Refractive indices n0 and n2 were 1.0 and 1.33, respectively in measurements on isolated corneal lenses, as shown in Figs. 1 and 2. The refractive index n1 was unknown. It has been reported in several insects that a refractive index
of the corneal cuticle is around 1.5–1.6 (e.g., Dethier, 1942). If 1.6 is substituted for a tentative refractive index n1, f0 was estimated to be either 424 ± 21 lm or
434 ± 25 lm, for sections and wholemount preparations, respectively (Table 1). The distance from the posterior principal plane to the posterior margin of the lens (dH0 in
Fig. 1) was obtained from the following equation (Born & Wolf, 1964).