Mutualism, cooperative interaction between different species, is widespread in nature. Despite its ubiquity and recognised importance for ecological communities, how mutualism evolves and persists remain major unresolved questions in evolutionary biology. The reason for this is that mutualistic interactions can often be exploited by parasites, and high degrees of species interdependence can increase extinction risk. Applying established network theory to interpret biological details of mutualistic interaction networks has great potential to reveal crucial system properties such as enhanced network resilience through species interdependence.I propose an interdisciplinary study combining network theory with detailed empirical analyses of a so far uncharacterised mutualistic network of subterranean ants, aphids and microbes. The ants protect aphids from predators, and in turn harvest honeydew, processed plant sap excreted by the aphids. In addition, symbiotic gut bacteria in both ants and aphids synthesise essential amino acids and assimilate nitrogen. These bacteria are crucial for upgrading the sugar rich but otherwise nutritionally poor plant sap and honeydew.My project will improve our understanding of mutualisms by unravelling the metabolic and behavioural mechanisms underlying species interdependence in this specialised symbiotic network. To this end, I will (i) assess covariance between bacterial communities of associated ants and aphids, (ii) determine the functional roles of bacterial communities in honeydew composition, and (iii) assess ant behavioural specificity in choice of aphid partners. I will then (iv) further interpret my findings in a broader context by applying network theory to this focal network and other comparable mutualistic networks, thereby adding to our general understanding of the evolution and stability of mutualism.