The sister groups Hexapoda and Myriapoda form the Uniramia. The Myriapoda is a polyphyletic assemblage of the Pauropoda (centipede-like animals having branched antennae and nine or ten pairs of legs), Diplopoda (millipedes), Chilopoda (centipedes) and Symphyla (also similar to centipedes, but possessing 10 to 12 pairs of legs). The Hexapoda, by far the largest taxon of the Uniramia, contain the Collembola, Protura, Diplura and Insecta. The uniramians are mainly terrestrial and occur worldwide, some living in fresh water, but very few in marine environments.
The name Uniramia refers to the fact that the appendages are basically unbranched and there is no evidence that they were derived from a branched condition. This subphylum consists of arthropods with one pair of antennae and with mandibles. The mandibles are supposed to be formed from a whole limb which bites, transversely at the tip. The Pauropoda and Diplopoda have mandibles and one pair of maxillae. The Chilopoda, Symphyla and Hexapoda have mandibles and two pairs of maxillae.
In contrast to the marine origin of the other three taxa making up the Arthropoda, the uniramians appear to have their origin on land. Increasing evidence based on comparative morphology and embryology indicates that the uniramians, and indeed all four of the arthropod groups, had a separate origin from different annelidan or near-annelidan ancestors and that arthropodization (i.e., the arthropod features of a chitinous exoskeleton and jointed appendages) evolved independently four times. |
