Phoresy is an interspecific and temporary relationship in which the phoretic organism (traveller) actively seeks out its vehicle (carrier) for dispersal out of unsuitable areas for further development of itself or its progeny. In nature, this unsuitability of areas might arise due to over-crowding, habitat deterioration, sibling rivalry or unavailability of mates. These conditions generally lead to the development of a close association between the traveller and the host and the traveller might therefore also show synchrony with the host life cycle. The traveller not only needs to locate and latch on to its carrier but also should not have a greatly detrimental effect on the host which would prevent it from being transferred to a new favorable environment. Only few studies in phoresy have indicated that the carrier might bear the cost of such a relationship but this cost has not been adequately quantified. The addition of a third interactant leads to the development of a tritrophic interaction which increases the complexity of the system and might require an increase in the traveller’s specificity towards the carrier. One system in which nematodes show a highly species-specific tritrophic phoretic interaction is the well known mutualistic fig–fig wasp system.

In fig–fig wasp–nematode system, the fig nematode is the traveller, the female pollinator wasp serves as the vehicle and the fig is the substratum for their development. The pollinator wasp is the most reliable carrier as non-pollinator wasps that also occur within this system do not enter syconia in most fig species; the male fig wasps, being wingless, do not fly at all and die within their natal syconium. Such a phoretic system requires great specificity between nematodes and female pollinator wasps. Therefore, this system gives us an excellent opportunity to investigate the cost of phoresy on the mutualism between the fig and the fig wasp and the mechanism of dispersal of these phoretic organisms between figs.