The superclass of hexapods (Hexapoda, greek = with six legs) constitutes the largest (in terms of number of species) grouping of arthropods and includes two classes: the Insecta and Entognatha. This last class includes three small orders of wingless hexapods: Collembola, Protura, and Diplura.
Description: The hexapod owe their name to the character that makes it easy to distinguish them from all other arthropods: a consolidated thorax with three pairs of legs. Almost all other arthropods have more than six feet.
The body of the hexapod is divided into three parts, the head (front), thorax (middle) and abdomen (rear). The head consists of a acron presegmentale, with proper eye, followed by five segments, all closely fused together, with the following appendices:
Segment II. Antennae (sensory), absent in Protura
Segment IV. Jaws
Segment V. Jaws (chewing)
segment VI. Lower lip
The mouth is located between the second and third segments and is covered by a projection of the first, the upper lip. In the true insects (class Insecta) the different parts of the mouth are external, while in other groups are internal. Like appendages are also found in the head of the Myriapoda and Crustacea, although these also have secondary antennas.
The chest consists of three segments, each of which bears a pair of legs. As is typical of arthropods adapted to terrestrial life, each leg, composed of five segments, lacking the gill branches that are found in some of the other groups of arthropods. In most insects, the second and third thoracic segments are also provided wings. It has been suggested that the wings are homologous to the gill appendages of crustaceans, or have evolved from extensions of the segments themselves.
The abdomen consists of eleven segments in all true insects, but in twelve Protura and Collembola in only 4:00 to 6:00. The appendixes are extremely small, limited to the external genitalia and, in some cases, a pair of sensory cerci on the last segment.
affinity 'with other groups: The Myriapoda are traditionally considered the closest relatives of hexapods, based on morphological similarities. New studies have questioned this point and according to some scholars (eg Giribet, Edgecombe and Wheelker, 2001) may be the closest relatives instead shellfish.
The non-insect hexapods (but once included in them) are variously regarded as a homogeneous group or a set of evolutionary lineages with different relationships with the true insects: the Diplura would be closer to them and Collembola, some say should even be excluded from the hexapod.
0 comments:
Post a Comment