A. Origin and migration of the BDU and ALM neurons. The ALM touch receptor neurons and the BDU interneurons are lineal sisters (the AB.arpppapp and AB.arppaapp cells divide to give ALMR/BDUR and ALML/BDUL, respectively). The ALMs are generated anterior to the BDUs but migrate posteriorly, while the BDUs migrate anteriorly so that each cell stops near the positions shown. B. Position and axonal morphology of the ALM, AVM and PLM touch receptor neurons. The scale shown at the bottom was used to measure ALM, BDU and AVM positions in the experiments shown below. The ALMs are located around position 9, the AVMs are located between positions 6 and 7 and the BDUs are located between positions 3 and 4 in wild type animals.