Spider egg sacs reveal how pockets of air can be used to conserve water

Controlling water transport across surfaces is essential for all living organisms. Spider egg sacs are multifunctional membranes that protect eggs and spiderlings from the external environment, but past research gives conflicting results about whether or not these mats of silk fibers can reduce evaporation of water in part because the diffusive resistance of any membrane cannot easily be measured independently of the system in which it is studied. We develop a model to describe water vapor transport across porous surfaces that reveals that the diffusive resistance across the gap space underneath the membrane and the boundary layer on the outside of the membrane can play much greater roles in controlling water vapor flux compared to diffusion across the membrane itself. The model accurately predicts diffusive resistance of a variety of synthetic surfaces from empirical studies, as well as the egg sacs of Latrodectus hesperus and Argiope aurantia. We show that "typical" spider egg sac membranes offer surprisingly low diffusive resistance to water because they are highly porous at microscopic scales. However, silk egg sacs still play key roles in controlling water loss by separating the diffusive resistance of the interior of the egg sacs from the outside boundary layer. Our model provides both a tool to explore diverse egg sac geometries and to facilitate comparison of diffusive resistance across membranes studied in diverse systems.