Fig. 4. Signalling via two receptor tyrosine kinases, Fgfr and Egfr, controls the directed outgrowth of the primordium of the dorsal air sac in the Drosophila larva. Fgf signalling (green), triggered by the presence of Bnl (blue) at the tip of the outgrowing air sac primordium, controls the direction of the outgrowth. Fgf signalling requires the Ras/MAPK cascade and Pointed (Pnt). Egf signalling is required more widely in the primordium, and regulates cell proliferation and cell death. Egf signalling does not require Pnt (for details, see Cabernard and Affolter, 2005).