Hexapod

Hexapods
Scientific classification
Kingdom: Animalia Phylum: Arthropoda Subphylum: Hexapoda
Classes & Orders
Class Insecta (insects) Order Diplura Order Collembola (springtails) Order Protura

Hexapods are a large group of arthropods, including the insects, as well as a few small groups closely related to them. They have bodies divided into an anterior head, thorax, and posterior abdomen. The head is composed of a presegmental acron that usually has eyes and six segments, all fused closely together, which have the following appendages:

1. None
2. Antennae (sensory), absent in Protura
3. None
4. Mandibles (crushing jaws)
5. Maxillae (chewing jaws)
6. 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 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, though 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.

The myriapods have traditionally been considered the closest relatives of the hexapods, based on morphological similarity, but new work has called this into question, and it appears their closest relatives may be the crustacean instead. There is also some evidence suggesting that the hexapod groups may not share a common origin, and in particular that the Collembola belong elsewhere.