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 .