Spinning behaviour and morphology of the spinning glands in male and female Aposthonia ceylonica (Enderlein, 1912) (Embioptera: Oligotomidae)

Evolutionary morphology of the antennal heart in stick and leaf insects (Phasmatodea) and webspinners (Embioptera) (Insecta: Eukinolabia)

The morphology of the antennal hearts in the head of Phasmatodea and Embioptera was investigated with particular reference to phylogenetically relevant key taxa. The antennal circulatory organs of all examined species have the same basic construction: they consist of antennal vessels that are connected to ampullae located in the head near the antenna base. The ampullae are pulsatile due to associated muscles, but the points of attachment differ between the species studied. All examined Phasmatodea species have a Musculus (M.) interampullaris which extends between the two ampullae plus a M. ampulloaorticus that runs from the ampullae to the anterior end of the aorta; upon contraction, all these muscles dilate the lumina of both ampullae at the same time. In Embioptera, only the australembiid Metoligotoma has an M. interampullaris. All other studied webspinners instead have a M. ampullofrontalis which extends between the ampullae and the frontal region of the head capsule; these species do not have M. ampulloaorticus. Outgroup comparison indicates that an antennal heart with a M. interampullaris is the plesiomorphic character state among Embioptera and the likely ground pattern of the taxon Eukinolabia. Antennal hearts with a M. ampullofrontalis represent a derived condition that occurs among insects only in some embiopterans. These findings help to further clarify the controversially discussed internal phylogeny of webspinners by supporting the view that Australembiidae are the sister group of the remaining Embioptera.

Pressure-induced silk spinning mechanism in webspinners (Insecta: Embioptera)

The articulated appendages of arthropods are highly adaptable and potentially multifunctional, used for walking, swimming, feeding, prey capture, or other functions. Webspinners (Order Embioptera) are a paragon in this context. In contrast to other arthropods producing silk, they utilize their front feet for silk production. However, employing the same leg for alternative functions rather than for pure locomotion potentially imposes constraints and compromises. We here present morphological and experimental evidence for a "passive" pressure-induced silk spinning mechanism induced by external mechanical stimuli. Furthermore, we demonstrate that, as a consequence of the conflicting functions for their front feet, webspinners have evolved a unique style of walking that reduces the potentially problematic contact between silk ejectors and the substrate. Here we answer for the first time a long-term question within this enigmatic group of insects-how webspinners can use their front feet to spin their nanoscale silk. This knowledge may open the door for experimental studies on an artificial spinning process and for future utilization in applied fields of robotics or chemistry.

Three-dimensional reconstruction on cell level: case study elucidates the ultrastructure of the spinning apparatus of Embia sp. (Insecta: Embioptera)

Spinning is a phenomenon not only present in spiders, but also in many other arthropods. The functional morphology and complexity of spinning organs is often poorly understood. Their elements are minute and studying them poses substantial methodological difficulties. This study presents a three-dimensional reconstruction of a silk gland of Embia sp. on cellular level, based on serial sections acquired with serial block-face scanning electron microscopy (SBFSEM) to showcase the power of this method. Previous studies achieved either high resolution to elucidate the ultrastructure or satisfying three-dimensional representations. The high-resolution achieved by SBFSEM can be easily used to reconstruct the three-dimensional ultrastructural organization of cellular structures. The herein investigated spinning apparatus of Embioptera can be taken as an example demonstrating the potential of this method. It was possible to reconstruct a multinucleated silk gland containing 63 nuclei. We focused on the applicability of this method in the field of morphological research and provide a step-by-step guide to the methodology. This will help in applying the method to other arthropod taxa and will help significantly in adapting the method to other animals, animal parts and tissues.

Surface and Wetting Properties of Embiopteran (Webspinner) Nanofiber Silk

Insects of the order Embioptera, known as embiopterans, embiids, or webspinners, weave silk fibers together into sheets to make shelters called galleries. In this study, we show that silk galleries produced by the embiopteran Antipaluria urichi exhibit a highly hydrophobic wetting state with high water adhesion macroscopically equivalent to the rose petal effect. Specifically, the silk sheets have advancing contact angles above 150°, but receding contact angle approaching 0°. The silk sheets consist of layered fiber bundles with single strands spaced by microscale gaps. Scanning and transmission electron microscopy (SEM, TEM) images of silk treated with organic solvent and gas chromatography mass spectrometry (GC-MS) of the organic extract support the presence of a lipid outer layer on the silk fibers. We use cryogenic SEM to demonstrate that water drops reside on only the first layer of the silk fibers. The area fraction of this sparse outer silk layers is 0.1 to 0.3, which according to the Cassie-Baxter equation yields an effective static contact angle of ∼130° even for a mildly hydrophobic lipid coating. Using high magnification optical imaging of the three phase contact line of a water droplet receding from the silk sheet, we show that the high adhesion of the drop stems from water pinning along bundles of multiple silk fibers. The bundles likely form when the drop contact line is pinned on individual fibers and pulls them together as it recedes. The dynamic reorganization of the silk sheets during the droplet movement leads to formation of "super-pinning sites" that give embiopteran silk one of the strongest adhesions to water of any natural hydrophobic surface.

The spinning apparatus of webspinners--functional-morphology, morphometrics and spinning behaviour

Webspinners (Insecta: Embioptera) have a distinctly unique behaviour with related morphological characteristics. Producing silk with the basitarsomeres of their forelegs plays a crucial role in the lives of these insects--providing shelter and protection. The correlation between body size, morphology and morphometrics of the spinning apparatus and the spinning behaviour of Embioptera was investigated for seven species using state-of-the-art methodology for behavioural as well as for morphological approaches. Independent contrast analysis revealed correlations between morphometric characters and body size. Larger webspinners in this study have glands with greater reservoir volume, but in proportionally smaller tarsi relative to body size than in the smaller species. Furthermore, we present a detailed description and review of the spinning apparatus in Embioptera in comparison to other arthropods and substantiate the possible homology of the embiopteran silk glands to class III dermal silk glands of insects.

Structural characterization of nanofiber silk produced by embiopterans (webspinners)

Embiopterans produce silken galleries and sheets using exceptionally fine silk fibers in which they live and breed. In this study, we use electron microscopy (EM), Fourier-transform infrared (FT-IR) spectroscopy, wide angle X-ray diffraction (WAXD) and solid-state nuclear magnetic resonance (ssNMR) techniques to elucidate the molecular level protein structure of webspinner (embiid) silks. Silks from two species Antipaluria urichi and Aposthonia ceylonica are studied in this work. Electron microscopy images show that the fibers are about 90-100 nm in diameter, making webspinner silks among the finest of all known animal silks. Structural studies reveal that the silk protein core is dominated by β-sheet structures, and that the protein core is coated with a hydrophobic alkane-rich surface coating. FTIR spectra of native embiid silk shows characteristic alkane CH₂ stretchings near 2800-2900 cm^-1, which decrease approximately 50% after washing the silk with 2 : 1 CHCl₃ : MeOH. Furthermore, ¹³C ssNMR data shows a significant CH₂ resonance that is strongly affected by the presence of water, supporting the idea that the silk fibers are coated with a hydrocarbon-rich layer. Such a layer is likely used to protect the colonies from rain. FTIR data also suggests that embiid silks are dominated by β-sheet secondary structures similar to spider and silkworm silk fibers. NMR data confirms the presence of β-sheet nanostructures dominated by serine-rich repetitive regions. A deconvolution of the serine Cβ NMR resonance reveals that approximately 70% of all seryl residues exist in a β-sheet structure. This is consistent with WAXD results that suggest webspinner silks are 70% crystalline, which is the highest crystalline fraction reported for any animal silks. The work presented here provides a molecular level structural picture of silk fibers produced by webspinners.