Neural circuit architecture and evolutionary adaptations in the Drosophila olfactory system This thesis aimed to understand the details of the neural architecture of the olfactory system in flies of the genus Drosophila and its evolutionary adaptation to diverse species specific environmental conditions. In the insect olfactory first relay station the antennal lobe olfactory sensory neurons (OSNs) relay odorant previously induced signals to segregated coding units the olfactory glomeruli. At this level the system already extracts and encodes valuable information such as odor identity concentration or odor location. Most insect’s volatile odorants are encoded in a combinatorial manner i.e. one odorant activates many distinct glomerular coding units which in turn are activated by several odorants. Selected odorants with particular ecological importance are encoded in single and distinct glomerular circuitries. In the first part of the thesis I show in a dense connectome analysis in the fruit fly Drosophila melanogaster that these dedicated glomerular coding units have evolved specific circuit features that might be important to ensure improved accuracy and signal amplification in a single glomerular system. This thesis discovered furthermore a substantial amount of autapses self activating feedback synapses along the large dendrite of a uniglomerular projection neuron which are likely to play a role in neuronal communication and or modulation. In the second part I discovered deep invaginations nearby presynaptic sites of mainly OSNs formed by protrusions from neighboring neurons. The so called “synaptic spinules” play a role in neuronal communication and or modulation. In the last part of my thesis I contributed to a large scale analysis of over 60 species of the genus Drosophila with respect to their phenotypic properties behavior and their olfactory and visual systems. This work provides important information on synaptic circuits and architecture and on the question of how the system might has evolved in the best possible way.