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Abstract An orb web's adhesive capture spiral is responsible for prey retention. This thread is formed of regularly spaced glue droplets supported by two flagelliform axial lines. Each glue droplet features a glycoprotein adhesive core covered by a hygroscopic aqueous layer, which also covers axial lines between the droplets, making the entire thread responsive to environmental humidity.We characterized the effect of relative humidity (RH) on ability ofArgiope aurantiaandArgiope trifasciatathread arrays to retain houseflies and characterize the effect of humidity on their droplet properties. Using these data and those ofAraneus marmoreusfrom a previous study, we then develop a regression model that correlated glycoprotein and flagelliform fiber properties with prey retention time. The model selection process included newly determined, humidity‐specific Young's modulus and toughness values for the three species' glycoproteins.Argiopeaurantiadroplets are more hygroscopic thanA. trifasciatadroplets, causing the glycoprotein withinA. aurantiadroplets to become oversaturated at RH greater than 55% RH and their extension to decrease, whereasA. trifasciatadroplet performance increases to 72% RH. This difference is reflected in species' prey retention times, with that ofA. aurantiapeaking at 55% RH and that ofA. trifasciataat 72% RH.Fly retention time was explained by a regression model of five variables: glue droplet distribution, flagelliform fiber work of extension, glycoprotein volume, glycoprotein thickness, and glycoprotein Young's modulus.The material properties of both glycoprotein and flagelliform fibers appear to be phylogenetically constrained, whereas natural selection can more freely act on the amount of each material invested in a thread and on components of the thread's aqueous layer. Thus, it becomes easier to understand how natural selection can tune the performance of viscous capture threads by directing small changes in these components.