In biology, setae (singular seta ; from the Latin word for " bristle") are any of a number of different bristle- or

hair Hair is a protein filament that grows from follicles found in the dermis. Hair is one of the defining characteristics of mammals. The human body, apart from areas of glabrous skin, is covered in follicles which produce thick terminal and f ...

-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 ecol ...

setae are stiff bristles present on the body. They help, for example, earthworms to attach to the surface and prevent backsliding during

peristaltic Peristalsis ( , ) is a radially symmetrical contraction and relaxation of muscles that propagate in a wave down a tube, in an anterograde direction. Peristalsis is progression of coordinated contraction of involuntary circular muscles, which ...

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 Chitin ( C8 H13 O5 N)n ( ) is a long-chain polymer of ''N''-acetylglucosamine, an amide derivative of glucose. Chitin is probably the second most abundant polysaccharide in nature (behind only cellulose); an estimated 1 billion tons of chit ...

. 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 ...

. 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 Phytoplankton () are the autotrophic (self-feeding) components of the plankton community and a key part of ocean and freshwater ecosystems. The name comes from the Greek words (), meaning 'plant', and (), meaning 'wanderer' or 'drifter'. Ph ...

. It captures them and allows them to be eaten. 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. 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 ar ...

'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'') 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 Deuterostomia (; in Greek) are animals typically characterized by their anus forming before their mouth during embryonic development. The group's sister clade is Protostomia, animals whose digestive tract development is more varied. Some exampl ...

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

In mycology, "setae" refer to dark brown, thick-walled, thorn-like cystidia found in corticioid and poroid fungi in the family Hymenochaetaceae. Though mainly microscopic, the setae of some species may be sufficiently prominent to be visible with a hand lens.

Plant setae

In botany, "seta" refers to the stalk supporting the capsule of a

moss Mosses are small, non-vascular flowerless plants in the taxonomic division Bryophyta (, ) '' sensu stricto''. Bryophyta (''sensu lato'', Schimp. 1879) may also refer to the parent group bryophytes, which comprise liverworts, mosses, and hor ...

or liverwort (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 pr ...

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 ha ...

. 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 Chaetocerotaceae is a diatom family ( Bacillariophyta). This family comprise the three genera ''Attheya'' T. West, ''Bacteriastrum'' Shadbolt and ''Chaetoceros'' Ehrenberg. ''Chaetoceros'' is perhaps the largest and most species rich genus of m ...

, "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 Synthetic things are composed of multiple parts, often with the implication that they are artificial. In particular, 'synthetic' may refer to: Science * Synthetic chemical or compound, produced by the process of chemical synthesis * Synthetic o ...

are a class of synthetic adhesives 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