The costs and benefits for partners in interspecific, mutualistic relationships can vary from one interaction to the next, or change with time during an interaction, which might influence the mutualistic contributions. Such flexible behavioural responses could be important in regulating the interaction. Partners could also have special adaptations for the purpose of influencing each other. An individual might, for instance, signal its need for the services provided by the partner. In interactions between lycaenid butterfly larvae and ants, the larvae deliver nutritious droplets from a specialized gland and, in return, are protected from enemy attacks by the ants. Lycaenid larvae have several ant-related adaptations, one being a pair of eversible tentacular organs. In this study, the relationships between tentacle display, the delivery of droplets and ant attendance were investigated in a number of experiments, where the interaction between larvae of Polyommatus icarus and either Lasius niger or L. flavus ants was manipulated. High rates of both droplet delivery and tentacle display were found at the first contact with ants, after an interruption in attendance and, for low levels of attendance, after a simulated enemy attack. Under steady state conditions, displays were most frequent at low levels of attendance, but droplet delivery had a maximum at intermediate levels. Manipulation of a larva's ability to use its tentacles also showed that ants respond to tentacle display by increasing their attendance. Larval behaviour was interpreted as aimed at regulating the, number of attending ants. (C) 1996 The Association for the Study of Animal Behaviour
