What’s bugging her, it’s supposed to be a party!

Imagine the best party of the year – it’s cold outside, but hot inside. The food is great, all your friends are there, everyone is receptive to flirtations by the opposite sex, it lasts for two nights in a row where it is, and then picks up in a new location again and again. Where is it and how do I get invited?

The heating of the P. selloum spadix evaporates and spreads a pheromone that attracts male Cyclocephala beetles. Pollination by beetles (cantharophily) is not one of the most common mechanisms for the spread of pollen to ovules; pollination by bees (melittophily), butterflies (psychophily), or even the wind (anemophily) is more common. In most cases of cantharophily, the flowers are big, white, strong-smelling, and the male flowers are usually eaten but the ovaries are protected. This is exactly the scenario in P. selloum.

The Cyclocephala beetles, but there are other examples. Orchids are famous for finicky polli beetles follow the pheromone back to the flower, and after partying, the flower closes down and kicks them out.

As the party winds down in this first P. selloum, the temperature is reduced and the spathe starts to close down on the spadix, forcing the beetles out – it’s closing time, you don’t have to go home, but you can’t stay here (lyrics by Third Eye Blind). As the beetles leave the flower two things happen, first they pass the viscidium, which coats the beetles with a sticky substance, and then they pass by the pollinia, which covers them with pollen grains.

Pollinators can various animals.Non-animal  pollinators work in some cases too, such as wind and water.

This is one type of pollination party, but not the only one. Most plants invite a variety of different pollinators, or a plant might self-pollinate. In some orchids of colder regions where pollinators are particularly rare, self-pollination can be a last resort. If the flower is not visited by any pollinator, the caudicle (the stalk on which the pollen resides) will shrivel up in a particular shape, dropping the pollen directly on the stigma (containing the eggs).

The more pollinators a flower depends on, the more energy the plant must spend on attractants. For instance, some flowers use color and UV patterns, as on the left. Some flowers use nectar and visible light patterns, as with the pitcher plant. The red flower is rafflesia, the largest flower in the world. It smells like rotting flesh to attract flies. Some flowers use mimicry, like the bee orchid on the right, it looks like a female bee and attracts males that will try to mate with it.

Another advantage is seen after the pollen is gathered on the pollinator. In order for a pollination to be successful, the pollen must be delivered to the female organs of a plant of the same species. If a pollinator species has developed a relationship with a certain plant (attracted by a specific odor or color, etc.) then the chances are higher that it will visit another plant of the same species after gathering pollen. This increases the chances of cross-pollination.

Many orchids use a single pollinator. Orchids are the most  diverse flower group, as show by the Dracula orchid,  the spectacle orchid, and the small tongue orchid, left to right.

P. selloum is an interesting exception to the rule of multiple pollinators, but there are other examples. For instance, orchids are famous for finicky pollination. There are >25,000 species of orchids, the largest group of plants (in contrast, all birds represent only ~10,000 species), and single pollinators are responsible for the propagation of thousands of them. In South America and South Africa, the number of single pollinator species is quite high, including many orchid species. (Why? I have no idea.)

The Darwin hawk moth wasn’t known when the star orchid was first described. Charles Darwin just predicted it must exist. Predicting the existence of a moth with 35 cm tongue didn’t win   Darwin many fans, but he was right.

In similar fashion, there are numerous plants that have co-evolved with a pollinator. The Angraecum sesquipedale orchid (star orchid) is a classic example. Charles Darwin was sent several examples of this flower and described them in an 1862 publication. Darwin noted that the nectar of this flower was located deep within a hollow spur. To reach the nectar, a pollinator would bump into the pollen and it would stick.

However, the tube was so narrow, that no known insect could have been considered a pollinator of this plant. Darwin predicted that an insect with a 30-35 cm proboscis (tongue-like appendage) would be found pollinating A. sesquipedale. He was ridiculed for such a bold proposition, but 40 years later, just such an insect was discovered, the Xanthopan morganii praedicta moth (named for Darwin’s prediction).