In arthropod

Arthropod

An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...

s, the maxillae are paired structures present on the head as mouthparts in members of the clade Mandibulata

Mandibulata

The Mandibulata or mandibulates is a clade of arthropods that comprises the extant subphyla Myriapoda , Crustacea and Hexapoda . Mandibulata is currently believed to be the sister group of the clade Arachnomorpha, which comprises the rest of arthropods...

, used for tasting and manipulating food. Embryologically

Embryology

Embryology is a science which is about the development of an embryo from the fertilization of the ovum to the fetus stage...

, 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.

Millipedes

In millipede

Millipede

Millipedes are arthropods that have two pairs of legs per segment . Each segment that has two pairs of legs is a result of two single segments fused together as one...

s, 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

Mandible (arthropod)

thumb|250px|The mandibles of a [[Bull ant]]The mandible of an arthropod is either of a pair of mouthparts used for biting, cutting and holding food. Mandibles are often simply referred to as jaws. Mandibles are present in the extant subphyla Myriapoda , Crustacea and Hexapoda...

 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

Osmoregulation

Osmoregulation is the active regulation of the osmotic pressure of an organism's fluids to maintain the homeostasis of the organism's water content; that is it keeps the organism's fluids from becoming too diluted or too concentrated. Osmotic pressure is a measure of the tendency of water to move...

 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.

Centipedes

In centipede

Centipede

Centipedes are arthropods belonging to the class Chilopoda of the subphylum Myriapoda. They are elongated metameric animals with one pair of legs per body segment. Despite the name, centipedes can have a varying number of legs from under 20 to over 300. Centipedes have an odd number of pairs of...

s, both pairs of maxilla are developed. The first maxillae are situated ventrally to the mandibles

Mandible (arthropod)

thumb|250px|The mandibles of a [[Bull ant]]The mandible of an arthropod is either of a pair of mouthparts used for biting, cutting and holding food. Mandibles are often simply referred to as jaws. Mandibles are present in the extant subphyla Myriapoda , Crustacea and Hexapoda...

 and obscure them from view. This pair consists of a basal plate formed from the fused coxae of each leg plus ventral sternite from this segment and is hence called a coxosternite and two pairs of conically jointed appendages called telopodites and coxal projections. The second maxillae, which partly cover the first maxillae, consist of only a telopodite and a coxosternite. The telopodite is recognizably leglike in structure and consists of three segments plus an apical claw. The second maxillae also have a metameric pore, which is the opening of the maxillary gland and maxillary nephridium homologous to those of millipedes.

Crustaceans

In crustacean

Crustacean

Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...

s, the two pairs of maxillae are called maxillula (1st pair) and maxillae (2nd pair). They serve to transport food to the mandibles but also frequently help in the filtration process and additionally they may sometimes play a role in cleaning and grooming. These structures show an incredible diversity throughout crustaceans but generally are very much flattened and leaf-like. The two pairs are normally positioned very close together and their apical parts generally are in direct contact with the mandible.

Hexapoda

The generalized condition in hexapods is for the first pair of maxillae to consist of a basal triangular sclerite called the cardo and a large central sclerite called the stipes from which arise three processes: the lacinia, the galea and the maxillary palp. The lacinia is often strongly sclerotized and toothed. It functions to cut and manipulate food in the mouth. The galea is a broad, scoop-like, lobe structure, which assists the maxillary palps in sampling items before ingestion. The maxillary palp is serially homologous to the walking leg while the cardo and stipes are regarded by most to be serially homologous to the first leg segment, the coxa.
The labium is immediately posterior to the first maxilla and is formed from the fusion of the second maxillae, although in lower orders including the Archaeognatha

Archaeognatha

Archaeognatha is an order of wingless insects, also known as jumping bristletails. They are among the least evolutionarily changed insects, appearing in the Devonian period along with the arachnids. The name Archaeognatha is derived from Greek Archaeos meaning "ancient" and gnatha meaning "jaw"...

 (bristletails) and Thysanura

Thysanura

Thysanura is an order of insects, encompassing silverfish and firebrats, known for their three long caudal filaments.The families Machilidae and Meinertellidae of the jumping bristletails were once included with Thysanura....

 (silverfish) the two maxillae are not completely fused. It consists of a basal submentum, which connects with the prementum through a narrow sclerite, the mentum. The labium forms the lower portion of the buccal cavity in insects. The prementum has a pair of labial palps laterally, and two broad soft lobes called the paraglossae medially. These paraglossae have two small slender lobes called glossae at their base.

Specializations

In many hexapods, the mouthparts

Insect mouthparts

Insects exhibit a range of mouthparts, adapted to particular modes of feeding. The earliest insects had chewing mouthparts...

 have been modified for different functions and the maxillae and labium can change in structure greatly. In bee

Bee

Bees are flying insects closely related to wasps and ants, and are known for their role in pollination and for producing honey and beeswax. Bees are a monophyletic lineage within the superfamily Apoidea, presently classified by the unranked taxon name Anthophila...

s, the maxillae and labium have been modified and fused to form a nectar-sucking proboscis. In the order Hemiptera

Hemiptera

Hemiptera is an order of insects most often known as the true bugs , comprising around 50,000–80,000 species of cicadas, aphids, planthoppers, leafhoppers, shield bugs, and others...

, the true bugs, plant hoppers, etc., the mouthparts have been modified to form a beak for piercing. The labium forms a sheath around a set of stylets that consist of the an outer pair of mandibles and an inner pair of maxillae. In lapping flies

Fly

True flies are insects of the order Diptera . They possess a pair of wings on the mesothorax and a pair of halteres, derived from the hind wings, on the metathorax...

, a proboscis is formed from mostly the labium specialized for lapping up liquids. The labial palps form a labella which have sclerotized bands for directing liquid to a hypopharangeal stylet, through which the fly can imbibe liquids. In Lepidopterans, the fluid-sucking proboscis is formed entirely from the galea of the maxillae although labial palps are also present. In Odonata

Odonata

Odonata is an order of insects, encompassing dragonflies and damselflies . The word dragonfly is also sometimes used to refer to all Odonata, but the back-formation odonate is a more correct English name for the group as a whole...

 nymphs

Nymph (biology)

In biology, a nymph is the immature form of some invertebrates, particularly insects, which undergoes gradual metamorphosis before reaching its adult stage. Unlike a typical larva, a nymph's overall form already resembles that of the adult. In addition, while a nymph moults it never enters a...

, the labium forms a mask-like extensible structure, which is used for reaching out and grasping prey.