Morphological and optical properties of the corneal lens and retinal structure in the posterior large stemma of the tiger beetle larva

The morphological and optical features of the corneal lens and retina have been examined in the posterior large stemma of the larva of the tiger beetle (Cicindela chinensis). A cup-shaped retina was positioned 55 ± 6 μm beneath the posterior margin of the corneal lens, which was 479 ± 20 μm in diameter and 391 ± 18 μm in thickness (n = 41). A light path through an isolated corneal lens showed that the object at infinite distance was focused on the distal margin of the retina. Geometrical optics gave a value of 334 ± 15 μm (n = 55) for the posterior focal length of the corneal lens. The refractive index of the corneal lens was estimated to be around 1.8, if the lens was considered to be homogenous in structure. The internal structure of the lens, including concentric lamellae, was presumed to contribute to such a high refractive index, because this was higher than that of insect cuticle. The retinal structure and how images were blurred at different focus levels were also examined. Data obtained for optics of the corneal lens and retinal structures are discussed with reference to the distinct visual behavior of the larva.