Flower visitation by animal pollinators is driven by the production of floral signals and rewards composed of an array of metabolites that entice the animals. ... However, deciphering animal behavioral responses to floral stimuli is not easy. ... Assessing animal responses to floral signals in a natural environment, although complicated by the numerous biotic and abiotic variables, still represents the best strategy to have a holistic view of animal responses to floral metabolic signals

In general, floral signals can be ‘honest signals’ and thus indicate the presence or amount of reward present in a flower. Direct honest signals emanate directly from the reward, such as the scent of nectar and pollen, or humidity  ... An interesting twist to this story is the fact that clearly not all floral signals are honest. For example, many plants with showy flowers never produce any rewards ... 

Such obligatory rewardlessness is especially common among orchids, but has independently evolved in several other plant families. Rewardlessness can be successful when pollination is highly efficient, and the plant does not require many pollinator visits for fertilization of its ovules, and/ or plants produce signals that are highly attractive even without an association with reward.

from 
Borghi, Monica, et al. "The Sexual Advantage of Looking, Smelling, and Tasting Good: The Metabolic Network that Produces Signals for Pollinators." Trends in plant science (2017).

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