A pollinator is the agent that moves pollen from the male anthers of a flower to the female stigma of a flower to accomplish fertilization or syngamy of the female gamete in the ovule of the flower by the male gamete from the pollen grain. Though the terms are sometimes confused, a pollinator is different from a pollenizer, which is a plant that is a source of pollen for the pollination process.
Types of pollinators
The most recognized pollinators are the various species of bees, which are plainly adapted to pollination. Bees are fuzzy and carry an electrostatic charge. Both features help pollen grains adhere to their bodies. Bees often also have a pollen carrying structure such as the corbicula of honeybees and bumblebees (also known as the pollen basket), or the scopa of the lower abdomen of megachilid bees, made up of thick bristles. Bees gather pollen, which is high protein food, to nurture their young, and inadvertently transfer some among the flowers as they are working. Bees need a steady source of pollen to multiply.
Lepidoptera (butterflies and moths) also pollinate. Because they are so long tongued and delicate, they are not major pollinators of our food crops, but are important for some wildflowers.
Many other insects accomplish some pollination. Wasps, bombyliid flies and syrphid flies are important pollinators. Beetles, and even thrips or ants can sometimes pollinate self fertile flowers. Blue bottle or carrion flies are important for some flowers, usually ones that exude a fetid odor.
Bats are important pollinators of some tropical flowers. Birds, particularly hummingbirds also accomplish much pollination, especially of deep throated flowers. One can often guess what are the primary pollinators of a particular blossom by its characteristics, the size, the depth of the corolla, the color (including patterns called nectar guides that are visible only in ultraviolet light), the scent, etc. Flowers have even been known to "trick" a pollinator, for example by emitting a sex pheromone for a bee or moth that entices the male to try to mate with the flower, thus accomplishing pollination.
Even humans can be pollinators, as many gardeners have discovered that they must hand pollinate garden vegetables, because of pollinator decline. This can involve using a small brush or cotton swab to move pollen, or to simply tap or shake tomato blossoms to release the pollen for the self pollenizing. Tomato blossom are self fertile, but have the pollen inside the anther, and the flower requires shaking to release the pollen through pores. This can be done by wind, by humans, or by a sonicating bee (one that vibrates its wing muscles while perched on the flower), such as a bumblebee. Sonicating bees are extremely efficient pollinators of tomatoes, and colonies of bumblebees are quickly replacing humans with vibrators as the primary pollinators for greenhouse tomatoes.
Many kinds of pollinators, from blue bottle flies, to bumblebees, and leaf cutter bees are cultured and sold for managed pollination. Millions of hives of honeybees are also contracted out as pollinators by beekeepers.