Butterfly eyespot morphology appears to be the result of the evolution of an altered version of the regulatory circuit which patterns the wings of other insects. This rogue regulatory circuit is able to pattern both the anterior and posterior eyespots independent of the usual anterior/posterior wing compartmentalization restrictions seen in the fruit fly "Drosophila". The altered regulatory circuit redeploys early developmental signaling sources, like the canonical hedgehog (Hh) pathway, Distal-less (Dll), and engrailed (En), breaking the anterior/posterior compartmentalization restrictions through increased localized levels of Hh signaling. In turn, this raises expression of its receptor Patched (Ptc) and transcription factor. Normally, in "Drosophila", engrailed acts in the posterior compartment to restrict Ptc and "Cubitus interruptus" (Ci) expression to the anterior compartment by repressing transcription of Ci, thereby preventing Ptc expression. From the perspective of evolutionary developmental biology, understanding the redeployment and plasticity of existing regulatory mechanisms in butterfly eyespot locus development has given more insight into a fundamental mechanism for the evolution of novel structures.