A common predatory method among spiders is aggressive mimicry. The ant-eating Zodarion spider species performs this trick too: anatomically disguised as an ant, they are able to stealthily hunt and capture their preferred prey. However, this crafty technique by no means puts the ant-eating spider out of harm's way. Several members of the insect order Hymenoptera, which includes wasps, have been found to feed on spider hosts. The wasp larvae subsist by consuming the spider or the spider's eggs. In a study published in ZooKeys, lead author Stanislav Korenko from the Czech University of Life Sciences, Czech Republic, describes two wasp species, Calymmochilus dispar and Gelis apterus, and newly identifies them as parasitoids of the ant-eating spider Zodarion styliferum.
Korenko and colleagues set out on 31 field expeditions throughout Central and Southern Portugal during springtime with the aim of cataloging parasitoids of the ant-eating spider. The scientists investigated several diverse spider habitats including sand beaches, river banks, olive groves, woodland habitats and arid meadows, and collected larvae and pupae of hymenopteran parasitoids found within spider dens – small igloo-shaped stone hideaways constructed by spiders to protect them from enemies and the environment. Unfortunately for the spiders, their igloos do not prevail to be enemy proof. Scientists found that predatory wasps were able to attack the host inside the shelter by penetrating the stone walls with their distinctively long ovipositors (a specialized piercing organ used by the wasp to lay its eggs within a host's body) – depositing wasp eggs into the abdomen of juvenile spiders, and in turn providing the wasp's offspring with nutrients for their development.
The research team observed spiders being attacked during the daylight when they are typically inactive and hidden within their igloo retreat. In order to identify the diversity of spider and wasp species, the scientists collected whole igloos if there was a wasp pupa or parasitized spiders inside and transferred them to containers for host and parasite species verification. While researchers identified three different species of Zodarion spiders overall during their fieldwork, they found Z. styliferum to be most abundant. To their surprise, the team found wasp larvae of C. dispar and G. apterus specifically within the igloos of Z. styliferum – these wasps had never been found before in Portugal nor had they been previously known to attack Z. styliferum.
In order to calculate the parasitism rate at each recovery site, the number of parasitized spiders was compared with the number of all spiders observed at that location. The rate of parasitism was recorded for only localities where C. dispar and G. apterus parasitized spiders: one location for C. dispar and six sites for G. apterus. For example, in one southernmost locality, described as an arid slope next to a roadway, the team found two out of 10 spiders were parasitized by C. dispar wasps. From six other diverse field sites peppered along the south-eastern boarder with Spain, the scientists collected between 12 and 44 spiders at each site and found between one and two of these spiders to be infected by G. apterus.
This study represents the first documentation of these two particular wasp species being associated with Z. styliferum, but also gives scientists interested in these host–parasite relationships a good hunch of where to look next. Members belonging to the largest genus of ant-eating spiders have been found to be widely distributed across Europe, Asia and North America – and where hosts are found, predator parasites are sure to strike.