The maxilla is one of the head extremities that appear in crustaceans (Crustacea) and insects (Insecta) in the broader sense. In the case of insects and the higher-level group of trachea animals (tracheata), the maxillae are integrated into a head capsule and serve here as mouth parts .
In crustaceans, too, the maxillae are usually involved in food intake. The transition between the functions of the extremities is fluid in the crustaceans and varies between the swimming leg and the mouthpiece. In crustaceans, the maxilla forms the fourth and fifth extremities in the cephalothorax . One also speaks of maxillipedas when the two maxillae are joined by other trunk legs as mouthparts (in crayfish , for example, these are three pairs of maxillipedes).
Insects largely have highly differentiated and specialized mouthparts. In insects, the maxilla forms the third and fourth head extremities after the antennae and mandibles , since the second antennae have been lost in the course of evolution. Correspondingly, the maxillae of crabs and those of insects and millipedes are homologous structures.
In insects, the second maxillae are always fused into a uniform labium . The labium should not be confused with the labrum, which shields the mouthparts as the upper lip, but cannot be diverted from the extremities. In the case of stinging-sucking insects, the labium as the overgrown second maxilla forms the basis for the suction tube.