Drosophila melanogaster is an attractive model organism for the study of numerous fundamental processes including nervous system development and function. This is due to the power of Drosophila genetics combined with the high degree of similarity between this organism and vertebrate systems, not only at the molecular level but also at the cellular and behavioural levels. The first part of my thesis focused on trophic interactions occurring in Drosophila larval central nervous system. Specifically, it describes the interaction of serotonin (5HT)-producing neurons with other three groups of neurons: the larval photoreceptors expressing Rhodopsin 5 (Rh5), the photoreceptor subset expressing Rhodopsin 6 (Rh6), and the larval circadian pacemakers (LNv). I found that both Rh5-and Rh6-expressing fibers contact a 5-HT arborization in the larval optic neuropil, where the 5-HT processes also overlaps with the dendrites of the LNv. The results of my experiments also indicate that the Rh6-expressing terminus is the neural process providing the signal required for the outgrowth of the serotonergic arborization. Furthermore, proper branching of this arborization requires normal Rae function. These findings further support the importance of extrinsic and intrinsic signalling for the assembly ofthe nervous system. The remainder of my studies attempted to investigate candidate neurons modulating Drosophila larval photobehaviour. Using the larval response to light as a behavioural paradigm and neuronal silencing experiments, my results demonstrate that 5HT neurons located in the brain regulate the larval photoresponse during development. In addition, my findings suggest that this modulation occurs at a central level and that is mediated by 5-HT1A(Dro) receptors. These observations provide new insights into the functions of serotonergic neurons in Drosophila as well as how neuromodulators shape neuronal circuit function and ultimately behaviour. / Thesis / Doctor of Philosophy (PhD)