Remarkable crystal and defect structures in butterfly eye nano-nipple arrays

• Crystal structures in nano-nipple arrays of two butterfly species were identified.
• Lattice characteristics for the hexagonal unit cells are presented.
• The most common defect is the 5–7 coordination defect.
• 5–7 coordination defects form secondary defects such as domain boundaries.
• Some defects are similar to defects in materials.

The corneal nipple structures on the eyes of two nymphalid butterfly species (Nymphalis antiopa and Polygonia interrogationis) are analyzed in terms of nipple arrangements and associated defects. The nipple arrays in both species have close-packed hexagonal lattices with lattice parameters of about 200 nm. The most abundant defects observed are 5–7 coordination defects that generate dislocations, dislocation-type low angle and structural unit-like high angle grain boundaries, as well as closed-loop defects. These disordered structures are compared with imperfections found in other 2D and 3D crystal structures, and it is concluded that the defects in the nipple arrays are likely not due to random growth accidents. Instead, they could be the result of geometric constraints due to eye curvature or serve a yet undiscovered purpose in the optical properties of these eyes.

Extraordinary resemblance between arrays of biological sub-micrometer protrusions and atomic crystal structures is revealed. Hexagonal crystal structure is disrupted by 5–7 coordination defects (dislocations) in various configurations. The most common arrangement are grain boundaries, topologically similar to defects found in graphene and other materials. The origin and purpose of the nano-nipple arrays is discussed with reference to these counterparts.Figure optionsDownload as PowerPoint slide