(A) Schematic showing the FB, its main associated input and output neuropil, and the general types of information thought to be conveyed. Here, the FB is divided into nine vertical columns defined by protocerebral bridge-FB (PB-FB) neurons (see Figure 29), which map the nine glomeruli in the left and right PB to columns in the FB, as indicated by the color of each glomerulus/column (see Figure 30). However, unlike in the PB, the number of FB columns is not rigidly set, but depends on cell type. In addition to columns, FB tangential cells divide the structure into nine horizontal layers. The ventral FB (layers ~ 1–6) receives columnar input from the PB while the dorsal FB (layers ~ 7–9) does not. (B) Morphological renderings of individual columnar neurons (shown in black; red circles are presynaptic sites) from each of the four broad columnar neuron classes. PB-FB-*, FX, vΔ, and hΔ (where X and * stand for an additional, neuron type-specific neuropil). Each class contains many distinct neuron types. The population of neurons comprising each neuron type innervates all columns of the FB, but in a layer-restricted manner. As shown next to each anatomical rendering, a neuron type’s morphology can be summarized by illustrating the location of dendritic (rectangle) and axonal (circles) compartments for the nine FB layers and any associated neuropil. Here, each neuron type is colored according to its class (see legend in D). (C) Same as in (B), but for 2 of the 145 types of FB tangential cells. (D) Schematic showing the innervation pattern of every FB columnar neuron type, each illustrated as in (B). Columnar neurons can be roughly grouped into four putative functional groups: those that convey information from outside the FB to specific FB layers (columnar inputs; subset of PB-FB-* neurons), those that convey information between layers of the FB (intra-FB columnar neurons; vΔ and hΔ neurons), those that convey information out of the FB (columnar outputs; PB-FB-* and FX), and those that could perform a mixture of these functions (input/output). Columnar inputs have axons in every FB layer they innervate, intra-FB columnar neurons have processes confined to the FB, and columnar outputs have dendrites (and very few axons) in every FB layer they innervate. Note that while some columnar types are grouped (e.g., PFNm and PFNp), these types can be distinguished by their connectivity both within and outside of the FB (e.g., PFNm and PFNp receive distinct noduli [NO] inputs). In addition, tangential cells innervating the superior protocerebrum (SMP/SIP/SLP), crepine (CRE), and/or lateral accessory lobe (LAL) (and additional structures) provide input to (left panel) and output from (right panel) specific FB layers. Tangential cells in many different layers send processes to the SMP/SIP/SLP, CRE, and/or LAL, but only consistently target these regions in most cell types in the layers that are shown. See Figure 28—figure supplement 1 for average pre- and postsynaptic counts by region and columnar neuron type.