Labium

Definition

In insects and other arthropods, the posterior, median mouthpart derived from the fused pair of ancestral second maxillae. The labium forms the 'lower lip' of the mouthpart complex, positioned behind the mandibles and maxillae, and functions to manipulate and hold food during feeding. It typically bears a pair of three-segmented labial palps with sensory gustatory and tactile functions. Structurally, it consists of a proximal postmentum (submentum and mentum) and a distal prementum that carries the palps and often terminal glossae and paraglossae (the 'tongue' analogues).

Etymology

From Latin labium meaning 'lip'.

Example

In a honey bee worker, the labium forms the flexible floor of the proboscis; when extended, the prementum, glossae, and paraglossae together create a hairy, nectar-lapping 'tongue' while the labial palps fold alongside to sense food quality.

Synonyms

• lower lip (informal)
• second maxilla (developmental)

Related Terms

• Maxilla
• Mandible
• Labrum
• hypopharynx
• Clypeus
• postmentum
• prementum
• labial palp
• Glossa
• Proboscis
• mouthparts

Usage Notes

The labium is developmentally and serially homologous to the maxillae, not merely a 'lip' in the vertebrate sense. Its morphology varies dramatically across insect orders: reduced or absent in some Hemiptera, modified into a piercing stylet sheath in others, elongated as a nectar-sipping proboscis in Lepidoptera, and forming the flexible 'tongue' base in bees and wasps. The term 'labium' also applies to the homologous structure in other arthropods such as crustaceans and myriapods. Do not confuse with the unrelated hymenopteran genus Labium (Ichneumonidae) or botanical/musical uses of the word.