Can ant‐eating Zodarion spiders (Araneae: Zodariidae) develop on a diet optimal for euryphagous arthropod predators?

Abstract Little attention is paid to the behavioural and physiological adaptations of ant‐eating predators. It is expected that there should be a strong selection for traits related to prey handling, leading to the evolution of morphological, behavioural and nutritional adaptations. Such adaptations may then entail trade‐offs in handling and utilization of alternative prey. To investigate behavioural as well as nutritional adaptations and the occurrence of the corresponding trade‐offs in two ant‐eating spiders of the genus Zodarion [Zodarion atlanticum Pekár & Cardoso and Zodarion germanicum (C. L. Koch)], spiders are reared on two diets: ants (i.e. their preferred prey) and fruit flies (i.e. an alternative prey that is nutritionally optimal for euryphagous spiders). Food consumption is observed and several fitness‐related life‐history parameters are measured. Although spiders readily accept ants, more than one‐third of 35 spiders refuse to consume fruit flies and starve. Furthermore, severe hunger does not induce these individuals to accept fruit flies. Starving spiders die before moulting to the second stadium. Spiders that eat fruit flies increase only little and slowly in weight, and all of these die during the first two stadia. By contrast, spiders on an ant diet increase dramatically in weight, and develop up to the fourth stadium. These data indicate that fruit flies are not suitable for Zodarion, supporting the hypothesis that there are behavioural and nutritional trade‐offs. Taking into account the results of previous studies, it is suggested that nutritional trade‐offs are generally important for stenophagous spiders.