Schematic of the behaviour of MP2 longitudinal axons in different genetic backgrounds. In wild-type embryos, MP2 axons extend along the medial fascicle (A, arrows) and are displaced laterally on expression of robo2 (B, arrows). Lateral displacement still takes place in robo1 mutant embryos (C, arrows). However, if embryos are double mutant for robo1 and gcm and, therefore, lack functional glia, MP2 axons either do not grow (D, empty arrows) or they misroute out of the CNS towards the muscle (D, arrows).