CHEMORECEPTION AND THE BEHAVIOR OF INSECTS
171
but the suggestion has been made that olfactory stimulation is not merely
monitoring the central nervous system so that anemotaxis is carried out;
instead, olfactory stimulation at the peripheral level is taking a direct
part in orientation at the same time as the stimuli which are regulating
anemotaxis directly (Otto, 1951). Steiner (1954) has shown that anemotaxis is indeed operating in the instance of Drosophila orienting to an
odor stream. At least part of the stimulation is mediated through the
aristae because removal of the aristae results in a greater velocity of
odorous air current being required for the performance of orientation.
Steiner and Wette (1954) have also shown with the same insect that
the presence or absence of olfactory stimulation influences visual responses. As a consequence of these experiments it can be concluded
that olfactory stimulation activates a visual nonorienting mechanism as
well as anemotaxis.
In view of the foregoing considerations it is now possible to examine
once again the problem of orientation to sex odors produced by female
moths. As was stated at the beginning of this discussion, there is every
reason to believe that the effective distances have been greatly exaggerated. Accurate measurements of the distances over which insects can
orient to odors, including those emanating from females, give the following results: Bombyx mori males to females in flowing air, 200 cm., in still
air, 5 cm. (Sengiin, 1954); clothes moth males to females, 1.5 cm. (Roth
and Willis, 1952b); Drosophila to food, 35 cm. (Flügge, 1934); Habrobracon to host, 3 cm. (Murr, 1930); Geotrupes to dung, 70 cm. in still air
and 4 m. in moving air (Warnke, 1931); male June beetles
[Phyllophaga
lanceolata (Say)] to females and to iso-amylamine, 15-20 ft. in still air,
50-75 ft. in a strong wind (Travis, 1939). Geraniol-baited traps attracted
Japanese beetles over a maximum distance of 300-500 yd. Under these
accurately controlled conditions the effective distances of orientation were
strikingly shorter than distances quoted from other field observations. It
seems clear that the orientation of females from a distance in the field,
even with marked specimens, represents a random wandering until an
odor stream at the effective distance is encountered, whereupon proper
orientation commences. The principal function of the odor appears to be
that of releasing stream orientation. Removal of the odor results in
prompt elimination of stream orientation. Accordingly, a concentration
gradient is not necessary for distance orientation. Only close to the odor
source in a high concentration does a steep gradient seem to be necessary. Thus, in the absence of unshakable evidence that moths truly
orient from great distances it is not necessary to discredit a molecular
explanation of olfactory stimulation in orientation to females. As
Schwinck so rightly pointed out, the simplest and most satisfactory ex-
171
but the suggestion has been made that olfactory stimulation is not merely
monitoring the central nervous system so that anemotaxis is carried out;
instead, olfactory stimulation at the peripheral level is taking a direct
part in orientation at the same time as the stimuli which are regulating
anemotaxis directly (Otto, 1951). Steiner (1954) has shown that anemotaxis is indeed operating in the instance of Drosophila orienting to an
odor stream. At least part of the stimulation is mediated through the
aristae because removal of the aristae results in a greater velocity of
odorous air current being required for the performance of orientation.
Steiner and Wette (1954) have also shown with the same insect that
the presence or absence of olfactory stimulation influences visual responses. As a consequence of these experiments it can be concluded
that olfactory stimulation activates a visual nonorienting mechanism as
well as anemotaxis.
In view of the foregoing considerations it is now possible to examine
once again the problem of orientation to sex odors produced by female
moths. As was stated at the beginning of this discussion, there is every
reason to believe that the effective distances have been greatly exaggerated. Accurate measurements of the distances over which insects can
orient to odors, including those emanating from females, give the following results: Bombyx mori males to females in flowing air, 200 cm., in still
air, 5 cm. (Sengiin, 1954); clothes moth males to females, 1.5 cm. (Roth
and Willis, 1952b); Drosophila to food, 35 cm. (Flügge, 1934); Habrobracon to host, 3 cm. (Murr, 1930); Geotrupes to dung, 70 cm. in still air
and 4 m. in moving air (Warnke, 1931); male June beetles
[Phyllophaga
lanceolata (Say)] to females and to iso-amylamine, 15-20 ft. in still air,
50-75 ft. in a strong wind (Travis, 1939). Geraniol-baited traps attracted
Japanese beetles over a maximum distance of 300-500 yd. Under these
accurately controlled conditions the effective distances of orientation were
strikingly shorter than distances quoted from other field observations. It
seems clear that the orientation of females from a distance in the field,
even with marked specimens, represents a random wandering until an
odor stream at the effective distance is encountered, whereupon proper
orientation commences. The principal function of the odor appears to be
that of releasing stream orientation. Removal of the odor results in
prompt elimination of stream orientation. Accordingly, a concentration
gradient is not necessary for distance orientation. Only close to the odor
source in a high concentration does a steep gradient seem to be necessary. Thus, in the absence of unshakable evidence that moths truly
orient from great distances it is not necessary to discredit a molecular
explanation of olfactory stimulation in orientation to females. As
Schwinck so rightly pointed out, the simplest and most satisfactory ex-