CHEMORECEPTION AND THE BEHAVIOR OF INSECTS
169
windward of the dung, it soon resumes search flight. If it lands to leeward, it orients in the usual way on foot. It is not surprising that an
insect which orients to something which normally lies upon the ground
should have its landing controlled by odor and should be unable to
utilize its vision to prevent mistakes in unusual cases where dung is
suspended in the air. Species which seek out flowers and other sources
of odor above the ground usually employ vision to assist them in locating
the source in the perpendicular.
Schwinck (1954), in her studies of the orientation of males of the silkworm Bombyx mori to the odor of the female, showed that these moths
could orient to an odor source in the absence of an air current but that
orientation was much more efficient with currents. This fact is general.
Here also odor is only the releasing stimulus which initiates
orientation
to air currents. In air without odor, currents have some orienting effect;
but odor stimulation is generally necessary for efficient stream orientation. A strong drop in odor concentration while the insect is following
up an odor stream results in elimination of stream orientation. This
conclusion agrees with Ottp's (1951) observation of the behavior of
DrosophiM
and with the foregoing descriptions of Steiner's (1954)
observations.
It is clear from all of the observations cited that odor initiates stream
orientation, that insects orient to an odor source better in a current than
in still air but that orientation is possible in still air, that removal of the
antennae prevents orientation, and that insects with a single antenna are
still capable of orienting either in still air or in a current. The exact
mechanism of orientation has been the topic of considerable discussion
and experimentation. The questions are simply these: is orientation
accomplished by simultaneous comparison of intensities on both sides of
the body (tropotaxis), by successive comparisons in time (klinotaxis),
or by some type of phobic response whereby the insect is repelled by the
absence of stimulus. The most commonly employed test for tropotaxis
is that of observing the effect of unilateral antennal amputation on the
ability to orient. The parasitic wasp Habrobracon
(Murr, 1930), species
of the dung beetles Geotrupes
(Warnke, 1931), the flesh fly Calliphora
erythrocephala
(Härtung, 1935), and Drosophila (Flügge, 1934) are said
to crawl or fly toward the intact side if one antenna is amputated. This
behavior is construed by the workers mentioned as evidence of tropotaxis.
Otto (1951), on the other hand, pointed out that such behavior truly
cannot be construed as evidence of tropotaxis. Working with Drosophila,
Vespa, and Geotrupes, but principally with Drosophila, he maintained
that the bias to the intact side by unilaterally operated insects is slight
at most, that these insects locate the odor source just as quickly as un-
169
windward of the dung, it soon resumes search flight. If it lands to leeward, it orients in the usual way on foot. It is not surprising that an
insect which orients to something which normally lies upon the ground
should have its landing controlled by odor and should be unable to
utilize its vision to prevent mistakes in unusual cases where dung is
suspended in the air. Species which seek out flowers and other sources
of odor above the ground usually employ vision to assist them in locating
the source in the perpendicular.
Schwinck (1954), in her studies of the orientation of males of the silkworm Bombyx mori to the odor of the female, showed that these moths
could orient to an odor source in the absence of an air current but that
orientation was much more efficient with currents. This fact is general.
Here also odor is only the releasing stimulus which initiates
orientation
to air currents. In air without odor, currents have some orienting effect;
but odor stimulation is generally necessary for efficient stream orientation. A strong drop in odor concentration while the insect is following
up an odor stream results in elimination of stream orientation. This
conclusion agrees with Ottp's (1951) observation of the behavior of
DrosophiM
and with the foregoing descriptions of Steiner's (1954)
observations.
It is clear from all of the observations cited that odor initiates stream
orientation, that insects orient to an odor source better in a current than
in still air but that orientation is possible in still air, that removal of the
antennae prevents orientation, and that insects with a single antenna are
still capable of orienting either in still air or in a current. The exact
mechanism of orientation has been the topic of considerable discussion
and experimentation. The questions are simply these: is orientation
accomplished by simultaneous comparison of intensities on both sides of
the body (tropotaxis), by successive comparisons in time (klinotaxis),
or by some type of phobic response whereby the insect is repelled by the
absence of stimulus. The most commonly employed test for tropotaxis
is that of observing the effect of unilateral antennal amputation on the
ability to orient. The parasitic wasp Habrobracon
(Murr, 1930), species
of the dung beetles Geotrupes
(Warnke, 1931), the flesh fly Calliphora
erythrocephala
(Härtung, 1935), and Drosophila (Flügge, 1934) are said
to crawl or fly toward the intact side if one antenna is amputated. This
behavior is construed by the workers mentioned as evidence of tropotaxis.
Otto (1951), on the other hand, pointed out that such behavior truly
cannot be construed as evidence of tropotaxis. Working with Drosophila,
Vespa, and Geotrupes, but principally with Drosophila, he maintained
that the bias to the intact side by unilaterally operated insects is slight
at most, that these insects locate the odor source just as quickly as un-