Labrum

Definition

A flap-like, unpaired sclerite forming the anterior-most component of the insect or arthropod mouthparts, positioned above the mandibles and articulating with the clypeus. The labrum functions as a protective cover for the underlying mandibles and assists in manipulating and retaining food during feeding. In most insects it is a single, median structure derived from the anterior edge of the head capsule, though its musculature and mobility vary considerably across orders—reduced or fused in some groups, notably mobile and prominent in others such as beetles and orthopterans.

Etymology

Latin labrum, meaning 'lip' or 'edge'

Example

In a grasshopper, the large, shield-like labrum hangs down over the base of the mandibles and can be lifted to expose the cutting edges beneath when the insect begins to chew plant material.

Synonyms

• upper lip (informal)

Related Terms

• Clypeus
• Mandible
• Maxilla
• Labium
• Epipharynx
• hypopharynx
• Subesophageal ganglion
• mouthparts

Usage Notes

The term 'upper lip' is sometimes used in introductory texts but is anatomically imprecise; the labrum is not a true lip and is not homologous to vertebrate structures. In arachnids and crustaceans, the labrum is typically smaller and less conspicuous than in insects. The labrum should be distinguished from the labium, which forms the posterior 'lower lip' of the mouthpart complex. Some authors restrict 'labrum' to the external sclerite and use 'epipharynx' for its internal, membranous lining.