cercus

Cercus, singular for cerci, is the term used for one of a pair of appendages at the rear end of the abdomen in many insects. Cerci vary in size and shape across taxa, ranging from slender filaments to robust, forceps-like structures. In most insects, these paired appendages are primarily sensory, equipped with hairs and receptors that detect air movement, vibrations, chemical cues, and the insect’s own movements. In some lineages cerci also contribute to balance or aerodynamics during movement.

Examples and variation: In mayflies (order Ephemeroptera) and stoneflies (order Plecoptera) cerci are long, threadlike tails that extend beyond the abdomen. Earwigs (order Dermaptera) possess enlarged, pincer-like cerci used in defense, gripping objects, and in mating displays. In other insects cerci can be small or reduced, or even absent, reflecting diverse evolutionary paths and ecological roles.

Taxonomic and functional significance: The presence, absence, and morphology of cerci can aid in identifying and classifying insect groups, although their roles are diverse and not universal. Cerci are distinct from other tail-like structures in arthropods, such as the telson or uropods, which have different evolutionary origins and functions.