The relationship between the yucca moth (Genus: Tegeticula) and yucca plant (Genus: Yucca) is believed to have begun at least 40 million years ago and is one of the most cited examples of co-evolution. The yucca plant requires pollination by the yucca moth and moth larvae, in turn, require developing yucca seeds for food - a relationship known as an obligate mutualism. This is actually one of the few documented cases of active pollination because the moth purposefully places pollen on the plants' stigmas. The female yucca moth has specially adapted mouth-parts used for pollen handling. The moth drags its tentacles across the yucca's anthers and collects a large amount of pollen, which the moth then forms into a sticky ball and carries it between its tentacles and thorax. This pollen ball is very large - often made of nearly 10,000 grains of pollen - and can constitute up to 10% of the moth's weight. After collecting the pollen ball, the moth flies to a different yucca plant to deposit its eggs. The moth deposits its eggs in the flower's ovary and uses its tentacles to scrape the pollen ball onto the top of the yucca plant's stigma, pollinating the yucca plant. The moth then climbs to the new flower's anthers and collects pollen, and repeats the entire process.
References: Yuccas, Yucca Moths, and Coevolution: A Review (Olle Pellmyr In: Annals of the Missouri Botanical Garden, Volume 90, pp. 35-55, 2003), The Yucca Plant and the Yucca Moth (M. Ramsay and J. R. Schrock In: The Kansas School Naturalist, Volume 41, Number 2, June 1995), Pollination Partnerships Factsheet (Claire Hemingway, Flora of North America Association, 2004), and Yuccas and Yucca Moths (Lund University)