Role of Hygroscopic Low Molecular Mass Compounds in Humidity Responsive Adhesion of Spider's Capture Silk.

The aggregate glue in spider webs is composed of hygroscopic low molecular mass compounds (LMMCs), glycoproteins and water. The LMMCs absorb atmospheric water and solvate the glycoproteins to spread and adhere to flying insects upon contact. The glue viscosity varies with humidity and there is an optimum range of viscosity where the adhesion is maximum. LMMCs composition and the humidity at which glue viscosity is optimized vary greatly among spider species. These findings suggest that spiders adapt to forage in diverse habitats by "tuning" LMMCs composition or how LMMCs interact with glycoproteins to control water uptake and adhesion. To test these hypotheses, we analyzed the LMMCs for spiders from diverse habitats and performed water uptake studies on intact glue droplets, isolated glue constituents, and synthetic LMMCs. Even though glue droplets showed differences in water uptake among spider species, we found no differences among species in hygroscopicity of natural or synthetic LMMCs mixtures. This demonstrates that LMMCs composition alone is insufficient to explain interspecific differences in water uptake of spider glues and instead support the hypothesis that an interaction between LMMCs and glycoproteins mediate differences in water uptake and adhesion.