Box A new career for insects pollination

Pollinating insects existed before the Cretaceous and the radiation of the angiosperms, but their role was minor, feeding at the flowers of some of the advanced gymnosperms. During the Cretaceous, however, there is striking evidence for angiosperm-insect coevolution (Fig. 18.22). Groups of beetles and flies that pollinate various plants were already present in the Jurassic and Early Cretaceous, but the hugely successful butterflies, moths, bees and wasps are known as fossils only from the Cretaceous and Tertiary.

The composition of insect faunas changed during the Cretaceous and Tertiary. One group to evolve substantially at that time was the Hymenoptera - bees and wasps. The first hymenopterans to appear in the fossil record, the sawflies (Xyelidae), had been present since the Triassic. Some fossil specimens have masses of pollen grains in their guts, a clear indication of their preferred diet. The sphecid wasps that arose during the Early Cretaceous had specialized hairs and leg joints that show they collected pollen. Other wasps, the Vespoidea, and the true bees appear to have arisen in the Late Cretaceous.

The first angiosperms may not have had specialized relationships with particular insects, and may have been pollinated by several species. More selective plant-insect relationships probably grew up during the Late Cretaceous with the origin of vespoid wasps that today pollinate small radially symmetric flowers. These kinds of specialized relationships are shown by increasing adaptation of flowers to their pollinator in terms of flower shape, and the food rewards offered, and of the pollinator to the flower. Late Cretaceous angiosperms similar to roses had specialized features that catered for pollinators that fed on nectar as well as pollen.

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