Jimmy Diehl, age 14, of Denton, North Carolina, for his question:
Why don't snakes close their eyes?
Most animals avert their eyes when we gaze directly at their faces. Unconsciously¬we take this deference for granted. Perhaps this explains why a snake's unblinking stare gives many people the shivers. It also might explain why many people dislike cats. Though Miss Puss has blinkable eyelids, she stubbornly refuses to let any mere human being stare her down. A snake, however, does not go in for staring contests. His eyelids are transparent and sealed shut. He cannot blink or close his eyes even if he tries.
The snakes are the earth's most recent reptiles. Their ancestors developed from lizards about 60 million years ago. A lot of evidence suggests that the first of them descended from burrowing lizards and lived in darkness. Vision was not very important to them. Later in their history, most of the snakes deserted their burrows and crawled forth to face the world. Sight then became, an important feature and various species solved the problem to suit their way of life. Some developed round pupils, others slit pupils. But all of them developed eyelids that never close.
Snake eyelids are transparent membranes, permanently sealed over the eyeballs. When a snake sheds his old skin, the glassy eyelids go with it. The discarded husk looks somewhat like a spacesuit belonging to some scaly alien, complete with sealed .in glassy goggles. A snake leads a scuffy life, crawling among spiky grasses and jagged stones. His special spectacles protect the surface of his vital eyeballs. But like the skin, they get a lot of daily wear and tear. Periodically, the snake grows a new skin and eyelids under the old ones and discards his battered husk all in one piece.
Under the transparent lid, the snake's eyeball moves around and also focuses to take in the view. It has a lens that thickens and thins to adjust to distances. Scientists have found that many daytime snakes have yellow tinted lenses which may cut the glare. The tints are somewhat similar to special spectacles that hunters use to reduce the uneven refractions from the violet end of the light spectrum.
Most of the world's 2,400 snakes do their prowling by day and, as a rule, these species have round pupils. Those that do most of their hunting after dark usually have pupils shaped like vertical slits. Pupils of this type can be opened wide to admit every stray beam of dim light or closed to reduce bright light to a minimum. Snakes with slit pupils include the boas and vipers, the rattlers and other pit vipers. There is every reason why these snaky characters should give us the shivers the sort of shivers that send us scooting elsewhere.
Snakes can see close objects clearly and in detail. But their vision tends to blur with distance. This suits a way of life in which the important items are close by. A snake depends on slow, silent stealth to sneak up on his prey and a galloping gazelle in the distance is way beyond his range of existence.