An unusual silken nest built under the sand surface is described in Yllenus arenarius—a jumping spider inhabiting sandy dunes. In this open habitat, characterized by high temperature and humidity gradients as well as a lack of retreats, the nest probably plays a key role in the survival strategy of Y. arenarius that is numerically dominant among day-active, dune-dwelling spiders. These salticids built nests a few millimeters under the surface after burrowing in loose sand. Four types of nests of different size, structure and function were built: A) where eggs were laid and early instars developed, B) where spiders molted, C) where they overwintered and, D) the most common, where spiders spent the night. Different age groups produced different numbers of nests per time unit. Juveniles in their first season of life built many more nests than subadult spiders in their second season, which in turn built more nests than adult spiders. Various functions of the silken nests and the high numbers built by juveniles suggest that the structures may play an important role in surviving in the dune.