Maxilla

The maxillae (plural: maxillae /mækˈsɪliː/) are the two maxilla bones forming the upper jaw and palate of the mouth. The two halves are fused at the intermaxillary suture to form the upper jaw. This is similar to the mandible (lower jaw), which is also a fusion of two halves at the mandibular symphysis.

Sometimes (e.g. in bony fish), the maxilla is called "upper maxilla," with the mandible being the "lower maxilla." Conversely, in birds the upper jaw is often called "upper mandible."

Structure

Each half of the fused maxillae consists of:

Articulations

Each maxilla articulates with nine bones:

Maxilla (arthropod mouthpart)

In arthropods, the maxillae are paired structures present on the head as mouthparts in members of the clade Mandibulata, used for tasting and manipulating food. Embryologically, the maxillae are derived from the 4th and 5th segment of the head and the maxillary palps; segmented appendages extending from the base of the maxilla represent the former leg of those respective segments. In most cases, two pairs of maxillae are present and in different arthropod groups the two pairs of maxillae have been variously modified.

Myriapoda

Millipedes

In millipedes, the second maxillae have been lost, reducing the mouthparts to only the first maxillae which have fused together to form a gnathochilarium, acting as a lower lip to the buccal cavity and the mandibles which have been enlarged and specialized greatly, used for chewing food. The gnathochilarium is richly infused with chemosensory and tactile receptors along its edge. A pair of maxillary glands, also called nephridial organs, involved in osmoregulation and excreting nitrogenous waste open up to the gnathochilarium and wastes are passed entirely through the digestive tract before being evacuated. The neprhidial organs are thought to be derived from similar organs in annelids, although reduced in number since the open circulatory system of arthropods lessens the demand on separate excretory organs. The reason for their anterior location is probably because these organs must be developed early on in the embryo and millipedes and other arthropods develop mainly by proliferation of cells at the posterior of the embryo.