Although plants are able to live just about anywhere, they have a major Achilles heel: they can't move. Generally, plants can cope, but when it comes time to reproduce, this sessile nature can substantially impact a plant's ability to get a date. For a long time, plants relied on wind or water to carry the male gamete to the female gamete. Unfortunately, the unthinking forces of nature are generally poor matchmakers. Whether or not a plant that relies on these strategies is successful in producing offspring is dependent on a whole lot of luck. To increase their odds, they'll produce copious amounts of pollen, which often ends up covering cars and tickling noses.
When animals finally came along, plants found ways to put their movement abilities to good use, both as a way to deliver genetic information and then later a dispersal mechanism by seed spreading. The problem with animals, however, is that we have wants and needs of our own and need to be persuaded to do the plants' dirty work. Unlike the wind and the rain, which are free, animals have to eat somehow, and so a payment system was arranged. Typically the reward offered by plants is a sugar-rich nectar that's energy-intensive to produce. For the most part, however, it's worth it – having a courier deliver pollen from plant to plant drastically reduces the amount needed to achieve fertilization, meaning that what a plant spends on nectar it can save on pollen.
Nectar, is still a big risk for the plant. A rich source of energy just open for the taking would quickly be snatched away by undesirables. The nectary must instead be hidden and protected. Consequently, the pollinator requires directions. The result is flowers, and a profusion of them. The advent of animal pollination lead to an explosion in plant diversity as flowering plants began to occupy different niches, woo different partners, and reward them in different ways.
But in any economic system there are the con artists who want something for nothing. Enter the orchids. Either the most diverse or second most diverse plant family (after the daisies), and by the estimation of most the most beautiful, orchids have elevated pollinator attraction to an art form. As it happens, mastery of this skill frequently comes with the complete abandonment of one's morals. Many different species of orchids have developed creative, conniving, and tremendously jerky ways of getting pollinators without having to write a paycheck.
The strategy that is the topic of this post is the ability of some orchid species to trade on another plant's reputation. The trick here is to produce flowers that look like those of an honest species but conveniently forget to include a nectar reward. Because nectaries are typically hidden, the pollinator can't tell whether they'll be tricked until they visit the flower and are dusted with pollen. For example, of the two pictures below, one is the South African iris Watsonia lepida, and the other is its orchid mimic Disa pulchra. If you were the long-tongued fly that pollinates both flowers, which would you visit?
If you picked the flower on the left, you just wasted your visit. Try again. One of these plants is Polygala vulgaris, the other the perfidious and miserly Anacamptis pyramidalis.
Okay, that time I lied. The picture on the left was Anacamptis, and the one on the right was another cheating orchid that flowers at the same time, Orchis mascula. Here's Polygala vulgaris:
This just illustrates the problem with the cheating strategy: if too many hucksters try it, then they spread the marks too thin. Or worse, the marks get wise and just stop trusting both the rewarding and the non-rewarding flowers. Rewardless flowers are a jerk move that, at the same time, paint the plants who use them into a corner. All rewardless orchids are limited in population by the plant that supports their pollinators. If they grow too numerous, the pollinators will have a bad year, which will in turn crash the populations of both orchid and its innocent patsy the next. Some orchids get around this by going after generalist pollinators instead, but animals possess an advantage that the orchid, however cunning it may be, never will: brains. Generally the mimic con only works in the early portion of the growing season, at which point the pollinators wise up and stop visiting their flowers.