New Insight Into the Evolutionary Arms Race Between Spider Egg Sac Pseudoparasitoids and Active Maternal Care by the Spiders

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New Insight Into the Evolutionary Arms Race Between Spider Egg Sac Pseudoparasitoids and Active Maternal Care by the Spiders

ABSTRACT

Pseudoparasitoids can lead to high mortality in spider egg sacs, and in some cases, they reduce the reproductive success of a spider female to zero. On the other hand, a species that develops within the spider's egg sac uses a limited resource derived from a single egg sac for its larval development. Therefore, the most crucial behaviour that increases the fitness of free-living pseudoparasitoid females is choosing the best host for their offspring. We analysed various points of the counter-adaptations of the spider egg sac pseudoparasitoid and spiders exhibiting active maternal care, utilising the ichneumonid Hidryta fusiventris (Thomson, 1873) and the wolf spider Pardosa lugubris (Walckenaer, 1802). We showed that the oviposition decision of H. fusiventris is based on the spider's egg sac size and that the females of P. lugubris invest the most in the first egg sac—their relative reproductive effort is the highest at that time. We observed that there is a time shift of approximately 1 month between the first egg sac of P. lugubris and the presence of H. fusiventris in its egg sac, which can significantly impact P. lugubris fitness (tenable escape of the spider from the pseudoparasitoid). However, in the case of parasitisation, females of P. lugubris continue taking care of these egg sacs despite the absence of spider offspring and changes in the shape, weight and texture of the egg sacs with H. fusiventris pupae. This care is assumed to be, for many reasons, necessary for the proper development of this pseudoparasitoid. We observed that H. fusiventris larvae are able to feed on both the eggs and juveniles of spiders. They serve as long-term dynamic food sources; however, changes in chemical composition throughout the developmental stages of P. lugubris may affect the development of H. fusiventris and its fitness-related traits.

Kostro-Ambroziak, A., Suprunowicz, U., Piotrowska-Niczyporuk, A., Czajkowska, M., Sandurska, E., Szymański, D., & Szymański, D. (2026). New Insight Into the Evolutionary Arms Race Between Spider Egg Sac Pseudoparasitoids and Active Maternal Care by the Spiders. Ecology and Evolution, 16(5), e73581. https://doi.org/10.1002/ece3.73581