Coxa

Definition

The proximal, often short and stout, first segment of an arthropod leg, articulating directly with the thorax (or with the pleuron in insects) and bearing the trochanter distally. In insects, the coxa is typically the functional hip joint, capable of rotation and leverage for walking, jumping, or grasping; in arachnids and crustaceans, it may be subdivided or modified for feeding (as in the coxal gnathobases of some arachnids) or respiration (coxal glands).

Etymology

From Latin coxa, meaning 'hip' or 'hip joint'

Example

In a grasshopper's hind leg, the greatly enlarged coxa and trochanter together form a powerful lever system that stores elastic energy for jumping.

Synonyms

• hip joint (informal)
• basipodite (in some crustacean terminology)

Related Terms

• Trochanter
• Femur
• Thorax
• pleuron
• coxal gland
• gnathobase
• subcoxa

Usage Notes

The coxa is counted as leg segment 1 in the standard hexapod formula, though in some groups (e.g., Odonata nymphs) it may be small or fused with the trochanter. In arachnology, 'coxa' specifically refers to the basal segment of the pedipalp or walking leg, and the term 'subcoxa' is sometimes used for a precoxal sclerite in comparative morphology. Contrast with 'trochanter' (segment 2), which in some insects is immovably fused to the coxa.