Pheromones are any chemical signal used to communicate between the members of a species. The existence of pheromones has been studied most thoroughly with respect to insects, but more complex species likely also possess pheromones.
The above diagram, by Peter Fluri, shows a female of the codling moth, Cydia pomonella, raising her abdomen and protruding her sex gland. She takes this calling posture for a short period in the early evening hours.
Codling moth males casting through the apple orchards will eventually be caught in the plume of a calling female get aroused and steer upwind until they find her and mate with her. Same way, many other flies and moths raise their abdomen and release pheromones in order to attract their respective partner for mating. These pheromones can spread to a wide area. The male partners will recognize the pheromones released by their females and they will reach them.
But what do these pheromones actually contain? What is the size of it? Well, though our researchers have cracked many nuts, still there are many questions to be answered. The pheromones released by each organism are different. And pheromones are released for different functions at different circumstances also like trail pheromones, alarm pheromones etc. but yet, a generalized structure of the pheromones have been revealed by our fellow scientists.
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During the past 40 years, pheromones of hundreds of insect species have been chemically elucidated, including the sex pheromone of the codling moth. Its main component is (E,E)-8,10-Dodecadien-1-ol, Codlemone, a primary alcohol containing a straight chain of 12 carbons and two conjugated double bonds. Other moth pheromones are hydrocarbons, epoxides, acetates or aldehydes. These molecules all vaguely resemble fatty acids, from which they are indeed biogenetically derived.
The general size of pheromone molecules can be limited to about 5 to 20 carbons and a molecular weight between 80 and 300. This is because below 5 carbons and a molecular weight of 80, very few kinds of molecules can be manufactured and stored by glandular tissue. Above 5 carbons and a molecular weight of 80, the molecular diversity increases rapidly and so does the olfactory efficiency. Once you get above 20 carbons and a molecular weight of 300, the diversity becomes so great and the molecules are so big that they no longer are advantageous. They are also more expensive to make and transport and are less volatile. In general, most sex pheromones are larger than other pheromones. In insects, they have a molecular weight between 200 and 300 and most alarm substances are between 100 and 200.
Most pheromones consist of blends of two or more chemicals which need to be emitted at exactly the right proportions to be biologically active. The female effluvia or sex gland can contain additional compounds which are related to the pheromone components and whose biological function is often unclear. On the other hand, many attractants of male moths have been discovered simply by field screening. In several cases it could later be shown that the attractant found with this technique was identical to the natural pheromone produced by the female. In most others, the composition of the true pheromone is still unknown. |
