Original: Refractive cornea In the eyes of most terrestrial vertebrates (along with spiders and some insect larvae) the vitreous fluid has a higher refractive index than the air, relieving the lens of the function of reducing the focal length.
Simplified: Refractive cornea The eyes of most land-living vertebrates (as well as those of some spiders, and [[insect larvae) contain a fluid that has a higher refractive index than the air.