Seta

Animal setae

Protostomes

Depending partly on their form and function, protostome setae may be called macrotrichia, chaetae, scales, or informally, hairs. The setal membrane is not cuticularized, so movement is possible.

Annelid

Annelid setae are stiff bristles present on the body. They allow earthworms and their relatives to attach to the surface and prevent backsliding during peristaltic motion. These hairs make it difficult to pull a worm straight from the ground. Setae in oligochaetes (the group including earthworms) are largely composed of chitin. They are classified according to the limb to which they are attached; for instance, notosetae are attached to notopodia; neurosetae to neuropodia. The setae on polychaete worms are referred to as chaetae due to their differing morphology.

Crustacean

Crustaceans have mechano- and chemosensory setae. Setae are especially present on the mouthparts of crustaceans and can also be found on grooming limbs. In some cases, setae are modified into scale like structures. Setae on the legs of krill and other small crustaceans help them to gather phytoplankton. It captures them and allows them to be eaten.

Insect

Setae on the integument of insects are unicellular, meaning that each is formed from a single epidermal cell of a type called a trichogen, literally meaning "bristle generator". They are at first hollow and in most forms remain hollow after they have hardened. They grow through and project through a secondary or accessory cell of a type called a tormogen, which generates the special flexible membrane that connects the base of the seta to the surrounding integument. Some insects, such as Eriogaster lanestris larvae, use setae as a defense mechanism, as they can cause dermatitis when they come into contact with skin. Diptera setae are bristles present throughout the body and function as mechanoreceptors.

Deuterostomes

Vertebrates

The pads on a gecko's feet are small hair-like processes that play a role in the animal's ability to cling to vertical surfaces. The micrometer-scale setae branch into nanometer-scale projections called spatulae. A Tokay gecko's two front feet can sustain 20.1 N of force parallel to the surface using approximately 14,400 setae per mm². This equates to ~ 6.2 pN per seta, but does not sufficiently account for the overall stickiness behavior shown by the foot pads.

Classification uncertain

In 2017, a description of a new species of basal deuterostome called Saccorhytus was published. This animal appears to have seta in the pores along the side of its body. However, in 2022, Saccorhytus is considered to be an early ecdysozoan, and was described as having "lacked setae".