We, people, depend on light. Over 80 % of the information we get is visual. That's why during the night we employ artificial light when we do not sleep, in order to prolong our activity. The dark is associated by human beings with fear, uncertainty and even dread. It's a legacy from the monkey stage: as diurnal creatures, we have a poor night vision, while many of our ancient predators, like leopards, lions and hyenas, see very well in the dark and we would be easy prey in these conditions.
But sunlight is more than a sensory energy: it fuels life on the planet Earth. Using water, carbon dioxide and sunlight, plants store the energy of the sun in biochemical compounds. Plants are eaten by herbivorous animals, these one by carnivorous and this way energy trapped by plants is passed through the whole life chain. But even if animals cannot use the sunlight directly for their own metabolism, they are impacted by it. First, all animals are assigned in two great categories: diurnal, that are active only during the daylight, and nocturnal, that avoid the sunlight. To employ light for orientation, animals have developed sensory organs that capture the light reflected by the objects from their environment: the eyes. The simplest eyes appeared in jellyfish and the most complex are considered those of the cephalopods (cuttlefish, squid and octopus) and vertebrates.
In fact, the alternation day-night regulate life activity on Earth. Some plant saps have different properties before, during and after a sun eclipse.
Many mildews grow differently during the day and during the night, even if apparently they have nothing to do with the light, feeding on dead biochemicals from their environment. Many flowers open before the sunrise (like flowering tobacco), others, only in full sunlight (like the sunflower). Some tropical butterflies will suffer metamorphosis (will pass from caterpillar to adult) only if the day is 17 hours long. The decrease with just one hour of the day's length will stop the metamorphosis. In some animals, the number of immune white cells depends on the day length.
Even the wavelength of the light has its influence. Many algae grow better in blue light, and fungi release their spores under the action of the blue light.
The violet light stimulates the reproduction rate on birds and mammals. The honey bees know by the sun position which moment of the day is, as plants produce nectar more abundantly in specific parts of the day. When the sky is clouded, they employ the ultraviolet light. When a bee finds a food source, it executes the famous 8-shaped dance, with a tilt representing the angle formed by the source with the sun and the horizon. If between the insect and the sky we put a source of polarized light, the insect is clearly disoriented. The bees discern the sunlight spectrum in four portions, each formed by two colors between which they cannot make the difference: red and orange, green and blue, blue and violet and ultraviolet with green and blue. Bees carried from the northern hemisphere to the southern one can orientate correctly only after the third generation. Even hybrids between southern and northern forms have problems.
We know that many species of marine fish, squid or fireflies employ bioluminescence, produced by them or symbiotic bacteria, for attracting mates, catching preys or orientation.
Ants have been found to have a great orientation sense employing the sun position. If in their way the ants encounter a mirror that deviates the sun rays, they change immediately their way. If an ant that displaced itself with 90 degrees to the sun is closed in a box and released, it will have a new angle of displacement, corresponding to the new position of the sun on the sky.
The Arctosa wolf spider, which lives on the edge of the lakes, in case of falling in the water, it will get out always on the same direction from which it has fallen, employing the Sun position. The Geotrupes scarab beetle orientates employing the polarized light of the sky; Talitrus, a small crustacean living on the wet sand of the beaches, employs also polarized light to head itself to the wetness. Elysia, a sea slug, lives between algae; in lab conditions, if just a sole source of light penetrates the aquarium, the animal displaces into the light direction under an angle that varies between 45 and 135 degrees. The recently hatched offspring of the sea turtles head on to the sea, even if this is not visible.
It seems that the light's quality over the sea plays a great role in this orientation.
Birds are known now to rely greatly on the Earth's magnetism for their migrations, but birds kept on aviaries during the spring are often very disoriented if they are exposed to artificial summer or winter skies. Vertebrates have a special gland, epiphysis, which tunes, including in humans, the circadian rhythm. This gland is a rest of the pineal eye, located on the top of the head in primitive amphibians (and some reptiles) and probably could detect danger when the animals were emerging from the water. Only tuatara from New Zealand still has a pineal eye, hidden under the skin.
The epiphysis secretes a hormone called melatonin, which is active only during the night. This gland receives signals from the eye's retina which enables it to know when it is night and when is day. This way, the epihysis reglates the circadian rhythm of all vertebrates (diurnal and nocturnal), mating time, and many cell bioreactions. There are animals living all their lives in the dark: those from caves and those from the ocean and seas abysses. These animals are sensitive to light and many, when exposed to it, can die in a few days. Most are depigmented and blind (like Proteus, the cave salamander) but others (like some cave mosquitoes and abyssal fish and squid) produce bioluminescence.