An Eye in the Trees

What am I looking at?

This is the eye and partial third eyelid of a red-eyed tree frog (Agalychnis callidryas). You can see its pupil (1); iris (2); and eyelid (3); plus its third eyelid (4), called a nictitating membrane. Click on the right arrow to see the third eyelid covering its eye.

Biology in the background

The red-eyed tree frog is native to Central America and northwestern South America. Its large red eyes offer a wide field of vision, allow it to see in low light, and are very good at tracking motion. Those capabilities are adaptations given its nocturnal insectivore life.

Most frogs, including the red-eyed tree frog, have a third eyelid that is semitransparent and serves several purposes. First, it protects the eye while the frog is jumping or swimming, while still allowing it to see. Second, it hides the bright color of the eye from predators without obstructing the frog’s vision. And third, when the frog is resting on the surface of a leaf during the day, with the edges of its body pressed tightly against the leaf to minimize casting a conspicuous shadow, the lid can be drawn over the eyeball without compromising the frog’s ability to see approaching danger.

At rest, the frog can even retract its eyeballs into its skull, an ability that also helps it swallow its prey – yes, frogs use their eyes to help push food down their gullet. When they open their eyes, the eyeballs emerge from the sockets like a pair of pop-up headlights, which may startle and disorient a predator.

This frog can grow up to 7.5 centimeters in length, or roughly four times the width of a human thumbnail. Its eyes are about 1 centimeter in diameter, or roughly three-quarters the width of a human thumbnail.

Technique

These images were created using macrophotography.