In gcmΔP1 robo3 double mutants, some embryonic central nervous system Fas2-positive fascicles collapse across the midline, with full collapse observed in 12.5% of cases and collapse from one side of the embryo (i.e. one hemisegment) in 22.6% of cases. This phenotype is never observed in either mutant alone. In gcmΔP1 robo3 double mutants, 31% of Fas2-positive axons also misroute towards the muscle, which is a phenotype observed with less frequency in gcm mutants. In gcmΔP1 robo3 double mutant stage 12.1 embryos both dMP2/MP1 and vMP2/pCC fascicles are affected, as the vMP2/pCC fascicle collapses completely over the midline (100% penetrance) and the dMP2/MP1 fascicle either does not extend or misroutes towards the muscle (89%). In gcmΔP1 robo3 double mutant stage 14 embryos there is a synergistic effect on both fascicles: the vMP2/pCC fascicle is completely collapsed over the midline (96% penetrance) and the dMP2/MP1 fascicle is missing (97% penetrance). The effect of glial loss is in severe gcmΔP1 robo3 double mutant embryos in which the longitudinal fascicles either collapse along the midline (63.3% penetrance when visualised with Avic\GFPScer\UAS.T:Hsap\Myr2) or misroute severely towards the periphery and exit the central nervous system (13.3% penetrance when visualised with Avic\GFPScer\UAS.T:Hsap\Myr2). However, when MP2 axons in these embryos misexpress leaScer\UAS.T:Hsap\MYC, under the control of Scer\GAL415J2, in the MP2 neurons, they do not extend longitudinally. Instead, they either do not grow (36.6% penetrance) and remain stunted at the base of the commissures, or they leave the central nervous system and misroute towards the muscle (30.7% penetrance). Ectopic expression of leaScer\UAS.T:Hsap\MYC under the control of Scer\GAL415J2 in a robo3 mutant embryo can displace Fas2-positive axons laterally and longitudinally.