Compounds of Orientation: acting at very low concentrations

In the beautiful Italian language that I will never master, the letter s sometimes plays the same role that the a does in turning the word symmetric into its antonym. But sometimes it’s a little more subtle. The translation of spaesato  is disoriented. It’s rooted in the word paese, which means village, and with the s added, it literally means “out of one’s village”. When the word was coined in a map-less world, stepping out of one’s paese could very well have led to disorientation.  But what about the etymology of our own word disorient?  Why does “to orient oneself ” contain the word found in Oriental? Orient is derived from a Latin word signifying to rise, as in sunrise, which occurs in the east, where the Orient is located relative to Europe. If one knows the location of the east, it reveals the other cardinal points and helps with direction.

This somehow reminded me of an insect’s orientation plight. Trail pheromones are compounds that are released by some individuals and which then influence other members. Ants rely on them heavily for orientation but not exclusively. They are also known to rely on visual cues to figure out where to go.

Termites are not ants; they’re more related to roaches. Unless the former are winged, they are eyeless, live in the dark and rely heavily on trail pheromones.  termitesNo matter how they nest or look for food, every species releases the compounds from their abdomen’s sternal gland and uses them for orientation.  The quantities involved are minute: 1 nanogram and 0.02 nanograms for example are effective in the  species Constrictotermes cyphergaster.  In six different families of termites and over 60 species, only 8 different trail pheromone compounds have been identified.  These typically are unsaturated alcohols containing 12 carbons, and often the same compounds serve as sex attractants.

But in 2010, when entomologists tested all of these compounds on false termite workers of Glossotermes oculatus, none of them elicited a response. Instead the substance acting as a pheromone was a ketone. Famous ketones of the natural world include vanillin, spearmint, cortisone and the sex hormones progesterone and testosterone.  Again the amount of compound involved was extremely small, the threshold was only 0.01 nanograms per cm of the trail. A gram of the material–the weight of a 5 dollar bill— would be enough to create a trail a million kilometers in length, almost 1.5 round trips to the moon.

(10Z,13Z)-nonadeca-10,13-dien-2-one
(10Z,13Z)-nonadeca-10,13-dien-2-one, a ketone acting as a trail pheromone in Constrictotermes cyphergaster, a species of termites.

What concentration is involved? The average mass of a termite is 2 mg or 2 X 10 -6 kilograms and given that there are a trillion 10 12ng in 1 kg, the threshold concentration of 0.01 ng/(2 X 10 -6  ) kg is equivalent to 5000 parts per trillion or 5 parts per billion. The argument is often made that pollutants at low concentrations are only picked up because of sensitive instrumentation and that we are alarmed without basis. Such a line of reasoning is speculative. Although pheromones are not pollutants, we still have here an example of how sensitive organisms can be to a relatively small number of molecules on a day-to-day basis.

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