Features within a different region around the perimeter of the head were analyzed by using masks with a radial extent of ∼15 nm and excluding features within the head, thus “scanning” the perimeter of the head in search of GFP-based tag positions. The position and shape of the mask are evident from the noise fluctuations visible in the movie. Most positions show only noise, but occasionally globular densities appear corresponding to the inserted GFP and BFP moieties. This “scanning” classification confirms the positions of tags (colored arrowheads) obtained by difference mapping (Figure S2). See full legend in Supplemental Data for further details.