Euglena rubra with red eyespot.
The eyespot is a light-sensitive organelle present in
some unicellular organisms, such as euglenoids,
green algae, zoospores,
and some gametes. Eyespots can also be found
on more complex animals that have well-developed eyes. A dragonfly, for
example, has an eyespot on each side of its head in addition to its compound
An eyespot enables a creature to respond to changes in light and dark but cannot gather enough information to form an image.