|
The subphylum Hexapoda (from the Greek for ''six legs'') constitutes the largest number of species of arthropods and includes the insects as well as three much smaller groups of wingless arthropods: Collembola, Protura, and Diplura (all of these were once considered insects). The Collembola (or springtails) are very abundant in terrestrial environments. ''Hexapods'' are named for their most distinctive feature: 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 (absent in Protura and Diplura), 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 fourth and fifth segments and is covered by a projection from the sixth, called the labrum (upper lip). In true insects (class Insecta) 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. 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 (often reduced in number in many insect species), but in Protura it has twelve, and in Collembola only six (sometimes reduced to only four). The appendages on the abdomen are extremely reduced, restricted to the external genitalia and sometimes a pair of sensory cerci on the last segment. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「hexapoda」の詳細全文を読む スポンサード リンク
|