Drosophila connectome
The "Drosophila" connectome, once completed, will be a complete list of the roughly 135,000 neurons in the brain of the fruit fly "Drosophila melanogaster", along with all of the connections (synapses) between these neurons. As of 2020, the "Drosophila" connectome is a work in progress, being obtained by the methods of neural circuit reconstruction. A stack of EM images of an entire brain exist, suitable for sparse tracing of specific circuits. A full connectome of a large portion of the central brain is likewise available. Many of the 76 compartments of the "Drosophila" brain have connectomes available, and  the remainders are subjects of ongoing study. Why "Drosophila". Connectome research (connectomics) has a number of competing objectives. On the one hand, investigators prefer an organism small enough that the connectome can be obtained in a reasonable amount of time. This argues for a small creature. On the other hand, one of the main uses of a connectome is to relate structure and behavior, so an animal with a large behavioral repertoire is desirable. It's also very helpful to use an animal with a large existing community of experimentalists, and many available genetic tools. "Drosophila" looks very good on  these counts: Current status. A high-level connectome, at the level of brain compartments and interconnecting tracts of neurons, exists for the full fly brain. A version of this is available online. Detailed circuit-level connectomes exist for the lamina and a medulla column, both in the visual system of the fruit fly, and the alpha lobe of the mushroom body. In May of 2017 a paper published in bioRxiv presented an electron microscopy image stack of the whole adult female brain at synaptic resolution. The volume is available for sparse tracing of selected circuits. In 2020, a dense connectome  of half the central brain of "Drosophila" was released, along with a web site that allows queries and exploration of this data. The methods used in reconstruction and initial analysis of the connectome followed. A natural question is whether the connectome will allow simulation of the fly's behavior. However, the connectome alone is not sufficient. Additional information needed includes gap junction varieties and locations, identities of neurotransmitters, receptor types and locations, neuromodulators and hormones (with sources and receptors), the role of glial cells, time evolution rules for synapses, and more.