Karkosiak Katherine Q, Schwartz Ravi Z, King Hunter, Blackledge Todd A
Department of Biology and Integrated Bioscience Program, The University of Akron, Akron, OH 44325, USA.
Department of Physics, and Center for Computational and Integrative Biology, Rutgers University-Camden, Camden, NJ 08103, USA.
J Exp Biol. 2025 Jul 1;228(13). doi: 10.1242/jeb.250298. Epub 2025 Jul 14.
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. Past research gives conflicting results about whether these mats of silk fibers reduce evaporation of water in part because the diffusive resistance of any membrane cannot be measured independently of the system in which it is studied. We developed a model to describe water vapor transport across porous surfaces that includes the important roles of the gap space underneath the membrane and the boundary layer on the outside of the membrane in controlling water vapor flux, in addition to the relative impermeability of 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 the black widow Latrodectus hesperus and the garden spider 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 preserving and defining an internal region of stagnant air that often dominates the diffusive resistance of the whole system. Our model provides a tool to explore diverse spider egg sac geometries, but can also be adopted to fit a variety of systems to facilitate comparison and engineering of diffusive resistance across membranes.
控制跨表面的水分运输对所有生物来说都至关重要。蜘蛛卵囊是多功能膜，可保护卵和幼蛛免受外部环境影响。过去的研究对于这些丝纤维垫是否能减少水分蒸发给出了相互矛盾的结果，部分原因在于任何膜的扩散阻力都无法独立于其所处的研究系统进行测量。我们开发了一个模型来描述水蒸气在多孔表面的传输，该模型除了考虑膜本身的相对不透性外，还纳入了膜下方间隙空间和膜外部边界层在控制水蒸气通量方面的重要作用。该模型通过实证研究准确预测了各种合成表面以及黑寡妇蜘蛛（Latrodectus hesperus）和横纹金蛛（Argiope aurantia）卵囊的扩散阻力。我们发现，“典型的”蜘蛛卵囊膜对水的扩散阻力出奇地低，因为它们在微观尺度上具有高度多孔性。然而，丝质卵囊在控制水分流失方面仍起着关键作用，因为它们能保留并界定一个内部停滞空气区域，而该区域往往主导着整个系统的扩散阻力。我们的模型提供了一个工具来探索各种蜘蛛卵囊的几何形状，但也可用于适配各种系统，以促进跨膜扩散阻力的比较和工程设计。
