Autodetection in spiders: Female false black widow spiders behaviorally and physically respond to conspecific sex pheromone

Females of the false black widow Steatoda grossa can alter the architecture of their webs in relation to external cues to upturn prey-capture, safety, or mating opportunities. Here, we tested the hypothesis that female S. grossa respond to mate competition in that they change the architecture of their web and the amount of pheromone they deposit on it in response to the number density of webs in the microhabitat. In each of seven replicates of experiment 1, three females (low-number density) were allowed to build their webs in the same room for two days. Following a 12-day intermission, the same three females were then be allowed to build a new web for two days together with 27 other females (high-number density) present in the same room. The design of experiment 2 was similar except that females first build their webs in the high-number density setting and then in the low-number density setting. Lastly, we exposed low-density numbers to synthetic pheromone mimicking high female density numbers. The females’ web architecture in low- and high-number density settings were measured and all webs were extracted individually to quantify the amount of pheromone present. Our data support the hypothesis that females change their web architecture and the amount of pheromone they deposit on it in response to the number density of webs in the microhabitat.