Shown for all synapses are the pre- (rows) and post- (columns) synaptic cells, colour-coded by cell type (see Figure 1—source data 1) and arranged in their rostro-caudal sequence along the longitudinal axis (presented in Figure 16—figure supplement 1 is the same matrix with cells sorted into left and right sides). Each intercept is colour-coded for the cumulative depth of presynaptic contacts made by that neuron upon its postsynaptic partner (key, bottom). In the case of dyads or triads, all connections are plotted. Also included are muscle cells, and the basal lamina of the CNS, both of which are exclusively postsynaptic. Other cell types, particularly ependymal cells lacking axons, are excluded. Muscle cells of the dorsal and medial bands are pooled on each side, because these are connected via gap junctions (Bone, 1992). The matrix is bounded by nested boxes between specific cell types. The smaller boxes enclosed by dashed lines indicate the connections between neurons of the same subtype. These are enclosed within boxes bounded by coloured lines, which indicate connections between neurons of the same brain region. Neurons of the brain vesicle are segregated into sensory neurons (orange lines, Sensory), intrinsic interneurons (pink lines, BVIN) and relay neurons (green lines, RN). Remaining boxes are as follows, neurons of the: PNS (PNS); motor ganglion (MG); and tail neurons of the caudal nerve cord (CNC). For matrix file see (Figure 16—source data 1).