Vein M is the longitudinal vein that ends slightly posterior to the apex of the wing. Crossvein r-m extends between veins R4+5 and M, whereas crossveins bm-cu and dm-cu extend between veins M and Cu1. The vein M ratio is the distance from the junction of bm-cu and M to that of r-m and M divided by the distance from the junction of bm-cu and M to that of dm-cu and M.
