Thieves and freeloaders: Argyrodine kleptoparasites invading cobwebs (Theridiidae) in the arid southwestern USA

Obligate argyrodine kleptoparasites (Theridiidae, Araneae) exploit heterospecific spider webs for food and shelter. Argyrodine spiders are a model lineage for the study of kleptoparasitism and related strategies, yet data on the behavior of the majority of the over 250 argyrodine species is lacking. Here we help fill that knowledge gap by documenting the natural history of two poorly known species. We studied Argyrodes pluto and Neospintharus baboquivari in the webs of two other cobweb spiders, the western black widow ( Latrodectus hesperus ) and Tidarren sisyphoides , across four sites in southern Arizona. Argyrodes pluto completes its life cycle in L. hesperus webs and specializes on host egg sacs, displacing them to the periphery and feeding on eggs and juveniles; this behavior appears essential for its reproduction. Neospintharus baboquivari occurs gregariously in both host webs, gleaning small prey. In contrast, N. baboquivari is reportedly a solitary araneophage in the colonial orb webs of Philoponella oweni . We quantified egg-sac displacement, and describe foraging, mating, egg-sac construction, and interactions with parasitoids and predators. These findings reveal novel natural history information and expand our understanding of argyrodine behavioral plasticity