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 chaeta due to their differing morphology.
Crustacean Crustaceans have mechano- and
chemosensory setae. Setae are especially present on the mouthparts of crustaceans In some cases, setae are modified into scale like structures.
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 s mating on a vertical glass window and showing
lamellae under the feet 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". ==Fungal setae==