The compound lens eye of Strepsiptera: morphological development of larvae and pupae

Adult males of the insect order Strepsiptera have unusual eyes that combine design principles from compound and single-lens eyes. Each unit potentially functions as an independent, image-forming eyelet that contributes a partial image of the visual field. This peculiar organization has led to conflicting views on the origin of strepsipteran eyes, regarding whether their derivation involves ocelli, stemmata or eucone-faceted eyes. This paper investigates the morphological development of strepsipteran eyelets and their neuropils. It suggests that the strepsipteran eye is most likely an adaptation of the compound eye and that strepsipteran eyelets likely have evolved from facets. The strongest evidence derives from similarities in the sequence of eyelet development. This includes the posterior-to-anterior order of maturation of units across the eye, as well as the fact that retinula cell axons follow the innervation of the larval eye. Furthermore, both eye types are characterized by a developmental shift of the lamina and medulla, relative to each other. It is hypothesized that strepsipteran eye evolution involved a reduction in the number of facets, however it remains unclear if eyelets evolved by enlarging individual facets or by the fusion of several facets.