Insect reward systems: Comparing flies and bees

Many elements of animal behaviour are organised by an innate reward-seeking drive stemming from neurobiological reward systems. The behavioural concept of reward and its neurobiological substrates was initially developed in mammalian systems, and there it has become clear that several novel social behaviours evolved through the co-option of reward pathways. Only more recently has reward been explored in insects. In this review, we consider current knowledge about reward pathways in the two predominant insect models: Drosophila melanogaster and the honey bee Apis mellifera. These two models are phylogenetically distantly related and have vastly different ecologies: fruit flies are mostly solitary while honey bees live in complex societies involving social foraging and brood care. Initially, it was assumed the reward system was essentially similar between these two organisms, but more recent studies have appeared to highlight quite significant differences. Here, we critically evaluate apparent differences in the neurobiology of the reward system between these organisms. We discuss which differences may be real and which may be reflective of the very different modes of analysis applied in these two models. Finally, we discuss how modification of reward systems might have contributed to social evolution in insects.