Pax6 transcription factors are essential upstream regulators in the developing 
anterior brain and peripheral visual system of most bilaterian animals. While a 
single homolog is in charge of these functions in vertebrates, two Pax6 genes 
are in Drosophila: eyeless (ey) and twin of eyeless (toy). At first glance, 
their co-existence seems sufficiently explained by their differential 
involvement in the specification of two types of insect visual organs: the 
lateral compound eyes (ey) and the dorsal ocelli (toy). Less straightforward to 
understand, however, is their genetic redundancy in promoting defined early and 
late growth phases of the precursor tissue to these organs: the eye-antennal 
imaginal disc. Drawing on comparative sequence, expression, and gene function 
evidence, I here conclude that this gene regulatory network module dates back to 
the dawn of arthropod evolution, securing the embryonic development of the 
ocular head segment. Thus, ey and toy constitute a paradigm to explore the 
organization and functional significance of longterm conserved genetic 
redundancy of duplicated genes. Indeed, as first steps in this direction, recent 
studies uncovered the shared use of binding sites in shared enhancers of target 
genes that are under redundant (string) and, strikingly, even subfunctionalized 
control by ey and toy (atonal). Equally significant, the evolutionarily recent 
and paralog-specific function of ey to repress the transcription of the antenna 
fate regulator Distal-less offers a functionally and phylogenetically 
well-defined opportunity to study the reconciliation of shared, partitioned, and 
newly acquired functions in a duplicated developmental gene pair.