A team of Janelia researchers reports hitting a critical milestone: they’ve traced the path of every neuron in a portion of the female fruit fly brain they’ve dubbed the “hemibrain.” The map encompasses 25,000 neurons – roughly a third of the fly brain, by volume – but its impact is outsized. It includes regions of keen interest to scientists – those that control functions like learning, memory, smell, and navigation. With more than 20 million neural connections pinpointed so far, it’s the biggest and most detailed map of the fly brain ever completed.