Afocal apposition optics in butterfly eyes

In most apposition compound eyes there are two components to the optical system of the ommatidium1,2, the cornea and the crystalline cone. The focusing power of the cornea is well documented3,4 whereas the crystalline cone is usually regarded as a mere optical spacer5,6; consequently, the ommatidial optics will consist of a simple focusing lens. To the contrary, we now demonstrate the existence of a complete afocal telescope in each ommatidium of butterfly apposition eyes. The optical system is an extreme variant of that found in refracting superposition eyes, thereby providing a connection between butterflies and moths.