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

Morphological and Behavioral Adaptations of Silk-Lovers (Plokiophilidae: Embiophila) for Their Lifestyle in the Silk Domiciles of Webspinners (Embioptera)

The diversity of true bugs gave rise to various lifestyles, including gaining advantage from other organisms. Plokiophilidae are cimicomorphan bugs that live in the silk constructions of other arthropods. One group, Embiophila, exclusively settles in the silk colonies of webspinners (Embioptera). We investigated the lifestyle of Embiophila using microscopy to study the micromorphology and material composition of the leg cuticle, choice assays and retention time measurements based on different characteristics of the embiopteran galleries and tilting experiments with different substrates to quantify the attachment performance of the bugs. Embiophila neither explicitly preferred embiopteran presence, nor required silk for locomotion, but the bugs preferred fibrous substrates during the choice experiments. The hairy attachment pad on the tibia showed the best attachment performance on substrates, with an asperity size of 1 µm. Additionally, very rough substrates enabled strong attachment, likely due to the use of claws. Our findings suggest that Embiophila settle in galleries of webspinners to benefit from the shelter against weather and predators and to feed on mites and other intruders. The combination of behavioral and functional morphological experiments enables insights into the life history of these silk-associated bugs, which would be highly challenging in the field due to the minute size and specialized lifestyle of Embiophila.

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.