Sterile Capsule–Egg Cocoon Covering Constitutes an Antibacterial Barrier for Spider Parasteatoda tepidariorum Embryos

Coexistence of organisms and pathogens has resulted in the evolution of efficient antimicrobial defense, especially at the embryonic stage. This investigation aimed to substantiate the hypothesis that the layers of silk in a spider cocoon play a role in the immunity of the embryos against microorganisms present in the external environment. A two-step interdisciplinary attempt has been made. First, the eggs and empty cocoons of the spider Parasteatoda tepidariorum were incubated on lysogeny broth agar media for 3 d. In the samples of eggs, no growth of bacteria was detected. This indicated that the eggs inside cocoons were sterile. Therefore, in the second step, the cocoons and egg surface were analyzed using SEM, TEM, and LM. The obtained images demonstrated that both inner and outer layers of the silk are built of threads of the same diameter, set in an irregular manner, and randomly clustered into groups. The threads in the outer layer were packed more densely than in the inner one. TEM analysis revealed threads of two types of fibrils and their arrangement. The resultant thread tangle of the cocoon, possibly correlated with the ultrastructure of the fibers, seems to be an example of a structure-function relationship playing a crucial ecoimmunological role in spider embryonic development.