Who wins? Ray spiders (Theridiosoma gemmosum) (Araneae: Theridiosomatidae) versus egg sac parasitoids

This is the first report on the parasitoids of egg sacs of Theridiosoma gemmosum (L. Koch, 1877) (Theridiosomatidae). The parasitic wasps Gelis melanocephalus (Schrank, 1781), Gelis mangeri (Gravenhorst, 1815) (Ichneumonidae) and Pachyneuron solitarium (Hartig, 1838) (Pteromalidae) were recorded as solitary parasitoids of egg sacs of T. gemmosum in Poland. The degree of parasitism of T. gemmosum egg sacs in three different years was 21.4–42.9%, and the reproductive fitness of female spiders in all cases of parasitized cocoons was zero. It seems that the suspension system of the egg sacs of ray spiders is not a barrier against egg sac parasitoids, including apterous species. We hypothesize that, at least in the case of G. melanocephalus, the size of the spider's egg sac is implicated in the ovipositional decision of the female parasitoid.