Structural and wetting properties of nature's finest silks (order Embioptera)

Morphological transformation from fibers to sheets in embiopteran silk

Embioptera (webspinners) are insects that construct domiciles using silk produced from their front feet. This silk is the finest known with measured single fiber diameters in the 30-140 nm range. In the wild, some webspinner silk on trees is observed to have a clothlike or shiny sheetlike appearance. Both forms of silk shield the occupants from rain water effectively: presumably valuable in tropical environments. In this article we elucidate the mechanism by which silk fibers are transformed into these structures through interaction with water. We quantify the evaporation rates of single water droplets which have been suspended on unmodified as-spun silk for two Trinidadian arboreal species: Antipaluria urichi (Clothodidae) and Pararhagadochir trinitatis (Scelembiidae). These rates are compared to those of droplets suspended on rose petals due to similar wetting properties (both hydrophobicity and pinning). We observe that on sufficiently thick silk, droplet evaporation rates decrease with time. This behavior is a result of a thin soluble film developing on the drop surface that later becomes a solid residual film. Experimentally verified theoretical models are invoked to support the results.

Dispersal Risks and Decisions Shape How Non-kin Groups Form in a Tropical Silk-Sharing Webspinner (Insecta: Embioptera)

Relying on silk can promote sharing, especially when its presence means life and its absence, quick death. In the case of Embioptera, they construct silken tubes and coverings exposed on tree bark in humid and warm environments or in leaf litter and underground in dry habitats. These coverings protect occupants from rain and natural enemies. Of note, adult females are neotenous, wingless and must walk to disperse. Evidence is pulled together from two sources to explore mechanisms that promote the establishment of non-kin groups that typify the neotropical Antipaluria urichi (Clothodidae): (1) a review of relevant information from 40 years of research to identify potential drivers of the facultative colonial system and (2) experimental and observational data exploring how dispersal contributes to group formation. To determine risks of dispersal and decisions of where to settle, adult females were released into the field and their ability to survive in the face of likely predation was monitored. Additional captured dispersers were released onto bark containing silk galleries; their decision to join the silk or to settle was noted. An experiment tested which attributes of trees attract a disperser: vertical or horizontal boles in one test and small, medium, or large boles in another. While walking, experimentally released adult female dispersers experienced a risk of being killed of approximately 25%. Dispersers orient to large diameter trees and join silk of others if encountered. These results align with observations of natural colonies in that adults and late-stage nymphs join existing colonies of non-kin. Experiments further demonstrated that dispersing females orient to vertical and larger diameter tree-like objects, a behavior that matched the distribution of field colonies. The ultimate reason for the observed dispersion pattern is probably because large trees support more expansive epiphytic algae and lichens (the food for this species), although the impact of food resources on dispersion has not been tested. Finally, further research questions and other webspinner species (including parthenogenetic ones) that warrant a closer look are described. Given that this group of primitively social insects, with approximately 1,000 species known, has remained virtually unstudied, one hope is that this report can encourage more exploration.

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.

Interpreting nature's finest insect silks (Order Embioptera): hydropathy, interrupted repetitive motifs, and fiber-to-film transformation for two neotropical species

Silks produced by webspinners (Order Embioptera) interact with water by transforming from fiber to film, which then becomes slippery and capable of shedding water. We chose to explore this mechanism by analyzing and comparing the silk protein transcripts of two species with overlapping distributions in Trinidad but from different taxonomic families. The transcript of one, Antipaluria urichi (Clothodidae), was partially characterized in 2009 providing a control for our methods to characterize a second species: Pararhagadochir trinitatis (Scelembiidae), a family that adds to the taxon sampling for this little known order of insects. Previous reports showed that embiopteran silk protein (dubbed Efibroin) consists of a protein core of repetitive motifs largely composed of glycine (Gly), serine (Ser), and alanine (Ala) and a highly conserved C-terminal region. Based on mRNA extracted from silk glands, Next Generation sequencing, and de novo assembly, P. trinitatis silk can be characterized by repetitive motifs of Gly-Ser followed periodically by Gly-Asparagine (Asn-an unusual amino acid for Efibroins) and by a lack of Ala which is otherwise common in Efibroins. The putative N-terminal domain, composed mostly of polar, charged and bulky amino acids, is ten amino acids long with cysteine in the 10th position-a feature likely related to stabilization of the silk fibers. The 29 amino acids of the C-terminus for P. trinitatis silk closely resemble that of other Efibroin sequences, which show 74% shared identity on average. Examination of hydropathicity of Efibroins of both P. trinitatis and An. urichi revealed that these proteins are largely hydrophilic despite having a thin lipid coating on each nano-fiber. We deduced that the hydrophilic quality differs for the two species: due to Ser and Asn for P. trinitatis silk and to previously undetected spacers in An. urichi silk. Spacers are known from some spider and silkworm silks but this is the first report of such for Embioptera. Analysis of hydropathicity revealed the largely hydrophilic quality of these silks and this feature likely explains why water causes the transformation from fiber to film. We compared spun silk to the transcript and detected not insignificant differences between the two measurements implying that as yet undetermined post-translational modifications of their silk may occur. In addition, we found evidence for codon bias in the nucleotides of the putative silk transcript for P. trinitatis, a feature also known for other embiopteran silk genes.

A Multiscale Characterization of Two Tropical Embiopteran Species: Nano- and Microscale Features of Silk, Silk-Spinning Behavior, and Environmental Correlates of their Distributions

Embioptera display the unique ability to spin silk with their front feet to create protective domiciles. Their body form is remarkably uniform throughout the order, perhaps because they all live within the tight confines of silken tubes. This study contributes to an understanding of the ecology of Embioptera, an order that is rarely studied in the field. We conducted a census to quantify the habitats of two species with overlapping distributions on the tropical island of Trinidad in a search for characteristics that might explain their distinct ecologies. One species, Antipaluria urichi (Saussure) (Embioptera: Clothodidae), lives in larger colonies with more expansive silk in habitats throughout the island, especially in the rainforest of the Northern Range Mountains. The other, Pararhagadochir trinitatis (Saussure) (Embioptera: Scelembiidae), was found only in lowland locations. We quantified silk-spinning behavior and productivity of the two species and found that A. urichi spins thicker silk sheets per individual and emphasizes spin-steps that function to create a domicile that is more expansive than that produced by P. trinitatis. Their silks also interact differently when exposed to water: the smaller-diameter silk fibers of P. trinitatis form more continuous films on the surface of the domicile after being wetted and dried than that seen in A. urichi silk. This tendency gives P. trinitatis silk a shiny appearance in the field compared to the more cloth-like silk of A. urichi. How these silks function in the field and if the differences are partially responsible for the distinct distributions of the two species remain to be determined.

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.