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.Electronic supplementary materialThe online version of this article (doi:10.1007/s00359-011-0658-1) contains supplementary material, which is available to authorized users.
This account provides a detailed morphological and ultrastructural study of wing-locking mechanisms (LM) in some aquatic Heteroptera. Scanning and transmission electron microscopy were used to describe the functional significance of macro- and microstructures holding wings tightly against the body at rest and those involved in functional diptery in flight. There are two types of LM holding the forewings (hemelytra) at rest: 1) wing-to-wing LM, and 2) wing-to-body LM. The first type includes the brush-to-brush LM, the clavus-clavus clamp and the clavus-clavus locking ridge. The second type includes devices locking the hemelytra to the body: the subcostal border of the hemelytra to the lateral border of mesepimeron, the knob-and-socket locking mechanism of the hemelytra, and the clavus-locking mechanism to the scutellum groove. The hindwing is locked by a pair of microtrichial fields situated on the hindwing-articulated pad at the basal area of the hindwing and on the thoracic pad in the vicinity of the wing articulation. Morphological and ultrastructural data suggest that different LM are parts of one mechanism holding wings to the body at rest. An additional locking mechanism, connecting the hemelytra with the hindwing, is the only LM providing functional diptery in flight.
The structure and function of the hemelytra-to-hindwing locking mechanism of the bug Coreus marginatus were analysed. The system consists of a cuticular protrusion in the ventral side of the hemelytra, which locks the subcostal border of the hindwing in flight. The speed and distance slid by both surfaces against one another during flight were assessed using a combination of high-speed video recordings and a 2D geometrical model. The friction coefficient between sliding surfaces was assessed using a micromanipulator, coupled with force transducers. This was done under three experimental conditions: freshly dissected, air dried and rehydrated ethanol preserved samples. The results showed a high speed of sliding, approximately 0.18 m s(-1), with a relatively low friction coefficient (0.2 micro). There was no evident difference in the friction measured under the various treatments, with the exception of the rehydrated condition, which was lower. The surface morphology of the wing locking mechanism, namely outgrowths of one part having rounded edges, and completely flat surface on the counterpart, effectively aids in the reduction of friction at the microscopic level. The structure is effective even dry, and after being preserved in ethanol, suggesting that no cuticle secreted lubrication substance is responsible for its effectiveness. The ultrastructure presumably confers mechanical stability to the system under the high load it is subjected to in flight.
We examined the morphology of setae and microtrichia in Aquarius paludum during larval development using a scanning electron microscope. We then conducted immersion experiments with larvae and adults in oxygenated and deoxygenated water. The adult water strider body is covered by a pilose double layer consisting of upper long setae (30-80 microm) and lower filiform microtrichia (5-9 microm). Only setae are present on the legs. Microtrichia on the larval body are very short: 0.5-0.6 microm in first and second instars, and 0.8-1.7 microm in third to fifth instars. Larval body setae are approximately as long as those of adults (25-50 microm), but are much less dense at 1,800-5,750 setae per mm(2) versus 15,000-20,000 setae per mm(2) in adults. The density of setae on the legs remains relatively constant throughout development (larvae: 15,000-20,000 setae per mm(2); adults: 20,000-26,000 setae per mm(2)). Immersion experiments demonstrated that young instars may use cuticular respiration. First- and second-instar larvae survived underwater for several hours without a visible air supply, although they did not survive in deoxygenated water. We posit that the short body microtrichia have a waterproofing function in larvae, whereas they create a compressible air bubble in adults. In adults, waterproofing is accomplished by the setae. The density and length of setae on the legs of larvae was nearly the same as that on the body and legs of adults and is presumably optimized for waterproofing. Thus, a change in morphometrical parameters can result in a large functional change in the same structure. We discuss this interpretation in both ecological and physiological contexts.
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