Neuronal wiring diagram of an adult brain

A basic property of the fly brain is that cell bodies are spatially segregated from neurites. Cell bodies reside near the surface (‘rind’) of the brain (Fig. 3a), surrounding a synapse-rich interior that comprises mainly of entangled neurons and glia, fibre bundles or tracts, and tubules of the tracheal system (Fig. 1f and Supplementary Fig. 3a).

A typical non-sensory Drosophila neuron is unipolar and consists of a primary neurite (also known as cell body fibre) that leaves the cell body (soma), enters the neuropil, and branches into secondary and higher-order neurites (Fig. 3b). Secondary neurites can sometimes be classified as axons if presynapses clearly dominate, or as dendrites if postsynapses clearly dominate. Such an axon–dendrite distinction was made, for example, when defining VPNs and VCNs above.

However, in general, a mixture of presynapses and postsynapses is found on all non-primary neurites (Fig. 3b). In addition, the soma of insect neurons is separated from the main processes (Fig. 3b). Given this structure, the concept that signals pass from dendrites to soma to axon, which is often a good approximation for mammalian neurons, does not apply for non-sensory neurons in the fly.

Neurons vary greatly in size and shape (Fig. 3c). We computed skeletons for all reconstructed neurons (Fig. 3d) to measure neuronal path lengths. The median path length of an intrinsic neuronal arbor was 685 µm (Fig. 3d). It has been argued that branched arbors are optimal for achieving a high degree of connectivity with other neurons. Neurons with short path lengths are interesting exceptions, and can be found in both the optic lobes and central brain. Path length and volume of intrinsic neurons both varied over two orders of magnitude (Fig. 3d; path length percentiles: 0.1%, 0.138 mm; 99.9%, 19.15 mm; volume percentiles: 0.1%, 16 µm³; 99.9%, 3,001 µm³). The whole brain contains approximately 122 million attached presynapses with a total neuronal path length of around 149 m, an average of 0.82 presynapses per micrometre of path length.

Sizes vary significantly between different cell superclasses (Extended Data Figs. 3a–f and 4). Optic lobe neurons are on average much shorter than central brain neurons (0.69 mm versus 2.13 mm on average) and take up a smaller volume (0.0069 mm³ versus 0.0086 mm³ total neuronal volume), which is why the optic lobes dominate the brain by neuron number but not by volume or synapse count. Visual centrifugal neurons are among the largest in the brain, and are larger on average than VPNs (4.92 mm versus 1.56 mm path length on average). We measured much shorter path lengths and volumes for afferent neurons because only part of their axonal arbors is contained within the brain (Extended Data Fig. 3b,e), whereas arbors of efferent, motor and descending neurons which also have some of their arbor outside the brain, were among the largest we measured (Extended Data Fig. 3c,f).

A small fraction of brain volume is made up of glial cells, which are categorized into six types. We estimated that 13% of the cell bodies in the electron microscopy dataset are non-neuronal or glial. Only a few astrocyte-like glia have been proofread (Supplementary Fig. 3b). Sheet-like fragments of ensheathing glia are readily found near fibre bundles in the automated reconstruction. Further proofreading of glia could be prioritized in the future if there is community demand.