Segmentation in Drosophila depends upon the striped expression of pair-rule transcriptional regulators. Pair-rule transcripts have localisation signals that direct the mRNA to the apical cytoplasm at the blastoderm stage of development, and on p. 4251, Bullock et al. use functional and phylogenetic analyses to investigate the evolution of this localisation process in flies. They examined pair-rule localisation in multiple Diptera species, both in situ and by injecting Drosophila embryos with fluorescently labelled pair-rule mRNA, and found that pair-rule transcripts are targeted apically in flies with apically positioned nuclei. However, this localisation is less apparent in lower Diptera with more centrally located nuclei, indicating that localisation efficiency correlates with blastoderm architecture. The apical targeting of pair-rule transcripts in Drosophila apparently increases the reliability with which segmentation occurs, as individuals with inefficient pair-rule mRNA localisation are very sensitive to reduced pair-rule gene dose. The authors suggest that this localisation enhances pair-rule activity by increasing the nuclear concentration of the corresponding transcription factors.