Neuronal wiring diagram of an adult brain

Our connectome includes only chemical synapses; the identification of electrical synapses awaits a future electron microscopy dataset with higher resolution (Discussion). Therefore, we use the term ‘synapse’ to mean chemical synapse. A Drosophila synapse is generally polyadic, meaning that a single presynapse communicates with multiple target postsynapses (Fig. 1e). In FlyWire, a polyadic synapse is represented as multiple synapses, each of which is a pair of presynaptic and postsynaptic locations. Polyadic synapses are common in other invertebrate species, such as C. elegans, and exist in some mammalian brain structures (for example, retina).

We define a connection from neuron A to neuron B as the set of synapses from A to B. A connection typically contains multiple synapses, and the number of synapses can be large (Fig. 3e,f). Connections with fewer than 10 synapses are typical, but a single connection can comprise more than 100 synapses (n = 15,837) or even more than 1,000 synapses (n = 27). The strongest connection that we identified was from a VCN (LT39) onto a wide-field lobula neuron (mALC2), and contained more than 2,400 synapses.

Setting a threshold of at least five synapses for determining a strong connection is likely to be adequate for avoiding false positives in the dataset while not missing connections (Methods). We observed 2,700,513 such connections between 134,181 identified neurons. There are several reasons to focus on strong connections. First, a connection with many synapses is expected to be strong in a physiological sense, other things being equal. Second, strong connections are more reproducible across individuals. Third, higher accuracy (both precision and recall) of automatic detection is expected for strong connections, assuming that errors are statistically independent.

One of the most basic properties of a node in any network is its degree, the number of nodes to which it is linked to. To characterize the degree distribution in the Drosophila connectome, we focused on intrinsic neurons because unlike afferent and efferent neurons, they do not suffer from undercounting of connections owing to truncation.

For any neuron, in degree is defined as its number of presynaptic partners (input neurons), and out degree is defined as its number of postsynaptic partners (output neurons). The median in degree and out degree of intrinsic neurons are 11 and 13 (Fig. 3g), respectively, with the restriction mentioned above to connections involving five or more synapses. These median values do not appear to be substantially different from the median in degree and out degree of 10 and 19, respectively, for neurons in the C. elegans hermaphrodite, considering that it contains several hundred times fewer neurons than Drosophila.

The neuron in the Drosophila brain with maximum degree is a visual GABAergic (γ-aminobutyric acid-producing) interneuron (CT1), with 6,399 postsynaptic partners and 5,080 presynaptic partners (CT1 in the left hemisphere). Most neuropils of the Drosophila brain contain one or a few large GABAergic neurons private to that neuropil, with high in degree and out degree (see Lin et al. for further analysis on connectivity motifs); these neurons are considered to be important for local feedback gain control. The Drosophila brain contains neurons with much higher degree than—for example—the C. elegans hermaphrodite for which the neuron with maximum degree is a command interneuron (AVAL) with 110 postsynaptic partners and 64 presynaptic partners.

The number of synapses established by a neuron is correlated with its total neurite path length (R = 0.80 (presynapse), R = 0.89 (postsynapse); Extended Data Fig. 3g). Presynapse and postsynapse counts are similarly correlated per neuron (R = 0.81; Fig. 3h). We tested whether large neurons tend to use their many synapses to create stronger connections with individual neurons versus more connections with many different neurons. The total number of synapses established by a neuron was much better correlated with its in and out degrees (R = 0.93 and R = 0.94, respectively) than its average connection strength (R = 0.25 and R = 0.3, respectively; Extended Data Fig. 3h,i). This indicates that on average, neurons scale their number of target neurons much more than the strength of an individual connection. It remains to be tested whether the additional target neurons are from the same type or from different cell types.

Connections and neurons are not necessarily the functional units of neural computation. For certain large fly neurons, the arbors are composed of multiple compartments that function somewhat independently. These subcellular compartments, rather than whole cells, should perhaps be regarded as nodes of the connectome. In this case, CT1 would be replaced by many nodes with lower degrees, and the connection from LT39 to mALC2 would be replaced by many connections with fewer synapses between compartments of these neurons. A connectome of neuronal compartments can in principle be studied using our resource, which includes the location of every synapse.