Scales and Setae
Setae are multicellular protuberances on the arthropod cuticle used primarily for mechanoreception. In all groups of arthropods and especially insects, the role of the setae has evolved from simple mechanoreception to various other functions, including defense, locomotion, prey capture, pheromone dispersal, sexual display, preening, and camouflage. Setae are often highly modified, with one common modification being that they may be flattened into a broad, plate-like scale. This article specifically examines the tremendous diversity in shape, structure, and function of setae and scales used by different insect groups and at different life stages. This diversity in setae type is then characterized according to their four major functions in the biology of the insect: mechanoreception, camouflage, defense, and pheromone dispersal.
STRUCTURAL MORPHOLOGY
The terminology applied to scales and setae has historically been confused, with numerous, often interchangeable, terms used by researchers to describe the various types of structures (e.g., hairs, bristles, trichiae, aculei, chetae). Each seta, or trichoid sensillium, is a mul-ticellular protuberance with specifically differentiated cells, with the most diagnostic landmark being its socket. The term trichoid sensil-lum applies to chemoreceptors as well as mechanoreceptors. The term hair is incorrect when applied to insects because hairs are morphologically different from setae and are considered a characteristic of mammals and not arthropods. Trichobothria is a term occasionally used to describe large, nontapering seta often associated with the mouthparts and genitalia. Other types of cuticular protuberances not considered here include the various minute projections of the cuticle (e.g., pru-inescence, velutum, pollen), collectively termed microtrichia, and large sclerotized extensions of the cuticle (e.g., spines, tubercles).
Most setae form the covering of the body surface, which in very high densities is called a pile. Usually a mechanoreceptor, a typical seta, is composed of four cells: (1) a sensory cell, which innervates the seta, is surrounded by a (2) thecogen cell, which acts as an auxiliary cell by secreting the dendrite sheath. During seta development the (3) trichogen cell secretes the tapered or scale-like protuberance, whereas the (4) tormogen cell is responsible for formation of the socket. While most setae develop as mechanoreceptors, some will become secondarily noninnervated and lose their sensory function. These setae may take on a different role (such as aerodynamics, sexual display, preening) in the functioning of the insect.
Scales are modified setae that have a flattened blade with longitudinal ridges, sometimes with serrate edges (Fig. 1A and 1B). Scales are usually inclined relative to the cuticle, overlapping each other when present in sufficient densities. Each scale has a narrow pedicel, and the enlarged blade may be either gradually attenuated to an apex or truncated.
Scales are found in most groups of insects and serve a variety of functions. In true flies (Diptera), scales are not common but in certain species can be found on the wings (e.g., mosquito, Anopheles annulipes—Culicidae), legs (e.g., Trichopoda spp.—Tachinidae), or body (e.g., Metatrichia spp.—Scenopinidae). Scales are also found on the elytra of various beetle (Coleoptera) families (e.g., Elateridae, Curculionidae, Buprestidae), but the function of such scales in Coleoptera and in Diptera is unknown. Scales on the body and wings of Lepidoptera have been studied in greater detail and are known to function in cryptic coloration, thermal regulation (e.g., butterfly, Colias spp.), and aposematism (warning coloration). In butterflies, wing scales contribute to lift during flight but not to drag, thus enabling them to glide for longer periods.
Lepidopteran wing scales have a specialized structure and are divided into two types. Primitive-type scales, found in nonditrysian Lepidoptera, are solid with longitudinal ridges (Fig. 1C). Normal-type
FIGURE 1 Lepidopteran scales. (A) Truncated wing scale, (B) serrate body scale, (C) cross section of primitive-type wing scale, (D) cross section of normal-type wing scale. Scale line length, 0.1 mm.
scales, found in the ditrysian Lepidoptera, are composed of superior and inferior lamellae, with an internal lumen subdivided by internal supports called trabeculae (Fig. 1D ) . Whereas the inferior lamella is smooth, the superior lamella is usually covered with longitudinal ridges, interconnecting transverse ridges (flutes), and/or perforations (windows). Some groups of nonditrysian Lepidoptera have a mixture of the two types, with normal-type scales layered over the primitive-type scales.
Wing color in Lepidoptera is produced by wing scales. Individual scales are usually a single color which may be generated by any one or a combination of pterins (red, yellow, white), melanins (black), fla-vonoids (white), carotenoids (blue, yellow), papilliochromes (cream-yellow), and ommachromes (red). Scales also produce color by diffraction; regular longitudinal ridge spacing produces various diffraction colors, whereas irregular spacing produces overlap in color spectra and is perceived as white light. Interference colors, from which light is reflected from a series of superimposed surfaces separated by distances equivalent to light wavelengths, result in selective phasing in and out of various colors. This type of color is important for producing bright color hues in adult butterflies (e.g., Morpho spp.) and is produced from a series of longitudinal vanes along the scale blade. Each vane is supported by a series of thickenings, or mullions, which act in concert with other vanes to form a series of reflecting surfaces and produce interference colors.
FUNCTIONAL TYPES
The functions of setae are sensory (e.g., touch, taste, and smell), but in many insects the original sensory function of some setae has been discontinued though cell death that creates a noninnervated
seta. These setae often have more specialized roles, sometimes with great morphological modification. There is incredible diversity in the shape, structure, and function of both the seta and its morphological derivative, the scale. A detailed examination of almost any insect will reveal a myriad of seta or scale types, often with vastly differing roles in the biological functioning of the organism. The following classification is clearly artificial and categorizes the diversity of setae and scales into functional types for ease of discussion.
Mechanical
LOCOMOTION Elongate setae are used by numerous insect groups for flight and swimming. Elongation of marginal setae is an efficient mechanism to increase the surface area of locomotory structures (e.g., wing, leg), with only a slight increase in weight or developmental investment (Fig. 2) . Such elongation of setae along wing margins is common only in very small sized insects, as the degree of elongation is finite and insufficient to generate adequate lift for larger insects. Groups of insects that use this mechanism for flight include all Thysanoptera and various microhymenopterans (e.g., Mymar— Mymaridae), microlepidoptera (e.g., Macarostola—Gracillaridae), and Coleoptera (e.g., Acrotrichis—Ptilidae) (Fig. 2). Aquatic insects that are active swimmers often have paddle-shaped leg segments fringed with elongated setae for added propulsion through water. Examples include various families of Heteroptera (e.g., Naucoridae, Corixidae, and Notonectidae) and Coleoptera (e.g., Hygrobiidae, Haliplidae, Dytiscidae, and Gyrinidae).
FIGURE 2 Right forewing of Acrotrichis sp. (Ptilidae: Coleoptera). Scale line, 0.5 mm.
PLASTRON To extend their duration under water, some aquatic insects use a plastron, or air bubble trapped by fine hydro-fuge setae. The setae are used to hold a large air bubble by surface tension. The plastron enables them to live permanently submerged in water because oxygen passes by diffusion from the water into the plastron. Examples of plastron use by this method include the beetle families Elmidae and Hydrophilidae.
FOOD GATHERING AND PREY CAPTURE
In predatory insects, enlarged setae are an economical substitute for teeth or spines for holding prey. Stiff enlarged setae are present along the inner margin of mandibles of some larval Nymphidae and Myrmeleontidae (Neuroptera). Similar setae on the inner margin of fore femora of adult Leptopodidae (Heteroptera) are used for the same purpose. Aquatic insects may use elongated rows of setae around the mouthparts as filters to trap food particles in flowing water (e.g., Coloburiscoides sp.— Coloburiscidae: Ephemeroptera) or use setae-fringed foretarsi to sieve for prey in detrital ooze (e.g., Agraptocorixa—Corixidae: Hemiptera).
Bees (Hymenoptera) commonly have enlarged hind tibiae and basitarsi covered with brush-like setae (scopa), which are used to carry pollen.
WING COUPLING
Some insect groups use specialized setae to couple the fore- and hind wings together, so ensuring synchronous wing beats during flight. The basic pattern, which occurs in Choristidae (Mecoptera), is composed of retinacular setae along the jugal margin of the forewing interlocking with frenular setae along the basicostal margin of the hind wing. More advanced forms are found in Lepidoptera, in which the frenulum may be a large single seta or multiple slender setae. In Trichoptera, a row of large setae is present along the costal margin or the subcostal vein of the hind wing, which engages either the jugal lobe or a ventral ridge in the anal field.
DIGGING
Fossorial insects often have stout setae arranged in rows on the leg segments for digging. A comb of long setae (pecten) is commonly present on the foretarsi of ground-nesting aculeate wasps (e.g., Bembix—Sphecidae). Enlarged and thickened setae-like structures borne on acanthophorites are used for digging during oviposition by females of various asiloid Diptera (e.g., Therevidae, Apioceridae).
Camouflage Aids
Highly modified setae are commonly used by insect larvae that camouflage themselves by carrying soil, feces, and/or trash particles on their body. Some of the most elaborate of these may be found on the bodies of myrmeleontoid Neuroptera (i.e., Ascalaphidae, Nemopteridae, Psychopsidae, Nymphidae, and Myrmeleontidae). Other than entangling camouflage materials, the biological function of these highly modified setae, called dolichasters, is unknown. Dolichasters may be simple, scale shaped or highly ornate, star or cup shaped or recurved hooks (Fig. 3). Ascalaphid and nymphid lacewing larvae may also have abdominal extensions called scoli (Fig. 4), on which rows of enlarged scales or setae are used to entangle camouflaging materials.
FIGURE 3 Dolichasters from head and thorax of unidentified species of Ascalaphidae (Neuroptera). Scale line, 0.2 mm.
Defensive
Lepidopteran larvae of several families (e.g., Arctiidae, Notodontidae, Thaumetopoeidae) use specialized urticating (irritating) setae as a defense against potential predators. Urticating setae are modified setae with a poison cell associated with the trichogen cell; the former discharges venom when the tip of the seta is broken off.
FIGURE 4 Lateral scoli on abdomen of neuropteran larvae. (A) Osmylops sp. (Nymphidae), (B) unidentified ascalaphid. Scale line, 1.0 mm.
The urticating setae may be very long and scattered over the body surface or short and positioned on the apex of a scoli. In Limacodidae, the stinging setae are short and positioned at the apices of often brightly colored, eversible tufts that are extended when the individual is disturbed. Some adults and/or larvae of the coleopteran families Cleridae, Trogossitidae, and Dermestidae are often clothed in long fine setae, which may be recurved. In addition to a sensory function, these setae may be used defensively to obstruct or entangle potential predators or parasitoids.
Secretory Substrate
Setae are commonly used by insects as a substrate of high surface area from which a pheromone is dispersed by evaporation. Adult males of some nemopterid (Neuroptera) genera use distinctive tufts of fine setae (bulla) on the hind margins of the fore- or hind wings to disperse pheromone. Males of the antlion tribe Acanthoclisinae (Myrmeleontidae) have eversible sacs (hair pencils) covered with fine setae for a similar purpose, and many male butterflies use specialized setae (androconia) located in basal depressions along the forewing veins called androconial organs. During courtship pheromone is dispersed from the hairs, often with elaborate “calling” behavior, which can consist of hovering and diving at the prospective female and fanning pheromone in her direction.
