A recently published paper investigates how daisies get their spots.
Dark petal spots are a prominent and striking feature of some flowers and inflorescences that often function in pollinator attraction. They occur in a phylogenetically broad range of angiosperm families including the eudicots Asteraceae, Brassicaceae, Geraniaceae, Ranunculaceae, Papaveraceae, Malvaceae, Scrophulariaceae and the monocots Iridaceae, Liliaceae, and Orchidaceae. Several field observations and manipulation studies provide support for the hypothesis that dark petal spots on flower corollas attract pollinators (Free and Williams, 1978Go; Eisikowitch, 1980Go; Dafni et al., 1990Go; Westmoreland and Muntan, 1996Go; Johnson and Midgley, 1997Go; Goldblatt et al., 1998Go; Van Kleunen et al., 2007Go). However, there are many different mechanisms proposed for petal spot attraction. Dark petal spots can intensify the color and texture of the flower, emit scents, produce nectar, trap heat, reflect UV light, or create a tactile guide for navigating the corolla. There can also be a behavioral element of pollinator attraction when the dark petal spots mimic the pollinator itself, inducing aggregating or even mating behavior (Midgley, 1993Go; Johnson and Midgley, 1997Go). For example, the bombyliid fly Usia biocolour was found to preferentially visit flowers of Linum pubescens that had either an artificial ink spot or another fly on one of their petals (Johnson and Dafni, 1998Go). These preferences were particularly pronounced during the afternoon, when the flies switched from primarily feeding to primarily mating behavior. By attracting pollinators through both physiological and behavioral mechanisms, dark petal spots apparently represent an evolutionarily adaptive trait in many different flowering plant species, yet no study to date has investigated the development and micromorphology of petal spot structure.