Nectar is a Sweet Bribe for Insects
Nectar is a sweet juice produced by plants. Greek mythology held that nectar was the food of the gods. In the language of poets, sweet or inspiring beverages are sometimes referred to as nectar. For example Hood, in "Hymeneal Retrospections," wrote "Your mouth, it was then quite a bait for the bees, Such nectar there hung on each lip." This plant juice is obviously held in high regard by humans.
In reality, nectar has rather common origins. It is nothing more than a byproduct of the process where plants use sunlight to manufacture tissue from the raw materials carbon dioxide, water and minerals.
Nothing is wasted in nature. So in many plants this unneeded sweet liquid residue is put to good use — as a bribe for insect pollinators. Millions of years ago plants began to take advantage of insects as a way to transmit pollen from plant to plant.
But, insects don't carry pollen around out of the goodness in their little hearts. They need an incentive for providing this service. Plants pay for pollen delivery by providing nectar. The sweet liquid is a wonderful energy source for insects. As a result, insects have a "will work for food" scheme with the plants.
Plants generally use flowers as nectar dispensers. Everything about flowers — their color, shape and even time of blooming — is related to doling out nectar and pollen.
The shape of flowers is such that insects must contact the pollen-bearing anthers on their way to retrieving the nectar. Thus flowers might have long tubes that force the insects to crawl down or extend a tongue down to reach the nectaries. Or the anthers surround the nectaries so the insect must brush them in getting to the nectar.
In general, the colors of flowers are conspicuous targets for flying insect pollinators. Sometimes the color we see is seen by the insects, but many flowers also have patterns that reflect ultraviolet light that we can't see but the insects can.
Some flowers also display ultraviolet markings known as pollen and nectar guides. These markings are like signposts directing the insect to the food resources. While all flowers do not have such guide markings, many more flowers have them than do not. Research has shown that flying insects are first attracted by the overall shape and color of the flower, but once they are close they follow the guide marks to the nectar source.
All of this is a remarkable system used by plants to make sure pollen is carried from plant to plant. The insects are willing workers in the scheme; they just demand to be paid for their services. The nectar used for the payoff is just waste material to the plant, so to get the insect to haul it away and carry the pollen as well is a really sweet deal!