Adaptation Strategy In Attachment–Detachment Cycles When Faced With Changes In Incline And Orientation During Gecko Climbing

Background: Geckos are endowed with the extraordinary capacity to move quickly in various environments; they benefit from efficient control for the complex footpads. Research on the locomotor behavior and contact status in the attachment–detachment (A-D) cycle of the footpads for diverse challenges is linked to the revelation of regulatory strategy. At present, there is a lack of systematic research for the A-D cycle, which limits the understanding of the adhesive locomotion mechanism.Methods: The A-D cycle that facilitates the level and up–down locomotion on inclined and vertical surfaces of Gekko gecko was investigated to clarify the locomotion postures and durations in the release, swing, contact, and adhesion stages, respectively. This reveals the relationship between the structure and function of the attachment devices, and its regulation when faced with changing locomotion demands.Results: Despite changes in climbing demands, gecko foot locomotion posture (angle extremes and changing trends) in the swing stage, the posture (bending angle: fore 41°, hind 51°) and contact time ratio (7.42%) in the contact stage remain unchanged, which is in contrast with the adjustable postures in the stance phase. Furthermore, the variation range of the forefoot locomotion posture is larger than that of the hindfoot, and the forefoot angle changing trend is opposite to that of the hindfoot, indicating that the combination of anatomical structure and functional demands results in the differentiation in the adaptation mode of the A-D cycle for the fore- and hindfoot. Conclusions: Gecko’s fore- and hindfoot have evolved different structures to undertake differential functions. The function (adhesion) for various locomotion demands relates to footpad deployment in the stance phase but is unaffected by the regulations (postures and durations) in the swing and contact stages. The results demonstrate that the unified adaptation strategy reduces the diversity and complexity of the control. It advances the understanding of the adhesive locomotion mechanism, reflects the structural evolution and adaptation strategy of attachment devices for functional requirements and provides biological inspiration for effective design and control of adhesion robots.

Physical constraints lead to parallel evolution of micro- and nanostructures of animal adhesive pads: a review

Adhesive pads are functional systems with specific micro- and nanostructures which evolved as a response to specific environmental conditions and therefore exhibit convergent traits. The functional constraints that shape systems for the attachment to a surface are general requirements. Different strategies to solve similar problems often follow similar physical principles, hence, the morphology of attachment devices is affected by physical constraints. This resulted in two main types of attachment devices in animals: hairy and smooth. They differ in morphology and ultrastructure but achieve mechanical adaptation to substrates with different roughness and maximise the actual contact area with them. Species-specific environmental surface conditions resulted in different solutions for the specific ecological surroundings of different animals. As the conditions are similar in discrete environments unrelated to the group of animals, the micro- and nanostructural adaptations of the attachment systems of different animal groups reveal similar mechanisms. Consequently, similar attachment organs evolved in a convergent manner and different attachment solutions can occur within closely related lineages. In this review, we present a summary of the literature on structural and functional principles of attachment pads with a special focus on insects, describe micro- and nanostructures, surface patterns, origin of different pads and their evolution, discuss the material properties (elasticity, viscoelasticity, adhesion, friction) and basic physical forces contributing to adhesion, show the influence of different factors, such as substrate roughness and pad stiffness, on contact forces, and review the chemical composition of pad fluids, which is an important component of an adhesive function. Attachment systems are omnipresent in animals. We show parallel evolution of attachment structures on micro- and nanoscales at different phylogenetic levels, focus on insects as the largest animal group on earth, and subsequently zoom into the attachment pads of the stick and leaf insects (Phasmatodea) to explore convergent evolution of attachment pads at even smaller scales. Since convergent events might be potentially interesting for engineers as a kind of optimal solution by nature, the biomimetic implications of the discussed results are briefly presented.

Attachment devices and the tarsal gland of the bug Coreus marginatus (Hemiptera: Coreidae)

The present ultrastructural investigation using scanning and transmission electron microscopy as well as light and fluorescence microscopy describes in detail the attachment devices and tarsal gland of the bug Coreus marginatus (L.) (Hemiptera: Coreidae). In particular, the fine structure of pulvilli reveals a ventral surface rich with pore channels, consistent with fluid emission, and a folded dorsal surface, which could be useful to enhance the pulvillus contact area during attachment to the substrate. The detailed description of the tarsal gland cells, whose structure is coherent with an active secretory function, allows us to consider the tarsal gland as the plausible candidate for the adhesive fluid production. Scolopidia strictly adhering to the gland cells are also described. On the basis of the fine structure of the tarsal gland, we hypothesise a fluid emission mechanism based on changes of the hydraulic pressure inside the gland, due to the unguitractor tendon movements. This mechanism could provide the fluid release based on compression of the pad and capillary suction, as demonstrated in other insects. The data here reported can contribute to understanding of insect adhesive fluid production, emission and control of its transport.

Morphology of powerful suction organs from blepharicerid larvae living in raging torrents

Background Suction organs provide powerful yet dynamic attachments for many aquatic animals, including octopus, squid, remora, and clingfish. While the functional morphology of suction organs from some cephalopods and fishes has been investigated in detail, there are only few studies on such attachment devices in insects. Here we characterise the morphology and ultrastructure of the suction attachment organs of net-winged midge larvae (genus Liponeura; Diptera: Blephariceridae) – aquatic insects that live on rocks in rapid alpine waterways where flow speeds can reach 3 m s− 1 – using scanning electron microscopy, confocal laser scanning microscopy, and X-ray computed micro-tomography (micro-CT). Furthermore, we study the function of these organs in vivo using interference reflection microscopy. Results We identified structural adaptations important for the function of the suction attachment organs in L. cinerascens and L. cordata. First, a dense array of spine-like microtrichia covering each suction disc comes into contact with the substrate upon attachment, analogous to hairy structures on suction organs from octopus, clingfish, and remora fish. These spine-like microtrichia may contribute to the seal and provide increased shear force resistance in high-drag environments. Second, specialised rim microtrichia at the suction disc periphery were found to form a continuous ring in close contact and may serve as a seal on a variety of surfaces. Third, a V-shaped cut on the suction disc (“V-notch“) is actively opened via two cuticular apodemes inserting on its flanks. The apodemes are attached to dedicated V-notch opening muscles, thereby providing a unique detachment mechanism. The complex cuticular design of the suction organs, along with specialised muscles that attach to them, allows blepharicerid larvae to generate powerful attachments which can withstand strong hydrodynamic forces and quickly detach for locomotion. Conclusion The suction organs from Liponeura are underwater attachment devices specialised for resisting extremely fast flows. Structural adaptations from these suction organs could translate into future bioinspired attachment systems that perform well on a wide range of surfaces.

Sophisticated suction organs from insects living in raging torrents: Morphology and ultrastructure of the attachment devices of net-winged midge larvae (Diptera: Blephariceridae)

2.AbstractSuction organs provide powerful yet dynamic attachments for many aquatic animals, including octopus, squid, remora, and clingfish. While the functional morphology of suction organs from various cephalopods and fishes has been investigated in detail, there are only few studies on such attachment devices in insects. Here we characterise the morphology, ultrastructure, and in vivo movements of the suction attachment organs of net-winged midge larvae (genus Liponeura) – aquatic insects that live on rocks in rapid alpine waterways where flow rates can reach 3 m s-1 – using scanning electron microscopy, laser confocal scanning microscopy, and X-ray computed micro-tomography (micro-CT). We identified structural adaptations important for the function of the suction attachment organs from L. cinerascens and L. cordata. First, a dense array of spine-like microtrichia covering each suction disc comes into contact with the substrate upon attachment. Similar hairy structures have been found on the contact zones of suction organs from octopus, clingfish, and remora fish. These structures are thought to contribute to the seal and to provide increased shear force resistance in high-drag environments. Second, specialised rim microtrichia at the suction disc periphery form a continuous ring in close contact with a surface and may serve as a seal on a variety of surfaces. Third, a V-shaped cut on the suction disc (the V-notch) is actively peeled open via two cuticular apodemes inserting into its flanks. The apodemes are attached to dedicated V-notch opening muscles, thereby providing a unique detachment mechanism. The complex cuticular design of the suction organs, along with specialised muscles that attach to them, allows blepharicerid larvae to generate powerful attachments which can withstand strong hydrodynamic forces and quickly detach for locomotion. Our findings could be applied to bio-inspired attachment devices that perform well on a wide range of surfaces.