Reflections of an insect's lifestyle and habitat: Morphological and ultrastructural adaptations involving the eyes of insects

Unlike single-lens eyes, compound eyes consist of many ommatidial units. Compound eyes are the insects' most important photoreceptive organ, although in several orders three or two small additional ocelli are present on the frons of the insect's head. Larval eyes, which resemble compound eyes in hemimetabolous but not holometabolous orders, and extraocular photoreceptors complete the armamentarium of light-detecting structures in insects. For nearly 400 million years, compound eyes have served insects leading different lifestyles in different habitats. Specific eye adaptations in phylogenetically distinct taxa are hampered by the eye's small number of cell types and limited genetic plasticity. However, some generalizations are possible regarding aquatic and terrestrial species, nocturnal and diurnal predators, pollinators, phytophagous and parasitic species as well as minute insects and species inhabiting caves and soils or occurring in winter and on snow. As part of this review, we also draw attention to several still open questions.