Drosophila melanogaster is a model organism instrumental for numerous biological studies. The compound eye of this insect consists of some eight hundred individual ommatidia or facets, ca. 15 µm in cross-section. Each ommatidium contains eighteen cells including four cone cells secreting the lens material (cornea). High-resolution imaging of the cornea of different insects has demonstrated that each lens is covered by the nipple arrays - small outgrowths of ca. 200 nm in diameter. Here we for the first time utilize atomic force microscopy (AFM) to investigate nipple arrays of the Drosophila lens, achieving an unprecedented visualization of the architecture of these nanostructures. We find by Fourier analysis that the nipple arrays of Drosophila are disordered, and that the seemingly ordered appearance is a consequence of dense packing of the nipples. In contrast, Fourier analysis confirms the visibly ordered nature of the eye microstructures - the individual lenses. This is different in the frizzled mutants of Drosophila, where both Fourier analysis and optical imaging detect disorder in lens packing. AFM reveals intercalations of the lens material between individual lenses in frizzled mutants, providing explanation for this disorder. In contrast, nanostructures of the mutant lens show the same organization as in wild-type flies. Thus, frizzled mutants display abnormal organization of the corneal micro-, but not nano-structures. At the same time, nipples of the mutant flies are shorter than those of the wild-type. We also analyze corneal surface of glossy-appearing eyes overexpressing Wingless - the lipoprotein ligand of Frizzled receptors, and find the catastrophic aberration in nipple arrays, providing experimental evidence in favor of the major anti-reflective function of these insect eye nanostructures. The combination of the easily tractable genetic model organism and robust AFM analysis represents a novel methodology to analyze development and architecture of these surface formations.

Whirligig beetles (Gyrinidae) inhabit water surfaces and possess unique eyes which are split into the overwater and underwater parts. In this study we analyze the micro- and nanostructure of the split eyes of two Gyrinidae beetles genera, Gyrinus and Orectochilus. We find that corneae of the overwater ommatidia are covered with maze-like nanostructures, while the corneal surface of the underwater eyes is smooth. We further show that the overwater nanostructures possess no anti-wetting, but the anti-reflective properties with the spectral preference in the range of 450–600 nm. These findings illustrate the adaptation of the corneal nanocoating of the two halves of an insect's eye to two different environments. The novel natural anti-reflective nanocoating we describe may find future technological applications.

Nanoscale nipple arrays covering the corneal surface of many insects provide antireflection properties and have inspired industrial applications. Based on visual inspection, the dense packing of these nanostructures was initially described to adopt a regular hexagonal order.However, Fourier analysis revealed lack of order over larger distances of the lens cornea, with only patches of hexagonally organized nanostructures. Here we developed a formal mathematical analysis of nippled nanocoatings read by atomic-force microscopy (AFM). This analysis permits automatic assessment of the degree of order in nanostructural packings and its correlation with various characteristics of the nanoscale objects. We applied this analysis to corneae of 17 insect species from 6 orders. We find no correlation between the degree of order and the overall size of the lens. Instead, a strong correlation of the order and the density of the nipple packing exists.Surprisingly, we also see that order correlates with the height of the nanostructures. We discuss these findings in the context of the origin of order in the bio-nanoworld, where order may result largely from the dense packing of the nanostructures, rather than from specialized patterning mechanisms. Our findings uncover mechanisms of order formation, which may also apply to micro-and macro-structures.