Determinants of orb web spider glue droplet hygroscopicity

The adhesion of the viscoelastic protein cores within the glue droplets of an araneoid orb web's prey capture thread is ensured by droplet hygroscopicity. Atmospheric moisture attracted by low molecular mass compounds (LMMCs) in the aqueous layer that surrounds these droplets' cores tunes droplet performance to the humidity of a species' habitat. This study measured the hygroscopicity of 15 species' glue droplets at 20%, 37%, 55%, 72% and 90% relative humidity to reveal a wide interspecific range of droplet hygroscopic profiles. These profiles show that the hygroscopicities of individual LMMCs are combined in an additive manner rather than being averaged to determine droplet hygroscopicity. This feature allows droplet hygroscopicity to change in a non-linear manner across humidity. A comprehensive model of the determinants of droplet hygroscopicity supported the important role of LMMCs by showing that they were responsible for 44% of droplet hygroscopicity. However, four additional variables also affected droplet hygroscopicity: cube root droplet volume contributed 21%, the ability of core proteins to assimilate water contributed 16%, core volume relative to aqueous volume contributed 14%, and core protein elastic modulus contributed 5%. In each of the 15 species there was good agreement between modeled and measured droplet hygroscopicity. However, interspecific differences in the values of the five contributing variables caused the percentage droplet hygroscopic contributions to differ greatly. Moreover, values of the contributing variables differed when species were grouped by foraging humidity as: low humidity, mid-humidity, high humidity and nocturnal. These findings show that the volume and molecular composition of each glue droplet constituent that affects droplet hygroscopicity are related to the humidity of a species' habitat.