The sense of smell enables animals to react to long-distance cues according to learned and innate valences. Here, we have mapped with electron microscopy the complete wiring diagram of the Drosophila larval antennal lobe, an olfactory neuropil similar to the vertebrate olfactory bulb. We found a canonical circuit with uniglomerular projection neurons (uPNs) relaying gain-controlled ORN activity to the mushroom body and the lateral horn. A second, parallel circuit with multiglomerular projection neurons (mPNs) and hierarchically connected local neurons (LNs) selectively integrates multiple ORN signals already at the first synapse. LN-LN synaptic connections putatively implement a bistable gain control mechanism that either computes odor saliency through panglomerular inhibition, or allows some glomeruli to respond to faint aversive odors in the presence of strong appetitive odors. This complete wiring diagram will support experimental and theoretical studies towards bridging the gap between circuits and behavior.DOI: http://dx.doi.org/10.7554/eLife.14859.001

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Highlights d All ORNs share a common dose-response function with variable sensitivity across odors d Sensitivities across odorants and ORNs follow a power-law distribution d Correlation in sensitivities corresponds to a geometric molecular property d ORN-odorant responses share similar temporal filters

We develop a mean-field theory for Escherichia coli (E. coli) chemotaxis based on the coupled spatio-temporal dynamics of the cell population and the mean receptor methylation level field. This multi-scale model connects the cells’ population level motility behaviors with the molecular level pathway dynamics. It reveals a simple scaling dependence of the chemotaxis velocity on the adaptation rate in exponential gradients. It explains the molecular origin of a maximum chemotaxis velocity. Simulations of our model in various spatio-temporal stimuli profiles show quantitative agreements with experiments. Moreover, it predicts a surprising reversal of chemotaxis group velocity in traveling wave environments. Our approach may be used to bridge molecular level pathway dynamics with cellular behaviors in other biological systems.