The early retinal genes dachshund (dac), eyes absent (eya) and sine oculis (so) are key regulators of adult eye development in Drosophila. Expression data implicate homologs of all three transcription factor genes in vertebrate eye development. However, functional confirmation has thus far only been reported for so homologs. We therefore investigated expression and function of these genes in the red flour beetle Tribolium castaneum. Our results show that Tribolium so and eya are essential for both larval and adult eye development. Moreover, widespread co-expression and indistinguishable knockdown phenotypes suggests eya interaction with so in many tissues. dac expression, by contrast, overlaps with eya and so only in select tissues, which, however, includes the primordia of larval and adult eyes. Notwithstanding, dac knockdown has no detectable effect on larval eye development and causes strong but incomplete adult eye reduction. In a parallel study, we show that dac is essential for adult eye development in combination with the Pax6 transcription factors eyeless (ey) and twin of eyeless (toy). Taken together, our data reveal conservation but also evolutionary plasticity of the Drosophila retinal determination gene network in insects.