biology Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of cells that process hereditary ...

, setae (singular seta ; from the Latin word for "

bristle A bristle is a stiff hair or feather (natural or artificial), either on an animal, such as a pig, a plant, or on a tool such as a brush or broom. Synthetic types Synthetic materials such as nylon are also used to make bristles in items such as ...

") are any of a number of different bristle- or hair-like structures on living organisms.

Animal setae

Protostomes

Annelid The annelids (Annelida , from Latin ', "little ring"), also known as the segmented worms, are a large phylum, with over 22,000 extant species including ragworms, earthworms, and leeches. The species exist in and have adapted to various ecolo ...

setae are stiff bristles present on the body. They help, for example,

earthworm An earthworm is a terrestrial invertebrate that belongs to the phylum Annelida. They exhibit a tube-within-a-tube body plan; they are externally segmented with corresponding internal segmentation; and they usually have setae on all segments. Th ...

s 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 (a 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 In invertebrates, the term parapodium ( Gr. ''para'', beyond or beside + ''podia'', feet; plural: parapodia) refers to lateral outgrowths or protrusions from the body. Parapodia are predominantly found in annelids, where they are paired, unjointed ...

Crustacean Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. The crustacean group ...

s 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 Krill are small crustaceans of the order Euphausiacea, and are found in all the world's oceans. The name "krill" comes from the Norwegian word ', meaning "small fry of fish", which is also often attributed to species of fish. Krill are cons ...

and other small crustaceans help them to gather phytoplankton. It captures them and allows them to be eaten. Setae on the

integument In biology, an integument is the tissue surrounding an organism's body or an organ within, such as skin, a husk, shell, germ or rind. Etymology The term is derived from ''integumentum'', which is Latin for "a covering". In a transferred, or ...

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

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

Deuterostomes

Vertebrates

Hemidactylus_frenatus_mating,_ventral_view

The pads on a

gecko Geckos are small, mostly carnivorous lizards that have a wide distribution, found on every continent except Antarctica. Belonging to the infraorder Gekkota, geckos are found in warm climates throughout the world. They range from . Geckos are ...

'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. * Gekko's seta: According to Kellar Autumn, "Two front feet of a tokay gecko (''

Gekko gecko The tokay gecko (''Gekko gecko'') is a nocturnal arboreal gecko in the genus '' Gekko'', the true geckos. It is native to Asia and some Pacific Islands. Subspecies Two subspecies are currently recognized. *''G. g. gecko'' (Linnaeus, 1758): tr ...

'') can withstand 20.1 N of force parallel to the surface with 227 mm² of pad area (Irschick et al. 1996). The foot of a tokay bears approximately 3600 tetrads of setae per mm², or 14,400 setae per mm² (Schleich and Kastle 1986; pers. obs.). Consequently, a single seta should produce an average force of 6.2 pN, and an average shear stress of 0.090 N⋅mm⁻¹ (0.9 atm). However, single setae proved both much less sticky and much more sticky than predicted by whole animal measurements, under varying experimental conditions, implying that attachment and detachment in gecko setae are mechanically controlled (Autumn et al. 2000)."

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 Ecdysozoa () is a group of protostome animals, including Arthropoda (insects, chelicerata, crustaceans, and myriapods), Nematoda, and several smaller phyla. They were first defined by Aguinaldo ''et al.'' in 1997, based mainly on phylogenetic tr ...

, and was described as having "lacked setae".

Fungal setae

In mycology, "setae" refer to dark brown, thick-walled, thorn-like

cystidia A cystidium (plural cystidia) is a relatively large cell found on the sporocarp of a basidiomycete (for example, on the surface of a mushroom gill), often between clusters of basidia. Since cystidia have highly varied and distinct shapes that ar ...

found in

corticioid The corticioid fungi are a group of fungi in the Basidiomycota typically having effused, smooth basidiocarps (fruit bodies) that are formed on the undersides of dead tree trunks or branches. They are sometimes colloquially called crust fungi or pa ...

and poroid fungi in the family

Hymenochaetaceae The ''Hymenochaetaceae'' are a family of fungi in the order Hymenochaetales. The family contains several species that are implicated in many diseases of broad-leaved and coniferous trees, causing heart rot, canker and root diseases, and also es ...

. Though mainly microscopic, the setae of some species may be sufficiently prominent to be visible with a hand lens.

Plant setae

botany Botany, also called , plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist is a scientist who specialises in this field. The term "botany" comes from the Ancient Greek w ...

, "seta" refers to the stalk supporting the capsule of a moss or

liverwort The Marchantiophyta () are a division of non-vascular land plants commonly referred to as hepatics or liverworts. Like mosses and hornworts, they have a gametophyte-dominant life cycle, in which cells of the plant carry only a single set of ge ...

(both closely related in a clade called "Setaphyta"), and supplying it with nutrients. The seta is part of the

sporophyte A sporophyte () is the diploid multicellular stage in the life cycle of a plant or alga which produces asexual spores. This stage alternates with a multicellular haploid gametophyte phase. Life cycle The sporophyte develops from the zygote p ...

and has a short foot embedded in the gametophyte on which it is

parasitic Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson has ...

. Setae are not present in all mosses, but in some

species In biology, a species is the basic unit of Taxonomy (biology), classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of ...

they may reach 15 to 20 centimeters in height.

Chaetoceros setae

In the diatom family Chaetocerotaceae, "seta" refers to the hairlike outgrowths of the valve, i.e. of the face of the cells.Tomas, C. R., Hasle G. R., Syvertsen, E. E., Steidinger, K. A., Tangen, K., Throndsen, J., Heimdal, B. R., (1997). ''Identifying Marine Phytoplankton'', Academic Press. These setae have a different structure than the valve. Such setae may prevent rapid sinking and also protect the cells from grazing.

Synthetic setae

Synthetic setae are a class of synthetic

adhesive Adhesive, also known as glue, cement, mucilage, or paste, is any non-metallic substance applied to one or both surfaces of two separate items that binds them together and resists their separation. The use of adhesives offers certain advant ...

s that detach at will, sometimes called resettable adhesives, yet display substantial stickiness. The development of such synthetic materials is a matter of current research.Setae Research

References

{{reflist, 28em Plant anatomy Animal hair Fungal morphology and anatomy