Western Prairie Fringed Orchid

by

Harold Richman

While on a five-year voyage around the world on the HMS Beagle, Charles Darwin collected many plant specimens. One of the most interesting was an orchid he found on the island of Madagascar. The orchid had a nectar tube that measured 12 inches long. Darwin searched for an insect that could pollinate this orchid but was unsuccessful. When he reported his find he predicted that the pollinator must be a moth with a 12 inch proboscis. Fifty years later the moth was found and, just as he had predicted, it had a 12 inch proboscis. The orchid is now referred to as Darwin's Orchid (Angraecum sesquipedale). Sesquipedale means "a foot and a half" which is a slight exaggeration. The moth is called Xanthopan morganii praedicta. Praedicta refers to Darwin's prediction.

Recently a similar scenario took place at the tall grass prairie preserve near Tolstoi. As part of a research project, Dr. Bob Jones and Kristie Bertuski are studying the Western Prairie Fringed Orchid which grows in the area. The orchid is on the endangered list and they are trying to find out the conditions required to increase the success rate of survival if planted in other suitable areas. They were unable to find any information on how the orchid was pollinated so they enlisted the aid of Dr. Richard Westwood, an entomologist at the University of Winnipeg.

When Sherlock Holmes arrived on the scene of a crime, he would study the area and advise his companion, Dr. Watson, "We are looking for a white male, 5' 10" tall, wearing army boots, who has just recently returned from India." Dr. Watson would ask, "How could you possibly deduce that from the evidence?" And Holmes would proceed to explain how he had arrived at his conclusions.

Just as Sherlock Holmes would have done, Dr. Westwood studied the orchid and determined that it was pollinated by a night-flying insect. It had to be of a certain size and had to have a proboscis at least three inches long.

To understand his deductions, you have to be familiar with the construction of an orchid. Instead of stamens which contain the pollen, the orchid has a special structure called a pollinia. The pollen grains are contained in this structure and when an insect attempts to reach the nectar the pollinia attaches to the head of the insect. When the insect visits the next orchid the pollinia is deposited on the stigmatic surface of that flower. In this way all the pollen grains are transferred at one time.

Because the fringed orchid has two pollinia, one on each side of the nectar tube, Dr. Westwood knew that the pollinator would have to be large enough for both pollinia to adhere to the insect's head. The nectar tube is three inches long so the pollinator would have to have a proboscis long enough to reach the nectar.

Since the prairie fringed orchid is only scented at night, Dr. Westwood could narrow down the pollinator to a night-flying insect. Just as Darwin had predicted, Dr. Westwood predicted that the pollinator was a moth. By checking the moths that were known to inhabit the area, he found fifteen that were likely candidates. In order to confirm his hypothesis, he set up Malaise traps near the orchids. In plan view, these traps are in the shape of an 'H' constructed of fine mesh. The top of the 'H' has a cone-shaped structure leading to a small opening. A tube is connected to the opening which leads to a glass jar.

When a flying insect encounters a barrier it will fly upward in an attempt to reach the top. You have probably noticed this behaviour if you have had a bee or a wasp trapped in your car. They will try to escape by flying up to the top of the windshield.

Dr. Westwood set up about thirty traps. Every morning he checked to see if any of the moths that could pollinate the orchid were in the jar. As he had predicted, two species of the fifteen were present. These two belong to a family called Sphinx moths. Eight individual moths were caught red-handed. They still had the pollinia attached to their heads.

"Elementary, my dear Watson."

Postscript