The housefly interfacetal hair

The external and internal fine structure of the housefly interfacetal hair and its sensory dendrite was studied with the scanning and transmission (high and low voltage) electron microscopes. The hair shaft contains no dendrites, and is usually situated within a socket on the lens surface. Immediately beneath and directly connected to the base of each hair is a bipolar neuron whose dendrite tip is enveloped in a shealth cell which, in turn, is surrounded by a second sheath cell. Septate junctions are seen between all these cells and contiguous portions of a large pigment cell. At the hair base, the dendrite of the neuron terminates in a tubular body only 1.5 mum in diameter which is filled with about 400 microtubules in highly ordered (in parallel pentagonal and hexagonal) arrays and whose sides are fused to neurofilaments in parallel. Another filament (ca. 70 A diameter) is in the center of each microtubule-neurofilament polygon. Structures proximal to the tubular body are typical for a scolopoid sensillum, i.e., connecting cilium (9 times 2 + 0 microtubules) with rootlet and basal bodies, unmodified dendrite, perikaryon and axon. The axon has not been traced to its synapse. The high degree of internal organization and shortness of the tubular body, as well as its eccentric insertion into the hair shaft lead to the hypothesis that this hair may be a highly sensitive mechanoreceptor. On the basis of their single innervation, these hairs could monitor flight speed from the degree of hair deflection caused by wind in general or particular laminar air currents flowing past the eyes during flight.     

Fine structure of antennal mechanosensilla of adult Rhodnius prolixus stål (Hemiptera: Reduviidae)

Each antenna of both sexes of adult Rhodnius prolixus has approximately 570 mechanosensitive neurons that innervate five morphologic types of cuticular mechanosensilla: campaniform sensilla, tapered hairs, trichobothria, and type I and type II bristle sensilla. Each campaniform sensillum and tapered hair is presumably innervated by one mechanosensitive bipolar neuron and probably functions in proprioception. The campaniform sensilla being located at the base of the scape could monitor the position of the antenna. Tapered hairs are found at the distal margin of flagellar segment I and projecting laterally from the bases of the pedicel and scape. They probably provide information about the relative positions of the antennal segments. Seven trichobothrium are located on the pedicel and three on flagellar segment I. Each trichobothrium has a long filamentous hair inserted into the base of a socket that extends inwardly as a cuticular tube and is innervated by one bipolar neuron with a tublar body, a parallel arrangement of microtubules associated with electron-dense material. The trichobothria may respond to small variations in air currents. Type I bristles occur at the base of the antenna and are the most numerous type of mechanosensillum; an average of 452 occur on each antenna of females and 440 on males. The bristle is curved toward the antennal shaft and is serrated distally. Type II bristles are located distally and are the second most numerous type of mechanosensillum; an average of 88 were counted on each antenna of females and 94 on males. The type II bristle is straight with small, longitudinal, external grooves and projects laterally from the antennal shaft. Each type I and II bristle sensillum is innervated by a bipolar neuron whose dendrite is divided into an inner and outer segment. The outer segment is encased by a dendritic sheath which may be highly convoluted and distally contains a tubular body. Two sheath cells are associated with each sensillum. Both types of bristle sensilla have a tactile function. The tubular bodies of both types of bristle sensilla have a complex structure indicating that they are very sensitive. Variations in the amount and arrangement of the electron-dense material at the tip of the tubular bodies may reflect differences in viscoelastic properties that underlie functional characteristics.     

Sensitivity of an insect mechanoreceptor during moulting

ABSTRACT. The fine structure and the behavioural threshold for vibration sensitivity of the eight thoracic filiform hairs of Barathra brassicae caterpillars were investigated through an intermoult/moult cycle. Associated with each filiform hair is one bipolar sensory cell and three enveloping cells. The outer dendritic segment terminates in an ecdysial canal in the hair base and a tubular body lies at its distal end. Shortly before apolysis the dendrite elongates. By this means the connection between the sensory cell and the old cuticular apparatus is maintained while the epithelium and the old thoracic cuticle are separating. The new cuticular apparatus of the filiform hair is formed in the second half of the larval stage by the three enveloping cells. A second tubular body in the elongated outer dendritic segment is formed at the base of the replacement hair 10 h before next ecdysis, so that the new hair functions as soon as ecdysis is completed, the old cuticular apparatus with the old tubular bodies being torn away with the exuvia during ecdysis. Sensitivity to a 300 Hz tone was tested in the standing wave of a Kundt's tube. Throughout most of the larval instar the threshold was 2.0 ± 0.3 μm particle displacement amplitude until 1–2h before ecdysis when it rose to 6.8 ± 1.3 μm and at 10–30 min before the beginning of ecdysis no reaction to sound could be detected. Once the old cuticle was shed maximum sensitivity returned as soon as the replacement hairs were erect. The sensilla are therefore physiologically functional at all developmental stages except for 30–60 min during actual ecdysis.     

Fine structure of tarsal sensilla of Aedes aegypti (L.) (Diptera: Culicidae)

The tarsi of all three pairs of legs of both sexes of Aedes aegypti (L.) bear spine sensilla, five types of hair sensilla, which are designated A, B, C1, C2 and C3, and campaniform sensilla. Type A and B hairs, spines, and cam-paniform sensilla are innervated by one neuron with a tubular body, a characteristic of cuticular mechanoreceptors. In particular the hairs and spines are tactile receptors and the campaniform sensilla are proprioceptors. The C1, C2, and C3 hair sensilla have the morphological features of contact chemoreceptors. Type C1 and C3 hairs are innervated by five and four neurons, respectively, which extend to the tip of the hair. Type C2 is innervated by five neurons, one of which terminates at the base of the hair in a tubular body while the remaining four extend to the tip of the hair. The role of the type C hairs in oviposition behavior, nectar feeding, and recognition of conspecific females is discussed. Presumed efferent neurosecretory fibers occur near the spine and hair sensilla.     

Hair regeneration during moulting in the spiderCiniflo similis (Araneae,Dictynidae)

Summary The mechanoreceptive and chemoreceptive hairs on the legs of the cribellate spiderCiniflo similis were examined during the moulting cycle. In mechanoreceptive hairs the new hair shaft is formed around the extended dentrites, which emerge from near the tip of the newly forming hair and continue to the old sensillum within the extended dendritic sheath. Thus there is no ecdysial canal in the base of the hair shaft as found in insect mechanoreceptive hairs. The dendritic connection with the old hair is maintained until shortly before ecdysis by which time new tubular bodies have developed in the same dendrites at the base of the new hair. In chemoreceptive sensilla the new hair shaft is also formed around the elongated outer segment of the dendrites (19 chemosensitive and 2 mechanosensitive). The two mechanosensitive dendrites develop new tubular bodies at the base of the hair. As ecdysis occurs the old dendritic sheath and dendrites are snapped off at the tip of the new hair but the pore remains open. The ultrastructural evidence indicates that the roles of the three main enveloping cells are as follows: The dendritic sheath cell secretes the dendritic sheath, the middle enveloping cell forms the hair shaft while the outer enveloping cell forms the socket. This pattern corresponds closely to that observed in insecta sensilla. The extreme length of the chemoreceptive dendrites during moulting is mentioned in connection with receptor function. The unique multi-layered nature of the middle enveloping cell is seen as a device for the formation of regularly occurring rows of small spines on the shaft of the hair.     

The mechanism of sensory transduction in the sensilla of the trochanteral hair plate of the cockroach,Periplaneta americana

Summary The trochanteral hair plate of the cockroach leg contains approximately 60 hair sensilla that are deflected by a joint membrane during flexion of the leg. Previous work has shown that the organ is a mechanoreceptor which limits leg flexion during walking by reflex connections to flexor and extensor motoneurons. Functional analysis of the largest sensilla has shown that their behaviour may be well approximated by a velocity detector followed by a unidirectional rectifier.We report here the results of an examination of the largest sensilla by scanning and transmission electron microscopy in an attempt to correlate the structure with the known functional elements. Each hair is innervated by a single sensory dendrite which is surrounded by an electron dense dendritic sheath. The dendrite terminates below the hair shaft in a tubular body containing a parallel array of microtubules embedded in an electron dense matrix, while the dendritic sheath extends beyond the tubular body to form the walls of the ecdysial canal. At the proximal end of the tubular body the dendritic sheath and sensory dendrite are anchored to the cuticular socket by a fibrous dome which seems to form a fulcrum around which the tubular body can be deflected by movements of the hair. We suggest that the basis for the detection of velocity may be mechanical differentiation by a fluid space between the dendritic sheath and the tubular body. The structure is also discussed with relation to the mechanism of sensory transduction and the possible causes of the unidirectional sensitivity.Supported by the Canadian Medical Research Council. The authors gratefully acknowledge the expert technical assistance of Sita Prasad     

Innervation of the cercal sensilla on the ovipositor of the Australian sheep blowfly (Lucilia cuprina)

ABSTRACT. The ovipositor of the female sheep blowfly, Lucilia cuprina (Wied.) (Diptera: Calliphoridae), has a complement of cercal sensilla that includes long, medium and short tactile hairs, two campaniform domes, four olfactory pegs, and ten double-channelled gustatory hairs. This sensory array is suited to assess oviposition site resources, prior to and during the laying of an egg batch.
The tactile hairs and campaniform sensilla are each innervated by a single, tubular body containing dendrite. The olfactory pegs are each innervated by a single, moderately branched dendrite, which gains access to the external environment via pores at the bottom of deep grooves in the peg wall. The gustatory hairs fall into two categories. Four hairs have a single, tubular body containing dendrite at their base, and four unbranched dendrites running up to the hair tip which has a terminal pore. Six of the taste hairs have no tubular body containing dendrite at the base, and three unbranched dendrites running up to a terminal pore.     

Mechanosensory afferents innervating the swimmerets of the lobster

Feathered hair sensilla fringe both rami of the lobster (Homarus americanus) swimmeret. The sensory response to hair displacement was characterized by recording afferent impulses extracellularly from the swimmeret sensory nerve while deflecting sensilla with a rigidly-coupled probe or controlled water movements. Two populations of hairs were observed: "distal" hairs localized to the distal 1/3 of each ramus and "proximal" hairs near its base. Distal hairs are not innervated by a mechanosensory neuron but instead act as levers producing strain within adjacent cuticle capable of activating a nearby hypodermal mechanoreceptor. Hair deflections of 25 degrees or more are required to evoke an afferent response and this response is dependent on hair deflection direction. The frequency and duration of the afferent discharge evoked are determined by the velocity of hair displacement. Each proximal hair is innervated by a single mechanosensory neuron responding phasically to hair deflections as small as 0.2 degrees in amplitude. Deflection at frequencies up to 5 Hz elicits a single action potential for each hair movement; at higher frequencies many deflections fail to evoke an afferent response. These sensilla, which are mechanically coupled, may be activated by the turbulent flow of water produced by the swimmerets during their characteristic beating movements.     

The tibial thread-hairs ofAcheta domesticus L. (Saltatoria,Gryllidae)

Summary The ultrastructure of the thread-like hairs (sensilla) on the tibia of the front leg ofAcheta domesticus (Gryllidae) Saltatoria was examined by serial sectioning. The presence of a tubular body indicates that these sensilla are mechanosensitive; electrophysiological measurements also confirmed this. The opposing forces on the articulating apparatus of single hairs and the sensitivity of the single receptor cell were measured after deflection of the hair in different directions. The articulating apparatus is characterized by three cuticular elements: a joint membrane, suspension fibers, and a socket septum. These elements form the basis for a structural bilateral symmetry along whose plane of symmetry the direction line of both the minimum receptor sensitivity and the minimum opposing forces lie. The tubular body embedded in the tip of the socket septum is attached to the base of the hair shaft. The hair provides the leverage for displacing the tubular body and the socket septum limits the extent to which it may be laterally displaced.These investigations have been supported by the Deutsche Forschungsgemeinschaft     

The cercal sensilla of the blowfly Lucilia cuprina. I. Structure of the sockets and distal dendritic regions

The functions of the gustatory, olfactory, touch and stress receptors on the cerci of Lucilia cuprina Wied. (Diptera: Calliphoridae) are apparent from the morphology of their distal dendritic segments and associated cuticular structures. Each trichoid mechanoreceptor has a dendrite containing a tubular body at the base of the hairshaft. The suspension fibres and socket septum may be involved in transmitting a stimulus to the dendrite terminal and restoring the hair to its resting position. The campaniform sensilla are considered as trichoid mechanoreceptors with reduced hair shafts and socket structures, reflected in fusion of the suspension fibres into the inner cuticle of the dome and loss of the socket septum. Fusion and reduction of the socket structures is also apparent at the bases of the olfactory pegs. They differ from typical antennal olfactory sensilla in having a flexible socket and relatively thick walls; features which may protect them from damage during ovipositor probing of potential oviposition substrates. The two types of cereal gustatory sensilla differ in their complement of chemosensory dendrites, one has three, the other four, the latter type also has a mechanoreceptive dendrite at the base of the hair shaft. Both types have socket structures resembling those of the trichoid mechanoreceptors.     

A computational fluid dynamics model of viscous coupling of hairs

Arrays of arthropod filiform hairs form highly sensitive mechanoreceptor systems capable of detecting minute air disturbances, and it is unclear to what extent individual hairs interact with one another within sensor arrays. We present a computational fluid dynamics model for one or more hairs, coupled to a rigid-body dynamics model, for simulating both biological (e.g., a cricket cercal hair) and artificial MEMS-based systems. The model is used to investigate hair–hair interaction between pairs of hairs and quantify the extent of so-called viscous coupling. The results show that the extent to which hairs are coupled depends on the mounting properties of the hairs and the frequency at which they are driven. In particular, it is shown that for equal length hairs, viscous coupling is suppressed when they are driven near the natural frequency of the undamped system and the damping coefficient at the base is small. Further, for certain configurations, the motion of a hair can be enhanced by the presence of nearby hairs. The usefulness of the model in designing artificial systems is discussed.     

COMPARATIVE ANALYSES OF THE DINOFLAGELLATE FLAGELLAR APPARATUS. III. FREEZE SUBSTITUTION OF AMPHIDINIUM RHYNCHOCEPHALUM1

The flagellar apparatus of the marine dinoflagellate Amphidinium rhynchocephalum Anissimowa was examined using the techniques of rapid freezing/freeze substitution and serial thin section three dimensional reconstruction. The flagellar apparatus is composed of two basal bodies that are offset from one another and lie at an angle of approximately 150° The transverse basal body is associated with two individual microtubules that extend from the proximal end of the basal body toward the flagellar opening. One of these microtubules is closely appressed to a striated fibrous root that also extends from the proximal base of the transverse basal body. The longitudinal basal body is associated with a nine member microtubular root that extends from the proximal end of the basal body toward the posterior of the cell. The longitudinal microtubular root and the transverse striated fiber are connected by a striated connective fiber. In addition to the microtubules associated with the transverse and longitudinal basal bodies, a group of microtubules originates adjacent to one of the transverse flagellar roots and extends into the cytoplasm. Vesicular channels extend from the flagellar openings to the region of the basal bodies where they expand to encompass the various connective structures of the flagellar apparatus. The possible function and evolutionary importance of these structures is discussed.     

Ultrastructure of the peg and hair sensilla on the antenna of larval Aedes aegypti (L.)

The antenna of fourth instar larvae of Aedes aegypti has one peg organ of a basiconic type innervated by four neurons. The dendrites are ensheathed to near their terminations at the peg tip by an electron-dense dendritic sheath and by a cuticular sheath. They have easy communication by diffusion with the external environment only at the tip through a peripheral ensheathing membrane and six slit-channels. One of the dendrites resembles a tubular body proximally and may be mechanoreceptive. The peg generally appears to be a contact chemoreceptor. There are three antennal hairs of a typical sensillum trichodeum type innervated at the base by one neuron each. An intricate terminal mechanism at the insertion of the dendrite in the hair is described. These are believed to be tactile hairs. There are also three antennal hairs each innervated by two neurons. The dendrite from one terminates at the base similar to that of a tactile hair, and is believed to function in a similar mechanoreceptive manner. The dendrite from the second neuron extends naked along the length of the hair lumen. It is believed to be primarily chemoreceptive, in a slow-acting general sensory function. In all the sensilla there appear to be secretions produced in the junction body regions of the dendrites, and there is evidence for accumulation of secretory materials in the dendritic tips in some of the sensilla.     

Cortical microtubules and microfibril deposition in the cell wall of root hairs ofEquisetum hyemale

Summary The cell wall of root hairs ofEquisetum hyemale is shown to be composed of three different cell wall textures. The growing cell wall at the tip of the hair is composed of a dispersed texture of microfibrils, which continues along the outside of the whole hair. With increasing distance from the tip an increasing number of helicoidally arranged lamellae is deposited. These findings correspond with the observed isotropism of young hairs in polarized light.Hairs of approximately 4 days old become positive birefringent, indicating that longitudinally oriented layers prevail over layers with a transverse direction. This phenomenon starts at the base of the hair. Full-grown hairs are positive birefringent up to the tip and concordantly show a thick additional inner cell wall layer which forms a helical pattern the length of the hair, with a mean microfibril angle of 25 with the cell axis.Cortical microtubules, subjacent to the dispersed, the helicoidal and the helical wall texture are axially aligned and, thus, not in coalignment with the last deposited microfibrils.Coated and smooth vesicles are present in the cortical cytoplasm of both growing and full-grown hairs. Electron-dense profiles (20 nm in diameter), surrounded by a halo (of 50 nm) were observed on the wall-plasmalemma interface in full-grown hairs only. A relation of these structures with microfibril deposition could not be demonstrated. They might represent channels transporting material to the wall, which, in full-grown hairs, is heavily impregnated with a tawny brown substance.The general hypothesis that cortical microtubule orientation directs microfibril deposition is disputed.     

Reduction of mechanical sensitivity in an insect mechanoreceptor correlated with destruction of its tubular body

Summary The modified cilium (dendrite) of epithelial mechanoreceptors of insects contains microtubules in different arrangements: (1) microtubules distributed over the entire receptor and not fixed in a special configuration, therefore called free microtubules, (2) densely packed, interconnected microtubules called the tubular body, and (3) 9 doublet microtubules. These groups of microtubules have been discussed in relation to mechanotransduction.In a preceding paper the free microtubules were proved to be not involved in mechanotransduction. In this paper the hypothesis is examined that the tubular body may be essential to mechanotransduction. For this purpose the effect of the microtubule-disassembling drug vinblastine on both the tubular body and the sensitivity is examined in a femoral mechanoreceptor of the cricket Acheta domesticus.After 6- to 26-h exposure to vinblastine the tubular body is partially or totally destroyed. Simultaneously, mechanical sensitivity decays to zero. In contrast, the pacemaker property for nerve impulses of the apical dendritic segment is only slightly altered. We conclude from these results that the tubular body is essential to mechanotransduction. Three experiments in which a (small) response persisted, despite a totally destroyed tubular body, suggest that receptor potentials can in principle be evoked without an intact tubular body.In addition to the irreversible reduction of receptor sensitivity, vinblastine causes a reversible reduction during repetitive stimulation. This adaptation is supposed to be the consequence of altered properties of the tubular body.     

Light and electron microscopic investigation of Pelomyxa prima (Gruber, 1884) (Peloflagellatea, Pelobiontida)

Cell organization of a multinuclear pelobiont Pelomyxa prima has been studied at the light and electron microscopic levels. Motile individuals demonstrate a characteristic drop-like or pyriform shape and reach 550 microkm in length. The cell cover is represented by a well-developed, morphologically differentiated glycocalyx 80-100 nm thick. The cytoplasm contains many structural vacuoles. The nuclei are of vertical type, numbering up to several nuclei in large individuals. Numerous cytoplasmic microtubules are associated with the external membrane of the nuclear envelope. Separate non-motile flagella are distributed throughout the cell surface, being more numerous in the posterior body end and uroidal zone of the protist. Basal bodies of the flagella are extremely long, being deeply immersed into the cytoplasm. These bodies are surrounded by a muff of electron-dense material, with numerous microtubules radiating from it. A compact bundle of microtubules starts from the base of a basal body axially into the cytoplasm. Besides, a band-like lateral microtubular rootlet is present. The number of microtubules in the axoneme of undulipodia is unstable. Neither mitochondria, nor Golgi complex were found. Two species of bacterial endocytobionts are present in the cytoplasm in considerable numbers.     

通河林区黄鼬(Mustela sibirica)冬季被毛形态结构的功能适应性

被毛在哺乳动物适应性进化过程中执行保温和保护两个重要功能,其形态结构上存在的功能适应性特征因所处的部位不同而表现出适应性分化现象,由动物体躯干至四肢末端呈显著的梯度变化。以黑龙江省通河林区黄鼬东北亚种(Mustela sibirica manchurica)冬季雌雄成体各10只完整毛皮对象,研究了背中部、腹中部和后肢下部3个部位的直针毛、披针毛、绒针毛、绒毛,以及后趾部硬毛的被毛性状因子,统计分析表明:通河林区黄鼬相同身体部位4种类型毛的长度和细度指标均为直针毛披针毛绒针毛绒毛,相同部位4种类型毛长度的相关性极显著,直针毛细度与披针毛细度相关性极显著(P0.01),绒针毛细度与绒毛细度相关性极显著(P0.01),这种特征使得被毛在整体结构上为实施保温和保护功能奠定基础;同时,黄鼬被毛各性状的保温功能从背部向后趾部呈递减趋势,而保护功能则呈现递增趋势,被毛形态结构性状上的分化与动物机体异温性充分结合,对于黄鼬适应寒冷的森林生态环境具有重要意义。     

THE FLAGELLAR APPARATUS OF CHRYSOLEPIDOMONAS DENDROLEPIDOTA (CHRYSOPHYCEAE), A SINGLE-CELLED MONAD COVED WITH ORGANIC SCALES1

The flagellar apparatus of Chrysolepidomonas dedrolepidota Peters et Andersen is similar to that of other members of the Ochromonadales, Chrysophyceae. there are four microtubular roots (R_1-4) and a system II fiber (= rhizoplast). the R₁ root consists of three microtubules that nucleate many cytoplasmic microtubules. One compressed band of 10 or more cytoplasmic microtubules is directed black along the R1 root in an anti-parallel direction. The R₂ root consists of one to two microtubules, and it extends toward the distal end of the R₁ root. The R₃ root consists of six (?seven) microtubules near its proximal end. The “a” and “f” microtubules of the R₃ root are under the short flagellum, and the “f” microtubule loops back and under the basal body, extending down to the nucleus. The R₄ root consists of one to two microtubules extending along the left side of the shot flagellum and curving under the short flagellum where it terminates near the “a” microtubule of R₃ Both flagella have a transitional plate and a transitional helix with five gyres. There is a thin, second plate in the basal body at the level of the distal end of the “c” tubules of the basal body triplets. The tripartite flagellar hairs have long lateral filaments but lack short lateral filaments. We compare the flagellar apparatus with that of other members of the Ochromonadales and members of the Hydrurales and Hibberdiales.     

Structure of galeal sensory complex in adults of the red turnip beetle,Entomoscelis americana brown (Coleoptera,Chrysomelidae)

Summary The internal and external structure of the galeae of the adult red turnip beetle, Entomoscelis americana, was studied using SEM and TEM. The galea broadens from base to truncated tip and its sides are of thick, sculpted cuticle invested with pores and coarse spines. The tip is of thinner, flexible cuticle covered with 8–12 uniporous, blunt-tipped apical pegs and a single, aporous, sharply-pointed apical hair.The coarse spines are singly innervated probable mechanosensilla owing to the tubular body at the distal end of the dendrite. These sensilla likely act as tactile hairs monitoring galeal-effected movements of food particles into the functional mouth. The pores are associated with glands within the galea. The function of the presumed secretion is not known but may be to keep objects and dried saliva from sticking to the mouthparts.The apical pegs are innervated by five neurons, each producing a single dendrite. Four dendrites enter the single peg lumen and communicate with the terminal pore. The fifth differentiates into a tubular body that inserts into the peg base. These are typical insect contact chemosensilla that, because of their location, would taste incoming food.The apical hair has no pores but is innervated by two neurons, each extending a dendrite into the hair lumen in chemosensillar fashion. The sensory mode of this sensillum is unknown but is probably not mechanoor chemoreception. Many of its features, reminiscent of taste hairs, lead us to hypothesize that it represents a one-time chemosensillum recently modified to a new form and sensory mode.Because larval and adult E. americana share similar food plant requirements, we hypothesize that similarities will be seen in their mouthpart sensilla. Comparisons of the adults and larvae show the common features between their respective galeal taste hairs are only those of insect contact chemosensilla in general. However, the adult apical hair and the larval medial sensillum show striking specific structural similarities. We propose that these are true structural and functional homologues.     

Roles for Rac1 and Cdc42 in planar polarization and hair outgrowth in the wing of Drosophila

The wing of Drosophila melanogaster is covered by an array of distally pointing hairs. A hair begins as a single membrane outgrowth from each wing epithelial cell, and its distal orientation is determined by the restriction of outgrowth to a single distal site on the cell circumference (Wong, L., and P. Adler. 1993. J. Cell Biol. 123:209- 211.). We have examined the roles of Cdc42 and Rac1 in the formation of wing hairs. We find that Cdc42 is required for localized actin polymerization in the extending hair. Interfering with Cdc42 activity by expression of a dominant negative protein abolishes both localized actin polymerization and hair outgrowth. In contrast, Rac1 is important for restricting the site at which hairs grow out. Cells expressing the dominant negative Rac1N17 fail to restrict outgrowth to a single site and give rise to multiple wing hairs. This polarity defect is associated with disturbances in the organization of junctional actin and also with disruption of an intricate microtubule network that is intimately associated with the junctional region. We also find that apical junctions and microtubules are involved in structural aspects of hair outgrowth. During hair formation, the apical microtubules that point distally elongate and fill the emerging wing hair. As the hair elongates, junctional proteins are reorganized on the proximal and distal edges of each cell.