The silkmoth cocoon as humidity trap and waterproof barrier

To better understand how silkmoth cocoons maintain the correct internal moisture levels for successful pupation, we examined cocoons from the long-domesticated mulberry silkmoth Bombyx mori as well as from two wild silkmoth species, Antheraea pernyi and Philosamia cynthia ricini. We determined fluid-independent values for the porosity, tortuosity and permeability of the inner and outer surfaces of cocoons. Permeabilities were low and, with the exception of A. pernyi cocoons, inner surfaces were less permeable than outer surfaces. B. mori cocoons exhibited the highest permeability overall, but only at the outer surface, while A. pernyi cocoons appeared to show different patterns from the other species tested. We discuss our findings in light of the ecophysiology of the various species and propose a ‘tortuous path’ model to help explain our results. The model describes how the structure of the inner and outer layers of the cocoon allows it to function as both a humidity trap and a waterproof barrier, providing optimum conditions for the successful development of the pupa.