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Learn how pheromones play an important role in insects

Our knowledge of pheromones in mammals has been based on more than 32 years of research involving our understanding of insect sex attractants. The moth is a prime example of a species studied for the ...

Our knowledge of pheromones in mammals has been based on more than 32 years of research involving our understanding of insect sex attractants. The moth is a prime example of a species studied for the anatomical and biochemical nature of its chcmosensory system.

A moth has antennae, which, like our nose, is the primary detecting system designed for recognition of chemical stimuli that governs its behavior. It is of interest that the moth has additional pathways that are found in its mouth. In mammals, there are also several olfactory receptor systems, but they are all located anatomically inside the nose. Learn about pheromones communication research in humans.

The primary sensing organ of the moth’s feathery antenna consists of two basal segments involved in movement. However, the key antennal function is in the long segmented agellum that are the bilateral feathery extensions one sees protruding from the head of the moth. The agellum has one million separate organelles, the bulk of which are chemosensory and designed to detect odor.

The antenna in the insect is the equivalent of our human nose, and as in mammals, the moth has an antennal nerve that runs to its brain that is the equivalent of our mammalian olfactory nerve. Associated with this nerve in the insect’s brain, there are spheroidal nerve clusters (glomcruli) of specialized nerve cells that communicate with the insect’s brain centers. As is true in mammals the system for insect olfaction has its distinct functional organization independent of that involved in taste detection.

Taste in insects is found in mouth parts, antenna, and feet, depending on the species.

The olfactory sensory receptor organs are called “senilla” and these insect sensors do not differ in their functional role from the organs of Jacobson found in the vomeronasal organ of the nose that is the sexual odor receptor in mammalian species, we feel, including our own.

In moths, pheromone stimulation is processed by large “macroglomerular complexes” which are similar to the nuclei found in mammalian brains.

These are the centers in the brain that receive the olfactory stimulation and mediate physiologic responses.

The sensilla receptors respond to specic pheromones of dened chemical structure related to the sex of the insect. However, in moths one can graft embryonic cellular antellal disks of the opposite sex into the developing caterpillar. This can result in an experimentally induced disparity between the antennal sexual olfactory receptors and the brain centers responsible for processing and reacting to pheromones. This insect surgery creates what is called a gyandromorph, an insect whose features bear mixed structural elements of both sexes. In effect, by surgically grafting disparate sex receptors, one creates a “homosexual, or bisexual” insect, although this is never ordinarily seen as a normal consequence of insect development in the wild.

As is true in mammalian vomeronasal stimulation, the insect’s excitatory pheromone responses are very specic to the pheromonal odors of the opposite sex. Of interest to mammalian responses to pheromones, receptors can be triggered by particular focused chemical affinities. However, the male insects possess brain nuclei that respond best to the total blend of pheromones produced by the female indicating that there is an integration of responses to odor in the insect brain. The insect’s olfactory neurotransmitters are similar to what are found in humans, although the sensitivity of the system to chemical stimuli in insects is thousands of times more active.

For odor to be perceived in the insect, the pheromones are absorbed to hair-like surfaces on the antennae similar to what are found in the sensory receptors of the human vomeronasal system. The odor molecules penetrate the ne cellular hair wall to interact with nerve receptors that trigger the nerve impulses that send the scent signal to the brain.

There are literally almost as many pheromones governing behavior in insects as there are insects. These pheromonal olfactory stimuli are also pertinent to the sexual behavior of spiders, mites, ticks, and crustaceans, such as lobsters and crabs.

Examples of pheromonal attractants governing insect behavior have been I described in detail by Jacobson (1972) for a wide range of insect species. I A description of how sexual attractants produce their effects is taken from his text, “Male Tergal Glands (situated on the back and exposed by wing raising) in the cockroach serve to maneuver the female into the proper pre-copulatory position which arrests her movement (while she feeds on the tergal gland secretion, ‘or palpates the male’s back) long enough for the male to clasp her genitalia.” The tergal section not only attracts the female by odor, but she feeds on it as he copulates with her. This is perhaps a parallel evolutionary example of the origins of oral sex.

The male sexual odors of insects. are at times strong enough to be perceived by humans. In some cicadae, it has a cinnamon odor; in caddis ies it has a vanilla scent. Attractive substances can be volatile or adhesive to surfaces, and can consist of scales or waxy secretions.

There are specic anatomic sites, depending on the species, which govern sexual interaction and patterns of behavior, depending on the location of the emitters of these attractive scents. These odors are discharged from sites ranging from the tip of the abdomen to glands on legs, back, or head. As in mammals, odor is not the single stimulus to sexual attraction, as reies utilize odor as well as vision to attract each other.

Pheromones as insect signals are not only regulators of sexual attraction, but can stimulate attack and aggressive attitudes, as, for example, in bees. In addition, they provide directional clues to food or dwelling places (i.e., hives or ant hills) or they stimulate nursing behavior in adults to feed pupal embryos or to take care of eggs, as in ants and bees,

†The character of the substances that provide sexual stimulation vary greatly. They range chemically from alcohols to aldehhdes and fatty acids, and these substances are not exclusive to insect sex attraction as related chemicals are found to be sex attractants in mammals as well As an example, butyric acid, which is found in human vaginal secretions is detected by humans at a concentration of 1 x 9 (to the 9"‘) while honey bees can detect it at l.l x 1], representing one molecule diluted in over one billion parts of air ‘or water. A good description of the signicance of insect pheromones is seen in the now classic description by Lewis Thomas of female moth attraction, which, with the wind blowing in the right direction, can attract males from miles away.

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ABOUT THE AUTHOR


Alexander Pommett is an author who studies pheromones for fun.



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