Pheromones are powerful attractants. The angle at which each turn is executed is decreased and a zigzag ﬂight pattern results. The turning response, which results in the zigzag pattern, is possibly accomplished in the following manner. As the insect ﬂies across the chemical trail, toward the boundary of the active space, it perceives a decrease in either the chemical concentration or number of filaments; it is then stimulated to turn back toward the center of the trail. The direction in which it turns may be regulated by: 1) chemotactic mechanisms; 2) an anemotactic response; 3) detection of a concentration gradient of odor molecules or odor-filament pulses over time; 4) a preprogrammed turning response in which right turns are alternated with left turns; or 5) a combination of these factors. As the insect progresses along the length of the trail there more.
Fig. 5.3. Proposed ﬂight path of a hypothetical insect ﬂying in a schematic representation of an aerial trail of pheromone. A) area of low concentration and extremely wide active space relative to size of insect: anemotaxis or possibly other mechanisms involved for determining polarity of trail; relatively straight ﬂight path; high ﬂight speed. B) area of medium concentra- tion and narrow active space relative to size of insect: anemotaxis, following of elongate gradient, or other mechanisms involved for determining polarity of trail; determination of lateral boundaries of trail by zigzag ﬂight pattern; decrease in ﬂight speed. C) area of high pheromone concentration and extremely narrow active space relative to size of insect: activation of visual orientation; arrestment of ﬂight and landing.
This is a decrease in 1) its ﬂight speed; 2) the angle of each turn; and 3) the lateral amplitude of the zigzag pattern. When the insect perceives a high concentration of odor (occurring in the vicinity of the emitting insect) it is stimulated to arrest its forward progress and either alight or visually search for the emitting insect or for another appropriate object.
This hypothetical integrated system is based on bits and pieces of information gathered from observations of many different species of insects. It is not our intent to infer that only one mechanism is used by a species under all conditions. Even though it has been demonstrated that a particular species can orient along an odor trail without using anemotaxis, it would be a fallacy to assert that that species does not use anemotaxis under certain environmental conditions. On the contrary, many insects may have a wide potential repertoire of orientation mechanisms. The me- chanisms (singly or in combination) are likely to have evolved in a way that allows them to be varied according to the sensory input arising from the many different internal and external stimuli perceived.
Knowledge of how insects use olfactory orientation mechanisms is still rudimentary. Still other mechanisms have, undoubtedly, yet to be uncovered. The way in which the mechanisms integrate into the complex scheme of orientation for any one species is strictly hypothetical at this time. Many more questions need to be answered before we can accumulate enough information to understand just one integrated system for guidance of an insect to its pheromone-releasing mate.