Pollination

In a dark recess at the base of a fern, a tiny bee began the arduous duty of motherhood by constructing a nest. She first cleared a small area on the ground by using her toothed mandibles to remove, particle by particle, larger bits of soil, then began digging in earnest with both ront and middle pairs of legs. As the small pile of excavated soil became larger, the busy animal gradually disappeared inside the tunnel until finally only the ejected soil particles revealed the hy-menopteran's presence. Working day and night, the minute insect formed a cell that would serve as a nursery for its first progeny. She added the finishing touches by compacting the soil in the chamber with the tip of a blunt abdomen.

With the first task completed, this mother now had to concentrate on the perilous duty of filling the cell with pollen for any offspring. In the dim light of the following morning, the solitary bee prepared to depart. Before that first foray away from the nest, she flew up and circled the area three or four times to memorize the location and then whisked off through the forest. Winding a tortuous way between the huge araucarian trees with their glistening resin-spotted trunks and dodging the hanging lianas, the diminutive insect noticed some small, intricate white flowers wedged between clusters of blue-green leaves.

An assortment of climbing and hovering insects was already busy feeding on the blooms. Metallic-colored beetles with comb-like antennae devoured the pollen while long-legged flies mopped up nectar and miniscule thrips with flashing silvery wings scraped tissue from the petals. Avoiding these, the little bee settled on a group of recently opened flowers and began transferring as much pollen as possible onto the stiff hairs of her hind legs. Even with a full load, she would still need to make another five or six trips before collecting enough pollen to nourish just one larva to adulthood. Arriving back at the nest, the female quickly entered the nursery cell and began scraping off the pollen. As she left, she blocked the nest entrance to foil any thieving insects that might be searching for a free meal by partially filling the entrance hole with a plug of dirt and then placing some debris on top as camaflougue. Then it was back to the forest to search for more flowers.

The journey this time led to a clearing where there was already a tumult of activity, but not just from insects. Ceratopsians were grazing in the undergrowth, consuming everything in their path like lumbermen implementing a clear-cut operation. Browsing indiscriminately on ferns, cycads, horsetails, low-lying conifers, and shrubby angiosperms, the giant herbivores devoured the herbage and shrubs that had regrown since their last visit.

Oblivious to any competition, the hymenopteran went from flower to flower collecting pollen, stopping now and then for a sip of nectar-. Her hairy body was covered with pollen and with each visit to a neighboring blossom, some of these grains were left on the female flower parts. The simple act of cross-fertilization would insure that seeds would be set and a new generation would appear. This was a gift delivered in exchange for pollen and nectar that the bee needed for procreation.

With hind legs loaded to capacity, the hardworking bee returned to the nest. Having now gathered enough provisions for one offspring, some nectar was added to the pollen grains and the mixture was shaped into a little ball. Care was taken to keep this resource from touching the moisture-laden earthen walls since an attack of mold could make it inedible for the young. When satisfied, the female turned and laid a pearly white egg on the top of the food mass, then hastily backed out of the chamber and closed the entry securely by tamping down a layer of soil particles over the hole.

The most significant way insects aided the establishment and spread of flowering plants was by their pollination activities, something the dinosaurs were incapable of doing. Most of the early plants were wind and water pollinated, and neither dinosaurs nor insects took an active part in this process. However, this did not keep any number of insects, from beetles and thrips to flies and wasps, from feeding on pollen from cycads, cy-cadeoids, and conifers. Those ancient associations may have been strictly one-sided, with the insects just eating the pollen and leaving, but at the same time, some could have fortuitously transferred pollen from one plant to the other. We know that grasshoppers and sawflies consumed pollen from Mesozoic gymnosperms since they have been preserved with these grains in their guts.70 Insects that habitually fed on the pollen of certain plants may have become the major or even sole pollinators of those species when the associations became more stable.

By the mid-Cretaceous, many pollen-eating insects added the angiosperms to their list of food plants. While the rate of insect pollination of mid-Cretaceous flowering plants may not have been as high as 70%, which is roughly what it is today, certainly a number of beetles, flies, wasps, and thrips would have dined on this resource.71 Although many of these flower-visiting insects were chance pollinators, some plants undoubtedly depended on them. Probably the ancestors of belid, brentid, and molytine weevils and langurid beetles that currently fertilize cy-cads had already assumed that role in the Cretaceous.4872 The attraction offered these beetles are shelter, food, a place to mate, and a site for larval development. The belid weevils dine on the starch-rich structures (sporophylls) that support the pollen sacs, while the langurid beetles ingest the pollen directly. By thus partitioning the food resources, they can coexist on the same plant. Other types of cycads are pollinated by small molytine weevils whose adults feed on the pollen and larvae consume the starchy stems.

Other gymnosperms in the amber forests were probably pollinated, in part, by nemonychid, belid, carid, and brentid weevils, which today are associated with araucarians, cypresses, and podocarps. Even leaf beetles thought to have developed in the trunks of cycadophytes could have been pollinators.35 It is likely that the Lebanese amber nemonychid weevil may have polli nated the resin-producing tree since they are presently known to develop in male cones of araucarians and other conifers.34

Current associations reported between weevils and palms possibly began in the Cretaceous. Masses of African derelomine weevils gather to feed, mate, and deposit their eggs on male oil palm flowers. There they pick up and later carry the sticky pollen grains to female flowers. This coevolutionary behavior pattern has become so established that mites and nematodes feeding in the palm flowers have come to depend on the beetles to transport them from plant to plant.73 If the weevils suddenly disappeared, so would the mites, nematodes, and possibly the palms.

As angiosperms diversified, so did the pollinating insects. Early angiosperms competed not only with gymnosperms, but also with each other to win the attention of potential pollinators. They developed more tasty rewards, as well as visual and sensual clues in the form of attractive colors and fragrances to draw the insects. And as primitive moths, flies, wasps, and bees became more efficient, a greater percentage of flowers were fertilized and more seed was set and distributed, thus insuring that when new habitats became available, rapidly growing an-giosperms were able to colonize them before gymnosperms.

The most dependable pollinators are those that use this resource to supply protein to nourish their young. They constantly visit a succession of flowers, depositing pollen on the stigmas. Having a messenger deliver the grains, even if some are sacrificed as food, certainly is a superior method than releasing masses of pollen and counting on the wind to distribute them to the right location.

The first steadfast pollinators were probably wasps that substituted plant protein in place of arthropod prey for their young. These wasps could have transported pollen on various parts of their bodies, possibly on their facial hairs, which is how an extant wasp carries pollen to her young.74 The actual method of transfer presumably depended on the size and properties of the pollen (sticky or dry, clumped or separate, etc.).

At some point in the Cretaceous, hymenopterans adapted morphological features that facilitated the collecting of pollen, the most significant of which were branched hairs. Such plumose hairs served as pollen nets to retain the grains and made transportation to larval habitats much easier. Today, plumose hairs occur on all bees, and the transition from wasp to bee regarding this character apparently occurred in the Early Cretaceous. Just when bees appeared has been estimated by molecular studies to be about 125 million years ago.75 These early "protobees," like the one in Burmese amber,44 still retained a few wasp features (color plate 14B).

Cretaceous wasps and primitive bees spent a significant amount of time visiting and pollinating flowers, and the plants responded by making their flowers more attractive. Thus, showy and colored petals, fragrances, oils, and nectaries appeared, the latter not just positioned on or at the base of the petals, but along flower stalks, stems, and leaves. Such props would raise a flag and beckon, just like carnival barkers, "Come and visit me first."

At present bees, with thousands of species, are the most important pollinators in the world, and all depend on angiosperms for survival. Most bees are solitary, as was the Cretaceous Melit-tosphex, and collect pollen from specific plants or plant groups or from plants in special habitats.7677 They often stake out a home range within a specific ecozone like moors, dunes, meadows, or heaths. All tasks depend on the single female, which collects the pollen, carries it to the nest in the ground or decaying wood, stores the grains in cells, deposits an egg in each cell, and then seals up the chambers. Each egg develops into a larva whose food is limited to that stored with it, which usually amounts to just enough nourishment to reach adulthood.

Just when social bees appeared in the past is difficult to say because while the previously reported Cretaceous social bee (Cretotrigona) has been discredited,78 primitive stingless honeybees could have evolved by the Late Cretaceous. Their well-developed societies and requirements for nectar and pollen would have made them efficient pollinators of flowering plants, just as they are in the warmer parts of the world today.79

The new insect pollinators emerging in the beginning of the mid-Cretaceous definitely gave the angiosperms a tremendous advantage. This meant that the flowers did not need to produce copious amounts of pollen to be distributed by wind currents. A small amount of pollen delivered by a dependable insect that had just visited the same type of flower was a much more efficient system, and Melittosphex is proof that this happened some 100 million years ago.44

Just how would these pollinators have affected the dinosaurs? If angiosperms were uncommon at the beginning of the mid-Cretaceous and restricted in their distribution, as has been suggested,69 the dinosaurs would have fed mainly on gym-nosperms. By increasing angiosperm diversity through pollination, insects would have given the flowering plants a competitive edge over the gymnosperms and dinosaurs would have suffered. However, in the Late Cretaceous, when at least some dinosaurs fed on angiosperms, pollinating insects would have assisted them by ensuring additional variety and the spread of flowering plants. Insect pollinators were keystone species that not only benefited the angiosperms, but all insect and vertebrate herbivores that depended on these plants for food, as well as predators and parasites that relied on the herbivores for survival.

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