“…This requires some sort of hierarchical decomposition of these networks into manageable and intelligible subsystems, whose properties and behaviour can be analysed and understood in relative isolation (Simon, 1962; Riedl, 1975; Lewontin, 1978; Bonner, 1988; Raff, 1996; West-Eberhard, 2003; Schlosser and Wagner, 2004; Callebaut et al, 2005). If each subsystem possesses a clearly delimited and discernible function, the network can be subdivided into functional modules (Raff, 1996; von Dassow and Munro, 1999; Hartwell et al, 1999; Wagner et al, 2007; Mireles and Conrad, 2018). In the Introduction of our paper, we provide a careful argument showing that the most common approach to identify functional modules has severe limitations, and propose an alternative method, which we then use to dissect and analyse a specific pattern-forming network, the gap gene system of the vinegar fly, Drosophila melanogaster .…”