FIGURE 1. How fly eyes differ from those of other arthropods.

a, d, Surface view of a single eye facet (ommatidium) of a compound eye. b, c, e, Ommatidia seen as though sliced through their long axis. a, In the ancestral fly eye and in modern-day insects such as bees and beetles, the light-capturing surfaces (rhabdomeres) are fused in the centre of the ommatidium. b, In 'apposition' eyes, pigment cells sheath each ommatidium, so light is received only down the central axis and image resolution is improved. c, Insects that need good night vision have 'superposition' eyes, which lack most of this pigment, so light is received from all angles. In this case more photons are received, but the resolution of the image is reduced. d, Modern flies and some mosquitoes have open rhabdoms, so that instead of one picture point per ommatidium, they can perceive seven. This requires some rewiring of the underlying neurons so that light received from different angles in one ommatidium can be resolved (neural superposition, e). Zelhof et al.¹ find that expression of the Spacemaker protein is responsible for the opening up of the rhabdomeres.