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A Mechanism for Pheromones

Let us discuss male pheromones. Wright (1958) proposed a mechanism by which a ying insect could steer toward a distant pheromone chemical source without the use of anemotaxis. His hypothesis is based...

Let us discuss male pheromones. Wright (1958) proposed a mechanism by which a ying insect could steer toward a distant pheromone chemical source without the use of anemotaxis. His hypothesis is based upon observations of the physical nature of a chemical trail. Wright noted that the spaces between the odorous filaments of the trail are positively correlated with the distances from both the odor source and the longitudinal axis of the odor plume.

He proposed that an insect downwind from the source perceives a series of discontinuous pheromone chemical signals in the form of pulses as it ies through a sequence of laments. When the insect is headed in the direction of the source, it senses a decrease in the time between pulses and is inhibited from turning.

If the insect is headed away from the source, or if the insect approaches the outer boundary of the trail (thus sensing an increase in the time between pulses), it is stimulated to initiate a zigzag ight pattern until it once again senses a decrease in the time between pulses and is thus ying toward the pheromone odor source. Wright then tested his hypothesis on Drosophila melanogaster and found that this species did not fit his model but apparently utilized anemotaxis as its steering mechanism (Wright 1964).

A similar mechanism for detecting the direction of a pheromone odor source may be based on utilization of an elongate concentration gradient. Bossert and Wilson (1963) state that at the downwind limit of its theoretical communication distance (1820 m) a male gypsy moth. It could not conceivably detect a behaviorally significant difference in odor concentration by ying one meter either upwind or downwind. At that distance, the average concentration difference in one meter would be only 0.013%. However, if conditions are examined closer to the source, an insect may be able to detect a difference in concentration over time. The reality of pheromones is profound.

Our study illustrates the theoretical concentration gradient of pheromone emitted by a female cabbage looper moth, Trichoplusia m‘, in a wind velocity of 25 cm/sec. Flight by the male toward the source from six in downwind would result in an increase in concentration of approximately 41% over a one-m distance. A 25% de- crease in concentration would occur during a one-m ight away from the source. Thus, a mechanism allowing detection of the elongate, average concentration gra- dient over time could work within a few meters of the source to indicate the direction toward the source. Flight toward the source would result in a detectable average increase in concentration over some period of time: the insect would, perhapsComputer Technology Articles, not then be stimulated to change course. Flight away from the source would result in a detectable decrease in concentration; perhaps the insect would then be stimulated to turn and y in the opposite direction of the male pheromones.

 They indicate that such a time-dependent mechanism must involve a memory system; i.e. the organism must remember what concentration of chemical it was previously exposed to in order to initiate the appropriate response when it detects a higher or lower concentration.

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ABOUT THE AUTHOR


Alexander P is a blogger from Los Angeles who studies pheromones.



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