Pollination by insects (insect) probably occurred in primitive seed plants, reliance on other means being a relatively recent evolutionary (evolution) development. Reasonable evidence indicates that flowering plants first appeared in tropical rain forests during the Mesozoic Era (about 65,000,000 to 225,000,000 years ago). The most prevalent insect forms of the period were primitive beetles (coleopteran); no bees and butterflies were present. Some Mesozoic beetles, already adapted to a diet of spores from primitive plants, apparently became pollen eaters, capable of effecting chance pollination with grains accidentally spared. The visits of such beetles to primitive flowering plants may have been encouraged by insect attractants, such as odours (odour) of carrion, dung, or fruit, or by sex attractants. In addition, visits of the insects to the plants could be made to last longer and thus potentially be more valuable to the plant as far as fertilization was concerned, if the flower had a functional, traplike structure. Nowadays, such flowers are found predominantly, although not exclusively, in tropical families regarded as ancient; e.g., the water lily (Nymphaeaceae) and the arum lily (Araceae) families. At the same time, other plants apparently began to exploit the fact that primitive gall-forming insects visited the flowers to deposit eggs. In the ancient genus Ficus (figs and banyan trees), pollination still depends on gall wasps (gall wasp). In general, Mesozoic flowering plants could not fully rely on their pollinators, whose presence also depended on the existence of a complete, well-functioning ecological web with dung, cadavers, and food plants always available. More advanced flowers escaped from such dependence on chance by no longer relying on deceit, trapping, and tasty pollen alone; nectar became increasingly important as a reward for the pollinators. Essentially a concentrated, aqueous sugar solution, nectar existed in certain ancestors of the flowering plants. In bracken fern even nowadays, nectar glands (nectaries) are found at the base of young leaves. In the course of evolutionary change, certain nectaries were incorporated into the modern flower (floral nectaries), although extrafloral nectaries also persist. Flower colours thus seem to have been introduced as “advertisements” of the presence of nectar, and more specific nectar guides (such as patterns of dots or lines, contrasting colour patches, or special odour patterns 【see photographs-->】) were introduced near the entrance to the flower, pointing the way to the nectar hidden within. At the same time, in a complex pattern of parallel evolution, groups of insects appeared with sucking mouthparts capable of feeding on nectar. In extreme cases, there arose a complete mutual dependence. For example, a Madagascar orchid, Angraecum sesquipedale, with a nectar receptacle 20 to 35 centimetres (eight to 14 inches) long, depends for its pollination exclusively on the local race of a hawkmoth, Xanthopan morganii, which has a proboscis of 22¹/₂ centimetres (nine inches). Interestingly enough, the existence of the hawkmoth was predicted by Charles Darwin (Darwin, Charles) and Alfred Russel Wallace, codiscoverers of evolution, about 40 years before its actual discovery.

In the modern world, bees are probably the most important insect pollinators. Living almost exclusively on nectar, they feed their larvae pollen and honey (a modified nectar). To obtain their foods, they possess striking physical and behavioral adaptations, such as tongues as long as 2¹/₂ centimetres (one inch), hairy bodies, and (in honeybees (honeybee) and bumblebees) special pollen baskets. The Austrian naturalist Karl von Frisch (Frisch, Karl von) has demonstrated that honeybees, although blind to red light, distinguish at least four different colour (colour vision) regions, namely, yellow (including orange and yellow green), blue green, blue (including purple and violet), and ultraviolet. Their sensitivity to ultraviolet enables bees to follow nectar-guide patterns not apparent to the human eye (see photograph-->). They are able to taste several different sugars and also can be trained to differentiate between aromatic, sweet, or minty odours but not foul smells. Fragrance may be the decisive factor in establishing the honeybee's habit of staying with one species of flower as long as it is abundantly available. Also important is that honeybee workers can communicate to one another both the distance and the direction of an abundant food source by means of special dances.

Bee flowers, open in the daytime, attract their insect visitors primarily by bright colours; at close range, special patterns and fragrances come into play. Many bee flowers provide their visitors with a landing platform in the form of a broad lower lip (see photograph-->) on which the bee sits down before pushing its way into the flower's interior, which usually contains both stamens and pistils. The hermaphroditism of most bee flowers makes for efficiency, because the flower both delivers and receives a load of pollen during a single visit of the pollinator, and the pollinator never travels from one flower to another without a full load of pollen. Indeed, the floral mechanism of many bee flowers permits only one pollination visit. The pollen grains of most bee flowers are sticky, spiny, or highly sculptured, ensuring their adherence to the bodies of the bees. Since one load of pollen contains enough pollen grains to initiate fertilization of many ovules, most individual bee flowers produce many seeds.

Examples of flowers that depend heavily on bees are larkspur, monkshood, bleeding heart, and Scotch broom. Alkali bees (Nomia) and leaf-cutter bees (leaf-cutter bee) (Megachile) are both efficient pollinators of alfalfa; unlike honeybees, they are not afraid to trigger the explosive mechanism that liberates a cloud of pollen in alfalfa flowers. Certain Ecuadorian orchids ( Oncidium) are pollinated by male bees of the genus Centris; vibrating in the breeze, the beelike flowers are attacked headlong by the strongly territorial males, who mistake them for competitors. Other South American orchids, nectarless but very fragrant, are visited by male bees (Euglossa (euglossine bee) species) who, for reasons not yet understood, collect from the surface of the flowers an odour substance, which they store in the inflated parts of their hindlegs.

The evolution of moths and butterflies (Lepidoptera) was made possible only by the development of the modern flower, which provides their food. Nearly all species of Lepidoptera have a tongue, or proboscis, especially adapted for sucking. The proboscis is coiled at rest and extended in feeding. Hawkmoths (hawk moth) hover while they feed, whereas butterflies alight on the flower. Significantly, some butterflies can taste sugar solutions with their feet. Although moths, in general, are nocturnal and butterflies are diurnal, a colour sense has been demonstrated in representatives of both. Generally, the colour sense in Lepidoptera is similar to that of bees, but swallowtails and certain other butterflies also respond to red colours. Typically, colour and fragrance cooperate in guiding Lepidoptera to flowers, but in some cases there is a strong emphasis on just one attractant; for example, certain hawkmoths can find fragrant honeysuckles hidden from sight.

Although prevalent in the primitive cycads and in conifers, such as pine and fir, wind pollination (anemophily) in the flowering plants must be considered as a secondary development. It most likely arose when such plants left the tropical rain forest where they originated and faced a more hostile environment, in which the wind weakened the effectiveness of smell as an insect attractant and the lack of pollinating flies and beetles also made itself felt. Lacking in precision, wind pollination is a wasteful process. For example, one male plant of Mercurialis annua, a common weed, produces 1,250,000,000 grains of pollen to be dispersed by the wind; a male sorrel plant produces 400,000,000. Although, in general, the concentration of such pollen becomes very low about one-fourth mile (0.4 kilometre) from its source, nonetheless in windy areas it can cover considerable distances. Pine pollen, for example, which is naturally equipped with air sacs, can travel up to 500 miles (800 kilometres) although the grains may lose their viability in the process. Statistically, this still gives only a slim chance that an individual stigma will be hit by more than one or two pollen grains. Also relevant to the number of pollen grains per stigma is the fact that the dry, glueless, and smooth-surfaced grains are shed singly. Since the number of fertilizing pollen grains is low, the number of ovules in a single flower is low and, as a consequence, so is the number of seeds in each fruit. In hazel, walnut, beech, and oak, for example, there are only two ovules per flower, and, in stinging nettle, elm, birch, sweet gale, and grasses, there is only one. Wind-pollinated flowers are inconspicuous, being devoid of insect attractants and rewards, such as fragrance, showy petals, and nectar. To facilitate exposure of the flowers to the wind, blooming often takes place before the leaves are out in spring, or the flowers may be placed very high on the plant (see photograph-->). Inflorescences, flowers, or the stamens themselves move easily in the breeze, shaking out the pollen, or the pollen containers (anthers) burst open in an explosive fashion when the sun hits them, scattering the pollen widely into the air. The stigmas often are long and divided into arms or lobes, so that a large area is available for catching pollen grains. Moreover, in open areas wind-pollinated plants of one species often grow together in dense populations. The chance of self-pollination, high by the very nature of wind pollination, is minimized by the fact that many species are dioecious or (like hazel) have separate male and female flowers on each plant. Familiar flowering plants relying on wind pollination are grasses, rushes, sedges, cattail, sorrel, lamb's-quarters, hemp, nettle, plantain, alder, hazel, birch, poplar, and oak. (Tropical oaks, however, may be insect-pollinated.)

Because the study of mechanisms of pollination began in Europe, where pollinating birds are rare, their importance is often underestimated. In fact, in the tropics and the southern temperate zones, birds are at least as important as pollinators as insects are, perhaps more so. About a third of the 300 families of flowering plants have at least some members with ornithophilous (“bird-loving”) flowers—i.e., flowers attractive to birds. Conversely, about 2,000 species of birds, belonging to 50 or more families, visit flowers more or less regularly to feed on nectar, pollen, and flower-inhabiting insects or spiders. Special adaptations to this way of life, in the form of slender, sometimes curved, beaks and tongues provided with brushes or shaped into tubes, are found in over 1,600 species of eight families: hummingbirds (hummingbird), sunbirds (sunbird) (see The Rodent That Acts Like a Hippo and Other Examples of Convergent Evolution), honeyeaters (honeyeater), brush-tongued parrots, white-eyes (white-eye), flower-peckers (flowerpecker), honeycreepers (honeycreeper) (or sugarbirds), and Hawaiian honeycreepers (Hawaiian honeycreeper) such as the iiwi (see photo-->). Generally, the sense of smell in birds is poorly developed and not used in their quest for food; instead, they rely on their powerful vision and their colour sense, which resembles that of human (ultraviolet not being seen as a colour, whereas red is). Furthermore, the sensitivity of the bird's eye (eye, human) is greatest in the middle and red part of the spectrum. This is sometimes ascribed to the presence in the retina of orange-red drops of oil, which together may act as a light filter.

Types and agents of pollination are analyzed in K. Faegri and L. van der Pijl, The Principles of Pollination Ecology, 3rd rev. ed. (1979), a well-balanced work based on extensive experience in both tropical and temperate regions; Michael Proctor and Peter Yeo, The Pollination of Flowers (1973), an excellent work on the process of pollination; and Bastiaan Meeuse and Sean Morris, The Sex Life of Flowers (1984).Bastiaan J.D. Meeuse Ed.