168
V. G. DETHIER
Three sensory faculties could obviously be involved: vision, olfaction,
and tactile responses to air currents. Flügge (1934) first showed, and
Otto (1951) and Steiner (1954) confirmed, that Orosophila searching for
food does not employ vision but relies upon a combination of olfactory
and mechanical stimuli. In a current of odorless air Orosophila from
which the wings have been removed travels in paths which have no
particular relation to the air stream. If, however, the flies encounter an
air stream which carries the odor of food such as banana or pear, they
immediately proceed to crawl against the wind and eventually arrive
at the odor source. In an odor gradient in the absence of air currents
Orosophila responds to weak olfactory stimuli by an alerting reaction,
i.e., it moves its antennae, cleans various parts of its body such as legs
and abdomen, and makes a few false attempts at searching. If the odor
is sufficiently concentrated,
the fly can orient and eventually arrives at
the odor source. Under conditions where there is no specific stimulation the fly runs on the average about 1.1 cm./second. In the presence
of odorless wind it runs about 0.9 cm./second; in an odor gradient, about
1.4-1.9 cm./second; and, upon exposure to a flowing odor stream, 2.3
cm./second. Clearly the odor stimulates the fly to greater activity. This
picture holds true generally for most species of insects studied. Stimulation by odor results in orientation to an air stream; cessation of odor
stimulation results in a reversion to a random-like wandering. Steiner
(1953) has investigated this problem in free-flying specimens of the
beetle Geotrupes stercorarius, an insect which in nature seeks dung for
purposes of egg laying. Three phases of its flight were recognized:
the search flight, the approach, and the landing. In the first phase in
still air the beetle flies more or less in circular and figure-of-eight patterns.
These are deformed to varying degrees depending upon the velocity of
the wind. The velocity of flight and the altitude are regulated through
vision and are related to wind velocity. If in the course of this flying
the beetle encounters odor, it turns against the wind and proceeds to
fly upwind on a zig-zag course. During this approach the altitude of
flight is regulated by visual and olfactory stimuli; under normal conditions, mostly by visual cues. As the odor source is approached, flight
becomes slower and lower, so that in the normal course of events the
beetle would come in low over the dung and in the presence of strong
odor concentrations fold its wings and fall to the ground. In an experimental set-up in which dung is suspended in a container over the
ground, the beetle may undershoot and in so passing out of the odorous
area resume its search flight. The beetle can be made to fold its wings
and drop to the ground if a strong odor concentration is wafted upon
it. When the beetle falls to the ground and finds no dung or lands to
V. G. DETHIER
Three sensory faculties could obviously be involved: vision, olfaction,
and tactile responses to air currents. Flügge (1934) first showed, and
Otto (1951) and Steiner (1954) confirmed, that Orosophila searching for
food does not employ vision but relies upon a combination of olfactory
and mechanical stimuli. In a current of odorless air Orosophila from
which the wings have been removed travels in paths which have no
particular relation to the air stream. If, however, the flies encounter an
air stream which carries the odor of food such as banana or pear, they
immediately proceed to crawl against the wind and eventually arrive
at the odor source. In an odor gradient in the absence of air currents
Orosophila responds to weak olfactory stimuli by an alerting reaction,
i.e., it moves its antennae, cleans various parts of its body such as legs
and abdomen, and makes a few false attempts at searching. If the odor
is sufficiently concentrated,
the fly can orient and eventually arrives at
the odor source. Under conditions where there is no specific stimulation the fly runs on the average about 1.1 cm./second. In the presence
of odorless wind it runs about 0.9 cm./second; in an odor gradient, about
1.4-1.9 cm./second; and, upon exposure to a flowing odor stream, 2.3
cm./second. Clearly the odor stimulates the fly to greater activity. This
picture holds true generally for most species of insects studied. Stimulation by odor results in orientation to an air stream; cessation of odor
stimulation results in a reversion to a random-like wandering. Steiner
(1953) has investigated this problem in free-flying specimens of the
beetle Geotrupes stercorarius, an insect which in nature seeks dung for
purposes of egg laying. Three phases of its flight were recognized:
the search flight, the approach, and the landing. In the first phase in
still air the beetle flies more or less in circular and figure-of-eight patterns.
These are deformed to varying degrees depending upon the velocity of
the wind. The velocity of flight and the altitude are regulated through
vision and are related to wind velocity. If in the course of this flying
the beetle encounters odor, it turns against the wind and proceeds to
fly upwind on a zig-zag course. During this approach the altitude of
flight is regulated by visual and olfactory stimuli; under normal conditions, mostly by visual cues. As the odor source is approached, flight
becomes slower and lower, so that in the normal course of events the
beetle would come in low over the dung and in the presence of strong
odor concentrations fold its wings and fall to the ground. In an experimental set-up in which dung is suspended in a container over the
ground, the beetle may undershoot and in so passing out of the odorous
area resume its search flight. The beetle can be made to fold its wings
and drop to the ground if a strong odor concentration is wafted upon
it. When the beetle falls to the ground and finds no dung or lands to