Shedding Cycle Research Articles

Phase-Average Mean Flow and Turbulence Structure in a Staggered Cylinder Array Subjected to Pulsating Cross-Flow

The unsteady turbulent flow field in a staggered array of cylinders with streamwise and transverse spacing to diameter ratios of 2.1 and 3.6, respectively, was studied experimentally by means of laser-Doppler anemometry for a Reynolds number of 2300. Flow pulsations in the streamwise direction were employed to control the frequency of vortex shedding in the array as has been previously demonstrated in Konstantinidis et al. [1]. Under these conditions, it was possible to obtain velocity measurements and for each velocity sample assign the phase-angle with respect to the pulsation cycle. This methodology allowed reconstruction of the unsteady (periodic) mean flow and random turbulence fields over an average vortex shedding cycle. The experimental data obtained are analyzed and the results offer a unique insight into the flow processes that take place inside the cylinder array.

Hysteresis phenomenon in the galloping oscillation of a square cylinder

It is well known that a square cylinder with one side normal to a uniform stream will gallop when a critical flow velocity is exceeded. It is also quite well known that there is a hysteresis phenomenon in the variation of the cylinder's galloping amplitude with the flow velocity. However, little is known about the cause of this hysteresis phenomenon, and the objective of this paper is to study it more closely. In the present study, flow over a stationary square cylinder at different angle of attack ( α) and at Reynolds number (Re) of 250 and 1000 was investigated numerically by using a 2-D hybrid vortex computation scheme. The study reveals that the well known point of inflection which exists in the side force ( C y ) versus α plots at high Reynolds number only occurs at Re=1000, α=4° in the present numerical simulation. Nonlinear analysis further reveals that this point of inflection is the cause of the hysteresis phenomenon. By further analysing the computed flow field, it is noted that at Re=1000, α=4°, intermittent flow reattachment takes place at alternate vortex shedding cycle on one side of the cylinder. This results in larger side force fluctuation, and it is conjectured that such large side force fluctuation affects the increasing trend of the side force with angle of attack, resulting in the point of inflection reported earlier. The above-mentioned alternate cycle flow reattachment was much less prominent at α=2° and 6° (Re=1000), and was not observed at Re=250. Finally, dye flow visualization was carried out in a recirculating water tunnel and the results at Re=1000 confirms the existence of the intermittent flow reattachment. However, in the experiment, flow reattachment does not take place in a very regular alternate cycle manner as in the computation. Instead, it occurs intermittently, possibly due to three-dimensional effects in real flow.

X-ray measurements within unsteady cavitation

The purpose of this work is to describe the two-phase flow structure of cloud cavitation. The experimental study is performed in a cavitation tunnel equipped with a Venturi-type test section. The geometry studied is characterized by a convergent angle of 18° and a divergent angle of 8°. It leads to regular large vapour cloud shedding. The flow is investigated by means of an X-ray attenuation device. Twenty-four collimated detectors enable the instantaneous volume fraction of the vapour phase to be measured at different locations of the two-phase flow. The X-ray intensity measurement mode enables fast data acquisition (1,000 Hz). The results are compared with double optical probe measurements. The influences of the velocity and the size of the cavitation area on the time–space distribution of the vapour fraction are studied. The convection velocity of the vapour structures is estimated. The mean distribution of the volume fraction of the vapour phase during the shedding cycle is computed using the phase- averaging method.

Particle image velocimetry and visualization of natural and forced flow around rectangular cylinders

Particle image velocimetry (PIV) measurements and flow visualization in a water tunnel show that vortex shedding at the leading and trailing edges of rectangular cylinders can be simultaneously phase-locked to transverse velocity perturbations when the applied perturbation Stp is close to an impinging leading-edge vortex/trailing-edge vortex shedding (ILEV/TEVS) frequency. The transverse perturbations, analogous to β-mode duct acoustic resonances, are generated through harmonic oscillations of the sidewalls. When this occurs, the leading-edge vortices are found always to pass the trailing edge at the same phase in the perturbation cycle regardless of the chord-to-thickness (c/t) ratio. Applying perturbations at an Stp not equal to the natural global frequency also results in phase-locked vortex shedding from the leading edge, and a near wake with a frequency equal to the perturbation frequency. This is consistent with previous experimental findings. However, vortex shedding at the trailing edge is either weaker or non-existent. PIV results and flow visualization showed trailing-edge vortex growth was weaker because leading-edge vortices arrive at the trailing edge at a phase in the perturbation cycle where they interfere with trailing-edge shedding. The frequencies at which trailing-edge vortices form for different c/t ratios correspond to the natural ILEV/TEVS frequencies. As in the case of natural shedding, peaks in base suction occur when the leading-edge vortices pass the trailing edge at the phase in the perturbation cycle (and thus in the leading-edge shedding cycle) that allows strong trailing-edge shedding. This is the reason for the similarity in the Stvs.c/t relationship for three seemingly different sets of experiments.

Direct-mode interactions in the wake behind a stepped cylinder

A computerized flow analysis of wake phenomena caused by a discontinuity in cylinder diameter is presented. The vortex linkage and half-loop formation observed experimentally by Lewis and Gharib [Phys. Fluids A 4, 104 (1992)] have for the first time been reproduced numerically. The instantaneous vorticity and pressure fields provide a distinct picture of the direct-mode interaction, from which it is concluded that a new half-loop is formed every third shedding cycle.

Periodic mass shedding of a retracting solid film step

A semi-infinite, uniform film on a substrate tends to contract from the edge to reduce the surface energy of the system. This work studies the two-dimensional retraction of such a film step, assuming that the film evolves by capillarity-driven surface diffusion. It is found that the retracting film edge forms a thickened ridge followed by a valley. The valley sinks with time and eventually touches the substrate. The ridge then detaches from the film. The new film edge retracts to form another ridge accompanied again by a valley, and the mass shedding cycle is repeated. This periodic mass shedding is simulated numerically for contact angle α between 30 and 180°. For smaller α, a small-slope late-time solution is found that agrees with the numerical solution for α=30°. Thus, the complete range of α is covered. The long-time retraction speed and the distance traveled per cycle agree quantitatively with experiments.

Ultrastructural localization of alpha-keratins in the regenerating epidermis of the lizard Podarcis muralis during formation of the shedding layer

In the epidermis of lizards, alpha- and beta-keratins are sequentially produced during a shedding cycle. Using pre- and post-embedding immunocytochemistry this study shows the ultrastructural distribution of 3 alpha-keratin antibodies (AE1, AE2, AE3) in the renewing epidermis and in the shedding complex of the regenerating tail of the lizard Podarcis muralis. The AE1 antibody that recognizes acidic low MW keratins is confined to tonofilament bundles in basal and suprabasal cells but is not present in keratinizing beta- and alpha-cells. The AE2 antibody that recognises higher MW keratins weakly stains pre-keratinized cells and intensely keratinized alpha-layers. A weak labeling is present in small electrondense areas within the beta-layer. The AE3 antibody, that recognizes low and high MW basic keratins, immunolables tonofilament bundles in all epidermal layers but intensely the alpha-keratinizing and keratinized layers (mesos, alpha-, lacunar and clear). Keratohyalin-like granules, present in the clear cells of the shedding layer, are negative to these antibodies so that the cornified clear layer contains keratins mixed with non-keratin material. The AE3 antibody shows that the mature beta-layer and the spinulated folds of the oberhautchen are labeled only in small dense areas among the prevalent electron-pale beta-keratin material. Therefore, some alpha-keratin is still present in the beta-layer, and supports the idea that alpha-keratins (basic) function as scaffold for beta-keratin deposition.

Aquatic vertebrate locomotion: wakes from body waves.

Vertebrates swimming with undulations of the body and tail have inflection points where the curvature of the body changes from concave to convex or vice versa. These inflection points travel down the body at the speed of the running wave of bending. In movements with increasing amplitudes, the body rotates around the inflection points, inducing semicircular flows in the adjacent water on both sides of the body that together form proto-vortices. Like the inflection points, the proto-vortices travel towards the end of the tail. In the experiments described here, the phase relationship between the tailbeat cycle and the inflection point cycle can be used as a first approximation of the phase between the proto-vortex and the tailbeat cycle. Proto-vortices are shed at the tail as body vortices at roughly the same time as the inflection points reach the tail tip. Thus, the phase between proto-vortex shedding and tailbeat cycle determines the interaction between body and tail vortices, which are shed when the tail changes direction. The shape of the body wave is under the control of the fish and determines the position of vortex shedding relative to the mean path of motion. This, in turn, determines whether and how the body vortex interacts with the tail vortex. The shape of the wake and the contribution of the body to thrust depend on this interaction between body vortex and tail vortex. So far, we have been able to describe two types of wake. One has two vortices per tailbeat where each vortex consists of a tail vortex enhanced by a body vortex. A second, more variable, type of wake has four vortices per tailbeat: two tail vortices and two body vortices shed from the tail tip while it is moving from one extreme position to the next. The function of the second type is still enigmatic.

Epidermal differentiation in the developing scales of embryos of the Australian scincid lizard Lampropholis guichenoti.

Formation of the first epidermal layers in the embryonic scales of the lizard Lampropholis guichenoti was studied by optical and electron microscopy. Morphogenesis of embryonic scales is similar to the general process in lizards, with well-developed overlapping scales being differentiated before hatching. The narrow outer peridermis is torn and partially lost during scale morphogenesis. A second layer, probably homologous to the inner peridermis of other lizard species, but specialized to produce lipid-like material, develops beneath the outer peridermis. Two or three lipogenic layers of this type develop in the forming outer surface of scales near to the hinge region. These layers form a structure here termed "sebaceous-like secretory cells." These cells secrete lipid-like material into the interscale space so that the whole epidermis is eventually coated with it. This lipid-like material may help to reduce friction and to reduce accumulation of dirt between adjacent extremely overlapping scales. At the end of their differentiation, the modified inner periderm turns into extremely thin cornified cells. The layer beneath the inner peridermis is granulated due to the accumulation of keratohyalin-like granules, and forms a shedding complex with the oberhautchen, which develops beneath. Typically tilted spinulae of the oberhautchen are formed by the aggregation of tonofilaments into characteristically pointed cytoplasmic outgrowths. Initially, there is little accumulation of beta-keratin packets in these cells. During differentiation, the oberhautchen layer merges with cells of the beta-keratin layer produced underneath, so that a typical syncytial beta-keratin layer is eventually formed before hatching. Between one-fourth distal and the scale tip, the dermis under epidermal cells is scarce or absent so that the mature scale tip is made of a solid rod of beta-keratinized cells. At the time of hatching, differentiation of a mesos layer is well advanced, and the epidermal histology of scales corresponds to Stage 5 of an adult shedding cycle. The present study confirms that the embryonic sequence of epidermal stratification observed in other species is basically maintained in L. guichenoti.

Differentiation of the epidermis during scale formation in embryos of lizard.

The formation of the first epidermal generations and in particular of the shedding complex in the developing scales of the lizards Anolis lineatopus and Podarcis muralis was studied by electron microscopy. The initially linear and bilayered epidermis turned into symmetric papillae, which became asymmetric scales. The outer periderm was more electrondense than the following layer, provisionally named 'inner periderm' but probably derived from the basal layer. Coarse thick filaments progressively filled the cytoplasm of the inner periderm or formed reticulate bodies resembling avian peridermal granules. Peridermal cells cornified slightly and could be shed in ovo. From the basal layer various suprabasal epithelial layers were produced. The first layer contained keratohyalin-like granules and was identified as a clear layer. Beneath it a spinulated (Anolis) or serrated (Podarcis) oberhäutchen differentiated. The clear and 'oberhäutchen' layers constitute the first shedding complex. The first epidermal layers that were shed consisted of flaking periderm or periderm together with the clear layer. The differentiation of a mesos layer was under way before hatching, when the epidermal morphology resembled a stage 4-5 of the adult shedding cycle. Fibroblasts under the inner side of the scale made few contacts with the basement membrane and their cytoplasmic elongations were mostly oriented parallel to the dense lamina. Instead, fibroblasts under the basal layer of the outer scale surface (BLOS) made numerous contacts with the basement membrane, suggesting that more dermal-epidermal interactions take place on this side of the scale.

The tau mutation shortens the period of rhythmic photoreceptor outer segment disk shedding in the hamster

The outer segments of vertebrate retinal photoreceptors undergo periodic shedding of membrane from their distal tips. This circadian rhythm of disk shedding persists with a period of about 24 h in the absence of external time cues. A circadian oscillator controlling photoreceptor disk shedding may exist in the eye, but in addition, the circadian clock in the hypothalamic suprachiasmatic nucleus (SCN) may also influence ocular rhythms including that of disk shedding. The tau mutation directly affects the SCN, and shortens the period of locomotor activity from 24 h in wild-type hamsters to 20 h in homozygous mutants. Here we show that homozygous tau-mutant hamsters in a 20-h light/dark cycle exhibit a 20-h oscillation in the rate of disk shedding, with peak phagosome numbers in the retinal pigmented epithelium occurring just after light onset. The numbers of phagosomes are significantly elevated from mid-dark levels prior to light onset, indicating that the disk shedding cycle anticipates dawn. Under conditions of constant darkness, the disk shedding rhythm in tau-mutant hamsters persists with a period of approximately 20 h. These results indicate that a rhythm of retinal photoreceptor outer segment disk shedding exists in the hamster eye, and that the period of this rhythm is shortened by the tau mutation.

Phase-locked eduction of vortex shedding in flow past an inclined flat plate

Flow past an inclined flat plate at an angle of attack of 30° and a Reynolds number of 30 000 is investigated experimentally. The velocity field in the wake is measured with a laser doppler anemometer (LDA) in the region from one plate breadth downstream to three and a half-plate breadths downstream. Controlled forcing is applied to the wake by vibrating the plate in the across-wind direction at a frequency in the middle of the lock-in range. The forcing serves to enhance the regularity and two-dimensionality of vortex shedding from the plate. It also facilitates phase-locked averaging of the LDA data. The LDA bursts are sorted according to their arrival instants relative to a particular phase of the vortex shedding cycles. The phase-averaged velocity results reveal large-scale vortical structures in the wake. Dynamical properties of these structures such as coherent vorticity and Reynolds stress production are discussed. The wake is found to be strongly asymmetric. The flow dynamics in the wake are dominated by a train of counterclockwise vortices shed from the trailing edge of the plate. The development, shedding and subsequent convection of these vortices are studied by following the consecutive phases of the shedding cycle.

A model to determine sampling strategies and milk inoculum volume for detection of intramammary Staphylococcus aureus infections in dairy cattle by bacteriological culture

A model was developed to evaluate the effects that methods of obtaining milk samples and culture inoculum volumes had on the sensitivity of microbiological culture to detect Staphylococcus aureus intramammary infections (IMI). An assumption was made that milk from mammary quarters infected with S. aureus only contains bacteria intermittently. A modified sine wave function was used to model this intermittent shedding pattern. Specifications for the components of the shedding cycle used in this function were based on quantitative culture results from 54 experimentally infected S. aureus quarters, sampled daily for a period of 30–49 days. The components of the shedding cycle were length in days, peak number of CFU shed per milliliter of milk, and length of time in the cycle when no shedding occurred. These components were used to estimate the model's predicted distribution of S. aureus CFU ml −1 milk when individual quarter milk samples were cultured for S. aureus. The sensitivity of culture for several sampling methods was then calculated. The model predicted that culture of a single quarter milk sample had a sensitivity ranging from 60 to 87% for detection of S. aureus IMI depending on inoculum volume. Quarter milk samples taken on day 1 and repeated either on day 3 or day 4, and cultured separately using 0.1 ml of milk for culture inoculum, were predicted to have sensitivities of 90–95% and 94–99%, respectively. Other milk-sampling strategies examined included culture of a composite milk sample (equal-volume mixture of milk from four separate mammary quarters ) and pooled milk samples in which samples from different milkings (either quarter or composite samples) were mixed together and then cultured. The range of predicted sensitivities of these other sampling strategies was 30–97%. Factors having the greatest impact on the sensitivity of culture, in order of importance were: the type of milk sample, the volume of milk cultured, and the time interval between repeated milk sample collection strategies.

Bubble interaction in low-viscosity liquids

An experimental study investigated how freely rising ellipsoidal bubbles approach each other, make contact and coalesce or breakup. Pulsed planar swarms of 10–20 bubbles with Eötvös numbers from 6.0 to 27.5 were released simultaneously in aqueous solutions of 0–48 wt% sugar with Morton numbers from 3.2 × 10 −11 to 3.7 × 10 −6. Bubble interaction was recorded by a video camera following the rising bubbles. Essentially, all coalescence and breakup events occurred after, not during, wake-induced collisions by a complex process related to the bubble vortex shedding cycle. This same process was also found in multi-bubble clusters and may account for excess turbulent kinetic energy generation in bubbly flow.

Male Reproductive Cycle, Age at Maturity, and Cost of Reproduction in the Timber Rattlesnake (Crotalus horridus)

The seasonal sexual cycle of the timber rattlesnake (Crotalus horridus) was studied in the northern part of its range (northeastern New York) to determine when males prepare morphologically for seasonal mating, and to estimate the age of maturity and cost of reproduction in males. Testis and kidney tissues from accidental mortalities obtained over a 14-year period were examined histologically and sem- iniferous tubular diameters, spermatogenic stage, and sexual segment of the kidney (SSK) diameters were quantified. Males reached sexual maturity in the fourth, fifth, or sixth calendar year of life; mean age of maturity was 5.3 yr (range 4-7 yr, N = 12). Maximum spermatogenesis and seminiferous tubular diameter were reached in July and continued through September. The SSK was hypertrophied throughout the active season. Thus, although mating potentially could begin early in the season, a lack of female reproductive pheromones may account for an absence of spring mating in this population. Crotalus horridus did not begin its annual shedding cycle until June, and females may not become vitellogenic and sexually attractive until mid-July when heterosexual pairs indicated the mating season began. Rattlesnakes were most frequently encountered by humans during the mating season. Sightings and road-killed males significantly outnumbered females (male/female encounter ratio 3.9:1.0) and males had a much higher human-caused mortality than females (male/female mortality ratio 13:1). Among males, mortality was significantly greater during the mating season than during the non-mating season (3.25:1.0). A mating system in pit vipers described previously as prolonged mate-searching polygyny is associated with a high cost of reproduction in males caused by their increased movements and exposure to predators during the late-summer mating season.

Electron microscopic analysis of the regenerating scales in lizard

Abstract Ultrastructural analysis of the regenerating tail epidermis of lizards belonging to the genera Lampropholis and Anolis has shown that the apical blastematic and non scaled superficial epidermis (wound epidermis) is made of thin keratinized cells forming mesos and an α‐layers. Lipids and keratin (α, while the presence of the β type is uncertain) are deposited among and into these superficial cells. In more proximal regions, the interscale epidermis is superficially made of mesos and α‐cells and under them spinosus‐like cells containing α keratin are seen. Electronpale cells containing keratoialin‐like granules form a clear layer and under them a new Oberhautchen and β‐layers differentiate down along the forming outer surface of the regenerating scales. Regenerating scales derive from epidermal in‐foldings and their outer surface grows at faster rate than the inner surface. The outer surface produces typical epidermal generations toward the centre of the scale. Oberhautchen, ß, mesos, and α layers are produced like in the shedding cycle of the normal epidermis. Autoradiographical observation after 3H‐thymidine administration have shown that epidermal cells take about six days to migrate from the basal layer to the keratinized superficial layers (β, mesos or α). The inner surface and the hinge region doe not produce any β‐keratin layers but are covered by a mesos‐layer. In this way the regenerated scales are coated by a mesos layer that prevents water loss. The mesos layer covering the inner surface detaches from the Oberhautchen of the outer surface, allowing the emergence of the regenerated scale. In the keratinized scales of Lampropholis, after 30–40 days of tail regeneration, neither lacunar nor clear layers were seen under the differentiating α‐layer of the regenerated outer surface. Therefore, when the wound epidermis is shed, it leaves underneath an incomplete new epidermal outer generation to cover the regenerated tail.

The hair follicle density and seasonal shedding cycle of the muskox (Ovibos moschatus)

Descriptions of muskox skin are limited despite the fact that its fine undercoat, commonly known as qiviut, is in increasing commercial use. Accordingly, skin biopsies from two bulls, two castrates, and one nonpregnant female were examined histologically at approximately 1-month intervals between 2.2 and 3.2 years of age. The biopsy sites were all 3 cm from the midline: one was in the midcervical region (the mane), another in the middle of the back (the saddle), and the third midway between the tuber coxae and the tuber ischii (the rump). Following surgical preparation, the biopsies were taken under local analgesia using a sharpened 4-mm stainless steel tube mounted in an electric drill. In addition, miscellaneous skin samples were collected from other sources. A regular pattern of follicles similar to that seen in the domestic sheep was hard to discern though primary follicles were readily identifiable by their size and their association with the duct of a sweat gland. For the three skin areas combined, the ratio of secondary to primary follicles (s/p ratio ± SEM) was 37.3 ± 1.8: 1, about twice that seen in the most improved breeds of sheep. The s/p ratio was slightly less on the saddle than on the neck and rump (P < 0.05). The mean follicle density was 42.8 ± 1.4/mm2, which is similar to that seen in the more highly selected sheep. The density on the neck was a little less than that in the saddle and rump (P < 0.05). Sebaceous glands and apocrine sweat glands are associated with primary follicles. Sebaceous glands are associated with some secondary follicles but sweat glands are not. About 100 sweat glands/cm2 were present.

Scanning electron microscopy of changes in epidermal structure occurring during the shedding cycle in squamate reptiles.

Previous studies of squamate epidermal structure have focused on either histology and ultrastructure or oberhautchen surface texture as revealed by scanning electron microscopy (SEM). Using SEM data drawn from a variety of lizard taxa (primarily iguanids, but also agamids, chamaeleonids, and scincids), as well as amphisbaenians and colubrid snakes, we relate the surfaces encountered in gross dissection of squamate skin to histologically identifiable layers, and characterize their surface structure. Only the oberhautchen bears the repeating pattern of ornamentation noted by previous authors. Because the clear layer is a perfect template of the oberhautchen surface, it is the only layer with which the oberhautchen might be confused. However, the clear layer can be identified by its tendency to curl and crack during preparation. All other surfaces encountered were relatively featureless, except for impressions left by dermal "papillae" associated with mechanoreceptors. Using a method for examining preserved specimens to determine the stage in the shedding cycle, we assess two sources of variation in epidermal surface structure: stage in the shedding cycle and wear. Examination of immature renewal-phase epidermis suggests that the oberhautchen does not mature synchronously across a single scale or across body regions. Comparing inner- and outer-generation oberhautchen in sheddingphase epidermis, we conclude that changes in surface appearance caused by natural wear fall into two categories: discrete scratches and accumulation of debris. We see no evidence of overall "buffing" on a microscopic level, though surface structure may be obscured by scratches and gouges. Many squamate taxa show a gradient from low relief surface structure on elevated regions such as keels to high relief patterns at scale edges. This gradient is not due to wear; its significance is unknown.

An Investigation of the Wake Structures of a Circular Cylinder Using a Computer Aided Flow Visualization : 1st Report, Generation and Dissipation of the Vorticities

Wake structures behind a circular cylinder at Reynolds number of 3.7 × 104 are investigated using a computer aided flow visualization technique, which includes our newly developed conditional sampling procedure. Phase-averaged velocity distributions in the boundary layer on a cylinder are measured and the process of generation and shedding of vorticity are presented. Total circulation discharged from the cylinder surface in a shedding cycle is directly evaluated. Phase-averaged stream-lines and vorticity fields in the wake are obtained, and using these results the coherent structures of the wake flow, i.e., geometrical arrangement, convection speed, a total circulation of a shedding vortex and inner structures of vortices are described. Less than 40% of the discharged vorticity appears in a shedding vortex, and a vorticity distribution in the vortex in the near wake is quite similar to that in an isolated theoretical viscous vortex. But the spacing ratio, convection speed and the drag coefficient obtained do not agree with those calculated from a classical theory.

The influence of light on cone disk shedding in the lizard, Sceloporus occidentalis.

The lizard, Sceloporus occidentalis has an all-cone retina. In lizards maintained on a 12-h light:12-h dark (12L:12D) cycle, a burst of cone outer segment (COS) shedding occurs 2 h after light offset (1400 h circadian time) (Young, R.W., 1977, J. Ultrastruct. Res. 61:172-72). In this investigation, we studied the effect of different lighting regimes on the pattern of cone disk shedding in this species. When lizards entrained to a 12L:12D cycle are kept in constant darkness (DD), the shedding peak is advanced approximately 2 h and the magnitude of shedding is reduced to 30% of control. COS increased in mean length from 12 micron in controls to 14 micron after one cycle in DD and maintained this length during a second cycle in DD. In constant light (LL), disk shedding was damped to approximately 10% of control values. Shedding synchrony in LL was also perturbed and therefore cyclic shedding bursts could not be distinguished. During LL there was a much larger increase in COS mean length than in DD. After one cycle of LL, COS length was 15 micron and after two cycles COS length exceeded 17 micron. When lizards entrained to 12L:12D are shifted to a 6L:18D regimen, the first shedding cycle is biphasic. The first peak of 5% shedding occurs 2 h after light offset whereas a second larger peak (13%) occurs according to the entrained schedule (1400 h). This manipulation separates out a dark-triggered and circadian shedding component, which is normally superimposed in lizards entrained to a 12L:12D cycle. When entrained lizards are placed in 36 h of LL followed by light offset, the peak shedding response after light offset is double the control response (53% vs. 27%). After 30 h of LL (lights off 90 degrees out of phase), there is a biphasic shedding response similar to the 6L:18D regimen although this time the dark-triggered shedding component is greater in magnitude then the circadian component. COS turnover is estimated by extrapolating from COS mean length increases during LL. From this method we obtained a 2.7-micron increase in COS length during each day in LL. If COS growth is not augmented during LL, this would yield a 4-5-d turnover time for the average 12.5-micron COS.