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One of the major subdivisions of the arthropods, including the insects, centipedes, milipedes, and related forms. In these the body is divided into a fused head, composed of six segments plus a presegmental acron that usually has eyes, and a trunk, which may be subdivided into a thorax and abdomen. The appendages on the head segments are highly modified and more or less uniform throughout the group:
1. None
2. Antennae (sensory)
3. None
4. Mandibles (crushing jaws)
5. Maxillae (chewing jaws)
6. 2nd maxillae or Labium (lower lip)
The mouth lies between the second and third segments and is covered by a projection of the first called a Labrum (lower lips). The head appendages are similar to those found in the Crustacea, except that the second antennae are lacking, from which the group is sometimes called the Atelocerata. However, the mandibles have been shown to correspond to different parts of the regular appendages.
The legs on the rest of the body are typically uniramous, that is, have a single walking branch with no gill branch as in other arthropods, which gives the group its name. In insects legs have five segments. The uniramous condition probably stems simply from all extant Uniramia being terrestrial or secondarily aquatic. However, some embryological work has suggested that the wings in insects may in fact be homologous to the gill branch in other arthropods, which would require their ancestors to still be aquatic. More work needs to be done on this.
The Uniramia are divided into the following classes:
The hexapods are distinguished by having the trunk divided into a three-segmented thorax and a limbless abdomen. The remaining forms are called myriapods and are generally believed to form a monophyletic group, for reasons I would love for someone to add. Within the Arthropoda, the Uniramia are generally considered the sister group to the Crustacea, but this is not quite so certain, and in proposals where the arthropods are polyphyletic they are often taken to be closer related to the non-arthropod Onychophora.