Elastic modulus of tree frog adhesive toe pads

Previous work using an atomic force microscope in nanoindenter mode indicated that the outer, 10- to 15-μm thick, keratinised layer of tree frog toe pads has a modulus of elasticity equivalent to silicone rubber (5–15 MPa) (Scholz et al. 2009), but gave no information on the physical properties of deeper structures. In this study, micro-indentation is used to measure the stiffness of whole toe pads of the tree frog, Litoria caerulea. We show here that tree frog toe pads are amongst the softest of biological structures (effective elastic modulus 4–25 kPa), and that they exhibit a gradient of stiffness, being stiffest on the outside. This stiffness gradient results from the presence of a dense network of capillaries lying beneath the pad epidermis, which probably has a shock absorbing function. Additionally, we compare the physical properties (elastic modulus, work of adhesion, pull-off force) of the toe pads of immature and adult frogs

Surface ciliation and tail structure in direct-developing frog embryos: a comparison between Myobatrachus gouldii and Pristimantis (= Eleutherodactylus) urichi

Surface ciliation in two direct-developing anurans from unrelated lineages, the Australian myobatrachid Myobatrachus gouldii and the South American terraranan Pristimantis urichi, is shown to be broadly similar, persisting on some body regions until close to hatching, suggesting a common need for circulation of fluid inside the jelly layers. The tail of M. gouldii is tadpole-like at its maximum extent though considerably reduced in its axial core and musculature. Its surface epidermis is thin and highly folded in some areas, with blood vessels approaching very close to the surface, consistent with a respiratory role. The tail moves actively when well developed, which may assist with respiratory exchange. The tail in P. urichi has a novel construction, quite different from both M. gouldii and that reported for Caribbean lineage terraranans such as Eleutherodactylus coqui or E. nubicola. In P. urichi, the tail expands laterally and posteriorly, not dorsally and ventrally, and only has a short axial core at its base, suggesting very limited motility: it therefore seems not to be composed of axial core and dorsal/ventral fins. We suggest that this thin-walled vascular structure, applied close to the perivitelline membrane, facilitates respiratory exchange. Discovery of this novel structure suggests that the development of other terraranan embryos needs investigation

Evolutionary analysis of genes involved in cement gland development in anurans

The cement gland (CG) is a transient organ, found in most anuran embryos and early larvae and located normally on the front of the head. Its sticky secretion allows newly hatched larvae to attach to the egg jelly or to another support later and remain hidden and stationary until feeding starts. Our ultrastructural studies showed that prominence structure of the CG in some species exists, but is lacking in some others. Previous work has shown that a large number of genes have a role in CG development in Xenopus laevis. The aim of the present study is to find out whether the loss of cement gland formation for those species studied here occurs because of missing genes or for other reasons. In order to test whether some of these genes are present in other anuran species, especially in those where the CG does not form, genomic DNAs were examined for sequence similarity by low stringency hybridization. Sequences from three different genes with a role in controlling CG development in Xenopus (otx-2, xcg-1 and xag-1) were individually hybridised with genomic DNA of four species of anurans (X. laevies, Leptodactylus fuscus, Phyllomedusa trinitatis and Physalaemus pustulosus) and one species of rodent (Muss musculus domesticus strain C57Bl/ 6). The results showed that Xenopus probes can detect the presence of potential homologues of all three genes in the different species. For the two genes most specifically involved in CG development, xcg-1 and xag-1, both are clearly present, even in the two species which lack CG development, though in one of these, P. trinitatis, xag-1 shows considerable difference from the other species. At this stage, we can conclude that the missing cement gland for those species studied here is not due to a lack of the genes responsible for the gland development. In order to carry this work further, in situ hybridisation should be used to determine the actual expression patterns of these genes

External gill motility and striated muscle presence in the embryos of anuran amphibians

Anuran external gills were assessed for motility and striated muscle content in 16 species from seven families. Motility of three kinds was observed. Pulsatory movements related to heart beat rhythm were common. In embryos developing to a late stage before hatching, movements of the whole embryo were frequent, with gills rearranging as a consequence. The only clearly active movement, presumably muscle driven, was ‘gill flicking’, a posterior movement of the entire gill into the body either on one side only, or both together, followed by a return to the normal spread-out position. Some species may actively spread their gills when hanging from the water surface film, but we did not observe this. In some species, active gill movement developed over time, but we were not able to follow all species over such a developmental sequence. The relationship between active motility and muscle content was good in most cases. Observations on late stage embryos of the aromobatid Mannophryne trinitatis are presented for the first time. In one species, we noted spread external gills being used to adhere hatchlings to a surface

Evaluation of the sparse reconstruction and the delay-and-sum damage imaging methods for structural health monitoring under different environmental and operational conditions

In this paper, the performance of the sparse reconstruction (SR) and the delay-and-sun (DAS) methods for damage localization, were evaluated for various environmental and operational conditions, both numerically and experimentally. To assess these damage localization methods, a methodology based on the Taguchi method was used to make the experimental design, and a modified performance-index was defined to represent the quality of reconstructed images. Then, the robustness and the accuracy of each method, in a well-defined performance region relevant to in-service aerospace structures, were investigated using the Taguchi and analysis of variance methods. It was concluded that for the defined conditions, the robustness of the delay and sum method is better than the sparse reconstruction method for uncontrolled factors. However, the sparse reconstruction method is more robust to poor baseline subtraction than the delay and sum method, while the delay and sum method was more robust to factors that lead to a model mismatch. These results provide additional insight into the design of reliable accurate structural health monitoring systems. The outcomes of this work can be used in future reaserch into SHM imaging techniques.</p