Examples of antennal lobe projection neurons. (A, B, E, F, I and J) Maximum intensity projections of GAL4-driven GFP expression patterns; (A, E and I) include the reference staining and (B, F, and J) show only the GFP signal. (C, G, and K) show color-enhanced direct volume renderings of the signal channel where the signal of interest is false-colored in green and the rest of the pattern is in grey. (D, H and L) Surface reconstructions of the signals of interest in relation to a surface model of the brain. The images in panels (C, D, G, H, K and L) were generated using Amira from the original confocal stacks by segmentation of the neurons of interest using local thresholding, followed by direct volume rendering or surface reconstruction. R14A02 (A–D) and R17G06 (E–H) show examples of the two axon tracts used by most projection neurons, the medial antennal lobe tract (mALT) and the medio-lateral antennal lobe tract (mlALT), respectively. Both lines express in central antennal lobe (AL) glomeruli (R14A02: DL1, D1DA4m, VA7m, VA6; R17G06: DL1, DL5). In R14A02, the axons follow the mALT to the mushroom body (MB) calyx and the lateral horn (LH), while in R17G06 they reach the LH via the mlALT. These patterns are displayed with increased transparency in (D, H, L, M, N and O) where they serve to indicate the positions of the mALT (orange) and the mlALT (purple). R60H12 (I–L) shows a novel population of transverse antennal lobe projection neurons (ALtPN). The cell bodies of these neurons are located ventrally to the antennal lobe (AL) and project into the basal VP3 glomerulus establishing very dense arborizations in the dorso-lateral part of this region. From there they follow the path of the mlALT until they reach the level of the fan-shaped body, where they turn medially, corkscrew halfway around the MB peduncle and arborize densely in a very confined, bar shaped area of the anterior dorso-lateral MB calyx. This region appears not to be the accessory calyx, which is described to be located more medial. Therefore we suggest naming this area lateral accessory calyx (lACA). Earlier descriptions of ALtPNs (; ; ) show a certain similarity to these neurons but the detailed morphology of the cells in R60H12 differ from those in earlier descriptions. Following the nomenclature of we suggest calling these cells AL-t5PN1. In addition to these neurons, a subpopulation in this expression pattern projects along the mlALT to the LH. With the original GAL4 pattern, it is not clear if they are a different population or if the same neurons also bifurcate to the MB calyx and the LH. However, the difference in the diameter of the proximal tracts suggests this expression pattern consists two different cell types. (M–O) 3-D surface renderings from three different angles to display the morphologies of the neurons shown in L. The scale bars in (K) and (L) are 100 μm.