Researchers have published the first complete wiring diagram of the central nervous system of a male fruit fly, in a paper that appeared in Cell on 3 September. Earlier maps stopped at the brain. This one takes in the brain, both optic lobes and the ventral nerve cord, the insect's equivalent of a spinal cord, joined — in the Cambridge laboratory's phrase — "through an intact neck". It counts 166,700 neurons and 11,710 cell types recognisable from one animal to the next, wired by some 125 million synapses.
Getting there took most of two decades. The nervous system was cut into 66 pieces and scanned for a year, a slice at a time, each about a ten-thousandth of the thickness of a human hair. Software trained to follow shapes through those images stitched the pieces back into neurons, and people checked the result by hand. The teams put the total at the equivalent of 44 years of human labour. What came out is a map of something the size of a poppy seed, and anyone can open it and look.
The comparison with the female map is what the authors lead with. Around 95 per cent of the cells are common to both sexes, and fewer than five per cent of the male's neurons are sex-specific. But those few are busy: about 12 per cent of male neurons are wired differently, against four in the female. The differences are not spread evenly. Sensory and motor neurons are shared; the divergence sits in the regions that decide what to do, where dense clusters send the same incoming signal into different circuits.
The issue is FLY PAPER: THE WAY OUT, an original cartridge for the Nintendo Entertainment System, and it runs five acts and sixteen stages on one idea: a map of connections is not yet an act. One fly carries the whole chain. It wakes inside a diagram and turns connections until a signal reaches the far side; then it has to fly, and hide, and time a jump between moving leaves, and answer a song it has only heard. Each act asks the same fly for a different thing its body can do, and the garden at the end is where the map stops being a map and becomes somewhere the fly got to on its own.