Dodging sexual conflict?-Sub-adult females of a web-building spider stay cryptic to mate-seeking adult males

Adult males of web-building spiders often cohabit the webs of sessile sub-adult (i.e. penultimate instar) females and mate with them as they moult to adults. Often, males accrue benefits from this cohabitation (kleptoparasitism, avoidance of cannibalism, potential polygamy), whereas sub-adult females may either accrue benefits or incur costs such as curtailed opportunity for mate choice or mate cannibalism. Working with the false black widow spider, Steatoda grossa, we tested the hypothesis that webs of sub-adult females, unlike those of virgin adult females, lack sex attractant pheromone that mate-seeking males could detect and exploit for mate location. We tested our hypothesis in laboratory experiments by presenting adult males with binary choices between different types of webs (e.g., webs of adult virgin females, sub-adult females or sub-adult males), and methanol extracts of these webs. Males spent more time on webs, or web extracts, of adult virgin females than on webs or web extracts of any other type. Most males (95%) also displayed courtship only on webs, or web extracts, of adult virgin females. Our data demonstrate apparent semiochemical crypsis of sub-adult females or their webs to mate-seeking adult males that seem to find sub-adult females by chance encounter. This crypsis is likely adaptive to sub-adult females that are in sexual conflict with adult males cohabiting their webs.