Longitude distribution of source counts (in 1° bins) at 70, 160, 250, 350, and 500 μm colour-coded as in Fig. 9. The vertical dashed lines denote the longitude range close to the Galactic centre where SPIRE was used in bright-source mode to avoid saturation and non-linearities in detector response. Labelled dotted lines indicate major spiral arms, tangent point/intersections or star-forming complexes.