Fig. 3: Neuron and connection sizes.

a, The synapse-rich (synapses shown in blue) neuropil is surrounded by a layer of nuclei (random colours) located at the outside of the brain as well as between the optic lobes (purple) and the central brain (blue). b, An LPsP (lateral accessory lobe–posterior slope–protocerebral bridge)¹ neuron can be divided into morphologically distinct regions. Synapses (purple and blue) are found on the neuronal twigs and only rarely on the backbone. c, We selected seven diverse neurons as a reference for d–h. d, The morphology of a neuron can be reduced to a skeleton (left) from which the path length can be measured. The histograms show the distribution of path length (middle) and volume (right; the sum of all internal voxels) for all neurons. The triangles on top of the distributions indicate the measurements of the neurons in c. e, Connections in the fly brain are usually multisynaptic, as in this example of neurons connecting with 71 synapses. f, The number of connections with a given number of synapses. g, In degree and out degree of intrinsic neurons in the fly brain are linearly correlated (R = 0.76). The dashed line is the unity line. Coloured dots indicate measurements of the neurons in c. h, The number of synapses per neuron varies between neurons by more than one order of magnitude and the number of incoming and outgoing synapses is linearly correlated (R = 0.81). Only intrinsic neurons were included in this plot. The dashed line is the unity line. Coloured dots indicate measurements of the neurons in c. Scale bars: 50 μm, b (main image) and c; 10 µm, b (expanded views).