Thin Basal Layer Research Articles

Validation of a two-layer depth-averaged model by comparison with an experimental dilute stratified pyroclastic density current

Numerical results of a two-layer depth-averaged model of pyroclastic density currents (PDCs) were compared with an experimental PDC generated at the international eruption simulator facility (the Pyroclastic flow Eruption Large-scale Experiment (PELE)) to establish a minimal dynamical model of PDCs with stratification of particle concentrations. In the present two-layer model, the stratification in PDCs is modeled as a voluminous suspended-load layer with low particle volume fractions (lesssim {10}^{-3}) and a thin basal bed-load layer with higher particle volume fractions (sim {10}^{-2}) on the basis of the source condition in the experiment. Numerical results for the suspended load quantitatively reproduce the time evolutions of the front position and flow thickness in the experimental PDC. The numerical results of the bed-load and deposit thicknesses depend on an assumed value of settling speed at the bottom of the bed load ({W}_{mathrm{sH}}). We show that the thicknesses of bed load and deposit in the simulations agree well with the experimental data, when {W}_{mathrm{sH}} is set to about 1.25times {10}^{-2} m/s. This value of the settling speed is two orders of magnitude smaller than that predicted by a hindered-settling model. The small value of {W}_{mathrm{sH}} is considered to result from decreasing in the effective deposition speed due to the erosion process accompanied by saltating/rolling of particles at the bottom of the bed load.

Nature and origin of large-scale and intrasalt deformation within the Wieliczka salt mine, Poland

The Wieliczka salt mine, near Kraków in southern Poland, is a world-famous historical and modern destination for both geoscientists and tourists. Despite numerous publications, there is still a lack of consensus on the nature and origin of the large-scale folds as well as whether the small-scale structures represent tectonic or soft-sediment deformation. In this preliminary work, we offer new ideas on both aspects. At the large scale, we emphasize the mechanical stratigraphy of the layered evaporite sequence, which comprises a thin basal weak layer, a thin strong unit, a thicker weak layer, and a thicker strong unit. We suggest that the inclined to recumbent folds and thrusts formed tectonically due to overthrust shear between the basal detachment and the overriding Carpathian frontal thrust, with different structural styles decoupled by the thick weak layer. At the small scale, we suggest that there was early extension directed toward the east to north-east, followed by contraction vergent toward the north. We infer that there was early, syndepositional gravity gliding down a topographic slope dipping into the coeval Gdów depocenter to the east, and that subsequent north-directed Carpathian shortening generated the small-scale contractional structures as the larger folds were developing

Sedimentary and volcanic record of the nascent Izu-Bonin-Mariana arc from IODP Site U1438

Abstract The oldest known, intact sedimentary record of a nascent intraoceanic arc was recovered in a ∼100-m-thick unit (IV) above ca. 49 Ma basaltic basement at International Ocean Discovery Program Site U1438 in the Amami Sankaku Basin. During deposition of Unit IV the site was located ∼250 km from the plate edge, where Izu-Bonin-Mariana subduction initiated at 52 Ma. Basement basalts are overlain by a mudstone-dominated subunit (IVC) with a thin basal layer of dark brown metalliferous mudstone followed by mudstone with sparse, graded laminae of amphibole- and biotite-bearing tuffaceous sandstone and siltstone. Amphibole and zircon ages from these laminae suggest that the intermediate subduction-related magmatism that sourced them initiated at ca. 47 Ma soon after basement formation. Overlying volcaniclastic, sandy, gravity-flow deposits (subunit IVB) have a different provenance; shallow water fauna and tachylitic glass fragments indicate a source volcanic edifice that rose above the carbonate compensation depth and may have been emergent. Basaltic andesite intervals in upper subunit IVB have textures suggesting emplacement as intrusions into unconsolidated sediment on a volcanic center with geochemical and petrological characteristics of mafic, differentiated island arc magmatism. Distinctive Hf-Nd isotope characteristics similar to the least-radiogenic Izu-Bonin-Mariana boninites support a relatively old age for the basaltic andesites similar to detrital amphibole dated at 47 Ma. The absence of boninites at that time may have resulted from the position of Site U1438 at a greater distance from the plate edge. The upper interval of mudstone with tuffaceous beds (subunit IVA) progresses upsection into Unit III, part of a wedge of sediment fed by growing arc-axis volcanoes to the east. At Site U1438, in what was to become a reararc position, the succession of early extensional basaltic magmatism associated with spontaneous subduction initiation is followed by a rapid transition into potentially widespread subduction-related magmatism and sedimentation prior to the onset of focused magmatism and major arc building.

Development of contrasting folding styles in the western Yangtze block, South China: Insights from numerical modeling

Two contrasting folding styles are well developed along both sides of the Qiyueshan fault, South China. The western part is characterized by chevron anticlines and box synclines in the Eastern Sichuan domain, while the eastern part features typical chevron synclines and box anticlines in the western Hunan-Hubei domain. These contrasting structures are important in better understanding the intracontinental orogeny around the eastern Sichuan Basin. In this study, we conducted a series of 2-D numerical simulations using the finite difference method. The model results indicate that the key parameters include stratigraphic competence, number and distribution of intermediate weak layers, and thickness of basal weak layer. Among them, the thickness of basal weak layer plays the most important role in shaping the two distinct structural styles. In the models with thin basal weak layer (<1 km), high-quality chevron anticlines are developed with the presence of multiple intermediate weak layers. These folds are mainly characterized by fault-propagation folds or fault-bend folds. The basal weak layer corresponds to the relatively thin Cambrian evaporite layer in the Eastern Sichuan domain. In contrast, in the models with thick basal weak layer (>8 km), a series of coherent chevron synclines are developed, and mainly form décollement folds. These characteristics are similar to the structural styles revealed by seismic profiles in the western Hunan-Hubei domain. This weak layer may represent the accumulation of mechanically weak rocks (e.g., slate, schist and phyllite) in Neoproterozoic Lengjiaxi and Tieshajie Groups in the western Hunan-Hubei domain, which is also consistent with the geophysical observations (e.g., weak seismic reflection, high electrical conductivity, and Bouguer gravity low anomaly). Both the differential thickness of basal weak layer along the “flat-ramp-flat” décollement and the overlying multiple intermediate weak layers contribute to the formation of the contrasting folding styles around the eastern Sichuan Basin.

Seismic characterization of a rapidly-rising jökulhlaup cycle at the A.P. Olsen Ice Cap, NE-Greenland

AbstractRapidly-rising jökulhlaups, or glacial outburst floods, are a phenomenon with a high potential for damage. The initiation and propagation processes of a rapidly-rising jökulhlaup are still not fully understood. Seismic monitoring can contribute to an improved process understanding, but comprehensive long-term seismic monitoring campaigns capturing the dynamics of a rapidly-rising jökulhlaup have not been reported so far. To fill this gap, we installed a seismic network at the marginal, ice-dammed lake of the A.P. Olsen Ice Cap (APO) in NE-Greenland. Episodic outbursts from the lake cause flood waves in the Zackenberg river, characterized by a rapid discharge increase within a few hours. Our 6 months long seismic dataset comprises the whole fill-and-drain cycle of the ice-dammed lake in 2012 and includes one of the most destructive floods recorded so far for the Zackenberg river. Seismic event detection and localization reveals abundant surface crevassing and correlates with changes of the river discharge. Seismic interferometry suggests the existence of a thin basal sedimentary layer. We show that the ballistic part of the first surface waves can potentially be used to infer medium changes in both the ice body and the basal layer. Interpretation of time-lapse interferograms is challenged by a varying ambient noise source distribution.

Rare Moss-Built Microterraces in a High-Altitude, Acid Mine Drainage-Polluted Stream (Cordillera Negra, Peru).

The Rio Santiago in the Cordillera Negra of Peru is severely contaminated by acid mine drainage in its headwaters. In a strongly acid stream, at about 3800 m above sea level (masl), microterraces were found with terrace walls built up of dead moss, with encrustations and interstitial fine, creamy sediment. The stream water was turbid due to the presence of similar suspended sediment, which also occurred as a thin basal layer in inter-rim basins. The moss was identified as the rare bryophyte Anomobryum prostratum (Müll. Hal.) Besch. Chemical and mineralogical analyses show that green, living parts of the moss are gradually coated by Al/Fe (hydr)oxides, inducing their senescence and death. The necromass is covered by creamy crusts through precipitation of schwertmannite-type material from the stream water and simultaneous ‘capture’ of fine sediment. The latter consists of a mixture of precipitate and fine detrital primary minerals. These processes are held responsible for the formation of the microterraces, which regarding their composition and environment seem to be unique. Remarkable is the high As content of the creamy crusts and sediment, attributed to strong sorption of As, whereas its solute concentration is relatively low. This calls for more attention to suspended fine sediment in the assessment of environmental risks of stream water use. Lastly, the results raise serious doubts about the use of aquatic bryophytes as bioindicator for chemical pollution in acid mine drainage-polluted streams.Electronic supplementary materialThe online version of this article (doi:10.1007/s11270-015-2390-x) contains supplementary material, which is available to authorized users.

Seismic imaging of crust beneath the Dharwar Craton, India, from ambient noise and teleseismic receiver function modelling

We use cross-correlation of continuous 18 months (2009 February to 2010 August) ambient noise data recorded over 35 broad-band seismographs in the Archean Dharwar Craton and the adjoining granulite terrain to generate Rayleigh-wave group velocity maps in the period 5–28 s. This is supplemented with longer period data (40–70 s) from earthquake source. Combined group velocity measurement was inverted jointly with the teleseismic receiver functions obtained at 50 stations (includes 15 stations operated during 1998–2002) to produce shear velocity image of the crust. The velocity image reveals thinner crust (34–38 km) in the late Archean (∼2.7 Ga) Eastern Dharwar Craton (EDC), while all other terrains (mid-Archean and Proterozoic) have crustal thickness from 40 to over 50 km. The mid-Archean (3.36 Ga) greenstone belt of the Western Dharwar Craton (WDC) has the thickest crust (∼50 km). The average crustal Vs beneath the EDC is ∼3.70–3.78 km s−1 as compared to 3.80–3.95 km s−1 beneath the WDC. We observe significant lateral variation in the thickness of lower crust (Vs ∼ 3.8–4.2 km s−1): ∼10–15 km in the EDC compared to ∼20–30 km in the WDC. The lowermost part of the crust (Vs ≥ 4.0 km s−1) is thin (<5 km) beneath the EDC in contrast to more thickness (10–27 km) beneath the WDC. Our analysis suggests intermediate composition for the crust beneath the EDC similar to those for other cratons. In contrast, the mid-Archean exposed WDC crust has more mafic composition and exceptional thickness—a scenario at variance with the global observations. We interpret this thick mafic crust to represent undeformed geological segment of 3.36 Ga. The EDC with a nearly flat Moho, felsic to intermediate composition of crust and thin basal layer may represent a reworked terrain during the late-Archean.

Quantifying the basal conditions of a mountain glacier using a targeted full-waveform inversion: Bench Glacier, Alaska, USA

AbstractGlacier dynamics are inextricably linked to the basal conditions of glaciers. Seismic reflection methods can image the glacier bed under certain conditions. However, where a seismically thin layer of material is present at the bed, traditional analyses may fail to fully characterize bed properties. We use a targeted full-waveform inversion algorithm to quantify the basal-layer parameters of a mountain glacier: thickness (d), P-wave velocity (α) and density (ρ). We simultaneously invert for the seismic quality factor (Q) of the bulk glacier ice. The inversion seeks to minimize the difference between the data and a one-dimensional reflectivity algorithm using a gradient-based search with starting values initialized from a Monte Carlo scheme. We test the inversion algorithm on four basal layer synthetic data traces with 5% added Gaussian noise. The inversion retrieved thin-layer parameters within 10% of synthetic test parameters with the exception of seismic Q. For the seismic dataset from Bench Glacier, Alaska, USA, inversion results indicate a thin basal layer of debris-rich ice within the study area having mean velocity 4000 ± 700 m s–1, density 1900 ± 200 kg m–3 and thickness 6 ± 1.5 m.

Thaumatoporella ladders unraveled

Thaumatoporella (incertae sedis) represents a widespread constituent in Upper Triassic-Cretaceous shal- low-marine carbonates of the Tethyan realm and exhibits a wide morphological variability. Seemingly monostro- matic cell-layers intermingled with and incorporated into crusts (Bacinella irregularis) were described as Thaumatoporella bridges or in the literature. This appearance of Thaumatoporella also accounted for the creation of the genus Polygonella ELLIOTT and its misleading comparison with single-layered crustose coral- linacean alga, e.g., genus Lithoporella. In fact, these ladders belong to the roof part of thaumatoporellaceans en- crusting microbial crusts that subsequently became overgrown and thus incorporated. Often a rudimentary thin basal layer and an elongated sparitic void corresponding to the former body of Thaumatoporella, are present evidencing that its morphology is that of a variously shaped organism (free or attaching) enclosed by an all-round wall. Irregular lobate and thin-walled thaumatoporellacens occurring in bacinellid fabrics might be interpreted as crust-dwellers.

Cauterization of the germinal nail matrix using phenol applications of differing durations: A histologic study

Phenol has been used for more than 60 years in the treatment of ingrown toenails. Many reports have documented the safety and effectiveness of phenol matricectomy, although with variable treatment application times. We performed research to determine the application time required for phenol matricectomy for complete denaturation of the nail matrix to occur at a concentration of 88%. Identifying the optimal time required to destroy the germinal matrix without causing further soft-tissue damage should reduce the potential for complications. Thirty anatomic cadaveric fresh hallux samples were used to perform phenol matricectomy. The applications were for 1 to 6 minutes using 10 samples per time point and analyzed by hematoxylin-eosin staining to determine the presence or absence of the basal or germinal layer of the nail bed epithelium (NBE). After a 1-minute application of 88% phenol solution, only superficial damage to the NBE was noted, with the basal layer primarily intact. After a 2-minute application, the nail plate was avulsed with a thin basal layer remaining. After a 3-minute application, full-thickness necrosis of the NBE was noted in 6 of the 10 specimens. After 4-, 5-, and 6-minute applications, full-thickness necrosis of the NBE was noted and the basal layer was completely destroyed in all 30 specimens. The study was performed in cadaveric fresh specimens. Application of 88% phenol solution for at least 4 minutes is necessary for complete destruction of the nail matrix, creating an environment that avoids nail regrowth.

Evidence for Near-Horizontal Tensile Faulting at the Base of Gornergletscher, a Swiss Alpine Glacier

Abstract Using 3D Green’s functions we determine full and constrained moment tensor solutions of icequakes near the base of Gornergletscher, Switzerland. The seismic events were recorded in the summer of 2004 using a high-density seismometer array. The seismic velocity model used in the generation of Green’s functions is based on radio-echo soundings to approximate the basal topography, which beneath the study site exhibits a strong inclination. As the basal conditions are not well known, we try moment tensor inversions with seismic velocity profiles consisting of two and three media. The former case consists of homogeneous ice resting on bedrock, whereas the latter case includes a thin basal layer with slow seismic velocities representing eroded material or highly fractured ice. Effects of errors in Green’s functions are estimated by sensitivity studies in which we invert 1D and 3D synthetics using Green’s functions of wrong velocity models. The results show that calculations of source types and fault plane orientations of tensile cracks are rather robust with respect to errors in Green’s functions. However, the quality of the waveform fits depends on strike and dip of the synthetic source. When inverting seismograms, Green’s functions of the seismic model that includes the basal slow velocity layer are found to give the most realistic source types as well as the best waveform fits. The fault mechanisms derived from constrained moment tensor inversions are near-horizontal tensile cracks, which suggest a complex time-dependent basal stress field.

Influence of abrasion on biofilm detachment: evidence for stratification of the biofilm

The objective of this paper was to understand the detachment of multispecies biofilm caused by abrasion. By submitting a biofilm to different abrasion strengths (collision of particles), stratification of biofilm cohesion could be highlighted and related to stratification of biofilm bacterial communities using the PCR-SSCP fingerprint method. The biofilm comprised a thick top layer, weakly cohesive and composed of one dominant species, and a thin basal layer, strongly cohesive and composed of a more diverse population. These observations suggest that microbial composition of biofilms may be an important parameter in understanding biofilm detachment.

Community structure and physiological characterization of microbial mats in Byers Peninsula, Livingston Island (South Shetland Islands, Antarctica)

The community structure and physiological characteristics of three microbial mat communities in Byers Peninsula (Livingston Island, South Shetland Islands, Antarctica) were compared. One of the mats was located at the edge of a stream and was dominated by diatoms (with a thin basal layer of oscillatorian cyanobacteria), whereas the other two mats, located over moist soil and the bottom of a pond, respectively, were dominated by cyanobacteria throughout their vertical profiles. The predominant xanthophyll was fucoxanthin in the stream mat and myxoxanthophyll in the cyanobacteria-dominated mats. The sheath pigment scytonemin was absent in the stream mat but present in the soil and pond mats. The stream mat showed significantly lower delta13C and higher delta15N values than the other two mats. Consistent with the delta15N values, N2 fixation was negligible in the stream mat. The soil mat was the physiologically most active community. It showed rates of photosynthesis three times higher than in the other mats, and had the highest rates of ammonium uptake, nitrate uptake and N2 fixation. These observations underscore the taxonomic and physiological diversity of microbial mat communities in the maritime Antarctic region.

Comparative morphology of the egg of the castniid palm borer, Paysandisia archon (Burmeister, 1880) (Lepidoptera: Castniidae)

Paysandisia archon (Burmeister, 1880) is an attractive neotropical castniid moth whose presence in Europe was recently reported. Its larvae are endophagous, feeding inside the trunks and branches of several species of palm trees (Arecaceae). The present paper deals with the morphology and biometry of the egg of this moth, comparing them with those of other castniid species. The egg is a typical castniid egg, fusiform, upright sensu Döring, light cream or creamy pink when freshly laid, 4.69 ± 0.37 mm long and 1.56 ± 0.11 mm wide. Larvae emerge by gently splitting the chorion along one of the longitudinal ridges, on the half closer to the micropyle. SEM, TEM and LSCM photographs showing ultrastructural details of the egg are shown for the first time. The micropylar rosette (c. 54 μm in diameter) has generally 14–17 cells; in its centre lies the micropylar pit (c. 6 μm in diameter) which bears 12–16 micropylar canal openings (= micropyles) around its periphery. The pathways followed by those canals through the chorion have been figured. Eggs sampled in the wild (so laid by several females) were found to have a slightly variable number of ridges: most bore seven ridges (68.87%), although a significant portion (30.46%) bore eight and 1 egg (0.67%) bore only six; this against the currently accepted rule of five‐ridged eggs for Castniini (i.e. Neotropical castniids) to which Paysandisia archon belongs. It has also been found that the same female specimen has the capability of producing six‐, seven‐ or eight‐ridged eggs. Five types of egg irregularities affecting the longitudinal ridges are also figured and described. Transverse striae on the egg of P. archon are about 122. Aeropyles (c. 4 μm in diameter) occur on the ridges, at the intersections between the latter and two contiguous (left and right of the ridge) transverse striae, amounting to c. 854 on a seven‐ridged egg and to c. 976 on an eight‐ridged egg. Occasionally minute aeropyles ('microaeropyles') (c. 1.96–3.13 μm in diameter) also occur on transverse striae located close to both egg poles. The chorion of P. archon shows the typical ditrysian fine structure with very thin basal layer (C‐1), 0.3–0.2 μm thick, gas‐filled trabecular layer (C‐2), c. 0.9 μm thick, and lamellar layer (C‐3), its thickness varying between 18.5 and 13 μm due to the bumpy external surface of the chorion. Aeropylar canals, that penetrate layer C‐3, connect the air‐containing inner chorionic meshwork (the trabecular layer C‐2) with the surrounding air; their outer part forms a big bulbous cavity (which opens to the outside through the small opening seen in external SEM images) and, underneath, a narrow canal follows, leading into the trabecular layer (C‐2).

Model for dense granular flows down bumpy inclines

We consider dense flows of spherical grains down an inclined plane on which spherical bumps have been affixed. We propose a theory that models stresses as the superposition of a rate-dependent contribution arising from collisional interactions and a rate-independent part related to enduring frictional contacts among the grains. We show that dense flows consist of three regions. The first is a thin basal layer where grains progressively gain fluctuation energy with increasing distance from the bottom boundary. The second is a core region where the solid volume fraction is constant and the production and dissipation of fluctuation energy are nearly balanced. The last is a thin collisional surface layer where the volume fraction abruptly vanishes as the free surface is approached. We also distinguish basal flows with the smallest possible height, in which the core and surface layers have disappeared. We derive simple closures of the governing equations for the three regions with insight from the numerical simulations of Silbert et al. [Phys. Rev. E64, 051302 (2001)] and the physical experiments of Pouliquen [Phys. Fluids 11, 542 (1999)]. The theory captures the range of inclination angles at which steady, fully developed flows are observed, the corresponding shape of the mean and fluctuation velocity profiles, the dependence of the flow rate on inclination, flow height, interparticle friction, and normal restitution coefficient, and the dependence of the height of basal flows on inclination.

Temporal development of prograding beach–shoreface deposits: the Holocene of Kujukuri coastal plain, eastern Japan

Prograding beach–foreshore deposits with a basal ravinement bed are recognized from detailed AMS 14C dating and grain size analyses of Holocene deposits of the Kujukuri coastal plain facing the Pacific coast of Japan. The Holocene deposits are about 20 m thick and consist of an upward-shoaling beach–foreshore succession that has a basal unconformity overlain by a thin layer of relatively coarse sand. The unconformity is a downlap surface, which originated from a ravinement surface. The overall succession was formed by a prograding beach–foreshore system during the highstand in sea level of the last 6000 years. Poorly preserved fossil shells, enclosed in the thin layer of coarse sand at the base of the succession, date from the transgressive stage in sea level before 6000 cal yr BP. Grain size distribution patterns are polymodal in the basal sand layer and unimodal in the remainder of the succession. Modal classes of the basal part are fine (3.3–2.6φ), medium (2.3–1.8φ), and coarse (1.2–1.0φ). The fine mode and the unimodal grain sizes define an overall upward-coarsening trend, which correlates with the upward-shoaling succession. In contrast, the medium and coarse modes are restricted to the basal sand, and are interpreted as related to the ravinement process. The thin basal sand layer, an admixture of these modal classes, is interpreted as a type of ravinement deposit preserved during sea-level highstand. No transgressive deposit was preserved on the ravinement surface because sediment supply was so small that the sediment on this surface experienced thorough reworking by storm waves.

The thermochemical structure and evolution of Earth's mantle: constraints and numerical models.

Geochemical observations place several constraints on geophysical processes in the mantle, including a requirement to maintain several distinct reservoirs. Geophysical constraints limit plausible physical locations of these reservoirs to a thin basal layer, isolated deep 'piles' of material under large-scale mantle upwellings, high-viscosity blobs/plums or thin strips throughout the mantle, or some combination of these. A numerical model capable of simulating the thermochemical evolution of the mantle is introduced. Preliminary simulations are more differentiated than Earth but display some of the proposed thermochemical processes, including the generation of a high-mu mantle reservoir by recycling of crust, and the generation of a high-(3)He/(4)He reservoir by recycling of residuum, although the resulting high-(3)He/(4)He material tends to aggregate near the top, where mid-ocean-ridge melting should sample it. If primitive material exists as a dense basal layer, it must be much denser than subducted crust in order to retain its primitive (e.g. high-(3)He) signature. Much progress is expected in the near future.

Influence of the glacier bed lithology on the formation of a subglacial till sequence — ring-shear experiments as a tool for the classification of subglacial tills

A glaciogenic sediment sequence deposited by a local glacier in the Rhine catchment area near Sargans (Eastern Swiss Alps) was investigated in order to examine the shear behaviour of two different tills. In addition to ring–shear experiments, examination of clast lithology, clay mineralogy and physical properties of all samples were determined. SEM–analyses of artificially produced deformation patterns in the till samples show only a very thin shear zones of 2–3 μm thickness, despite long shear distances of up to 145 mm. A clear relationship between local bedrock and the formation of a thin basal layer of heavily deformed till can be demonstrated. This locally derived till layer exhibits distinctly different shearing properties compared to the massive overlying till unit that is composed of more distantly transported lithic material from the upper catchment area of the glacier. The difference in the angle of internal friction between the two tills is >10° at residual shear strength, although the clay mineralogy of both tills is similar. It can further be demonstrated that after the deposition of a 10–15 cm thin till layer directly on bedrock, the flow dynamics of the glacier tongue may have changed due to differences in glacier bed debris lithology. Differences in the shear behaviour of the sediments, allied to microscopic analyses, point to an important environmental change from a deformation till to an overlying “non–deformed till”.

Mosquito embryos and eggs: polarity and terminology of chorionic layers

The development of genetically modified vectors refractory to parasites is seen as a promising strategy in the future control of endemic diseases such as malaria. Nevertheless, knowledge of mosquito embryogenesis, a pre-requisite to the establishment of transgenic individuals, has been presently neglected. We have here studied the eggs from two neotropical malaria vectors. Eggs from Anopheles ( Nyssorhynchus) albitarsis and Anopheles ( Nyssorhynchus) aquasalis were analyzed by laser scanning microscopy and scanning electron microscopy and compared to those of Drosophila melanogaster. We verified basic conflicting data such as mosquito egg polarity and ultrastructure of eggshell layers. A 180° rotation movement of the mosquito embryo along its longitudinal axis, a phenomenon not conserved among all Diptera, was confirmed. This early event is not taken into account by several present groups, leading to a non-consensual assignment of eggshell dorsal and ventral poles. Since embryo and egg polarities, defined during oogenesis, are the same, we propose to consider the flattened egg side as the dorsal one. The structure of Anopheles eggshell was also examined. Embryos are covered by a smooth endochorion or inner chorion layer. Outside this coat lies the compound exochorion or outer chorion layer, assembled by a thin basal lamellar layer and external tubercles. The terminology related to eggshell layers is discussed.

Leaf cuticle ultrastructure of two czekanowskialeans from the Middle Jurassic Yima Formation of Henan, China

Leaf cuticles of Phoenicopsis and Arctobaiera collected from the Middle Jurassic Yima Formation, Henan Province, Central China were studied with transmission electron microscopy. The upper and lower cuticles are generally similar in ultrastructure. The cuticle is usually composed of two parts. The upper part is finely granular. The lower part is mainly fibrillar. In the upper cuticle there is normally a thin basal granular layer. The structure appears to be close to Type 3 of Holloway, but it has an inner granular layer and the fibrillae are usually more parallel in orientation. This type of cuticle is also different from those of the other fossil plants. This is the first time the ultrastructure of leaf cuticles of the Czekanowskiales has been studied. It is of interest that the two taxa of the same plant group bear a similar cuticular ultrastructure.