Surface morphology and asymmetric transmural diffusion of Diprion similis (Hymenoptera: Diprionidae) cocoons

For many insect species, pupation occurs within the protected walls of a tightly spun silken cocoon. It is generally accepted that the cocoon protects the inhabitant from environmental stress, predators, and pathogens.  In this study we examine diffusion of water vapor and respiratory gases across the cocoon wall of the introduced Pine Sawfly (Diprion similis (Hartig)).  Our results show that the relative rate of diffusion can be as much as 1.97x greater when water vapor is moving from the outer surface of the cocoon to inner surface compared to the opposite direction.  Similarly, the outer cocoon surface shows greater wettability relative to the inner surface.  Observations from light and scanning electron micrographs of the cocoon wall show a greater fiber density for the inner surface compared to the outer surface, thus suggesting that specific morphological features of the cocoon play a role in asymmetric diffusion of water vapor observed in this study.