chaetae

Chaetae, or setae, are small bristle-like structures composed mainly of chitin that project from the body of segmented worms, especially annelids. They arise from segmental pockets in the epidermis and extend outward from the body, helping to interact with the surrounding substrate.

In polychaetes, chaetae occur in bundles called chaetal fascicles on the paired parapodia. They vary in shape, including slender capillary chaetae and heavier blade-like falcate or acicular chaetae. Some species possess internal chitinous rods called aciculae that stiffen the parapodia and aid in lever action during movement.

In oligochaetes such as earthworms, chaetae are fewer and arranged in transverse rows, typically four pairs per segment, and function mainly as anchors to grip the substrate during locomotion. They help translate body contractions into forward movement by providing a grip point as the worm crawls.

Chaetae can serve functions beyond locomotion, including stabilization during burrowing and, in some contexts, sensory input. They are produced by specialized epidermal cells called chaetoblasts and grow as the animal increases in size.

The term chaetae covers a range of bristles across annelids and related groups; variation in chaetal morphology is used in taxonomy and reflects differences in habitat, lifestyle, and evolutionary history.