Eyespots on butterfly wings are a beautiful example of developmental patterning and likely function to scare off predators. Previous work had implicated the Distal-less (Dll) transcription factor as a crucial regulator of eyespot development, but a more recent study using CRISPR-Cas9 to target exon 2 proposed the opposite (Dll as an eyespot repressor). Now Heidi Connahs, Sham Tlili and colleagues combine genetic and theoretical analysis to interrogate the role of Dll. By injecting guide RNAs targeting distinct exons, the authors generate mosaic animals with clones of crispant cells. Targeting exon 3 leads to loss of eyespots, as well as to split eyespots when mutant tissue bisects eyespot centres. In contrast, targeting exon 2 leads to both loss and gain of eyespots, as well as to changes in eyespot shape; these constructs appear to induce exon skipping after alternative splicing, producing truncated proteins. The authors then describe expression patterns of genes in a putative eyespot patterning network, and integrate these into a reaction-diffusion model that links Dll expression to Wnt and BMP signalling. This model can produce eyespot-like patterns and, crucially, theoretically replicate the variety of Dll crispant phenotypes observed experimentally. This work thus defines Dll as a key activator of eyespots, and sets the scene for further analysis of the patterning networks that restrict its activity to the eyespot.