the hexapod groups may not share a common origin, and in particular that the Collembola belong elsewhere.

External link

• Tree of Life Project – Hexapoda