CHEMORECEPTION AND THE BEHAVIOR OF INSECTS
161
burdened ant returns in a straight line, MacGregor then investigated
other variations of oriented running, and found that as a nest remains
longer in a given territory looping decreases and direction improves. He
reasoned that, since a straight course is fashioned out of small segments,
the abolition of loops requires only intermittent stimulation at intersections. As ants continued to come home to a well-established nest they
always passed over one particular spot. The course was tortuous to the
spot, then straight. The spot was odorous, small, colony-specific, and
capable of giving the ant the proper sense of direction. The eventual
odor trail was made up of an accumulation of many of these spots. As
Carthy pointed out, however, the existence of a spot imparting direction
does not solve the problem of orientation in this species. After reaching
a spot the ant does not, according to Carthy, proceed in as straight a
line as would be expected if a kinesthetic sense were involved. Vowles'
(1950) experiments showing that this species can respond to polarized
light and Carthy's failure to demonstrate odor trails by dusting suggest
that visual cues are of considerably greater importance than MacGregor
believed, although MacGregor did demonstrate that at least in initial traillaying vision is of some use. On the other hand, he demonstrated conclusively that a blinded ant responded to the spot in the same manner as
a sighted one; therefore, the sense of direction imparted could not be
a visual one. He also showed that ants which missed the spot and
wandered in a looping path did not pick up the trails of ants returning
directly to the nests even though they crossed these trails. This observation suggests that there are no odor trails, at least at this particular stage.
Again the possibility cannot be overlooked that homing is different when
single ants are studied (as in Carthy's experiments with this species)
and when numbers of ants are employed (as in MacGregor's experiments ).
In any event this type of trail-laying is different from that of such
species as Acanthomyops.
Schneirla (1953), in reviewing the orientation of ants, offered a possible explanation of the use of directional odor
spots on the basis of maze studies with Formica incerta. He found that
this ant uses a learned dependence upon directionalized light, in particular, with a specific reliance upon olfactory cues at difficult critical
points. If a maze had been learned on the basis of light coming from one
direction and then at a junction between a correct turn and a blind
alley an adjacent piece of floor of true pathway and one of blind alley
were exchanged, the ant was greatly disturbed and tended to turn into
the blind alley. In contrast, an exchange of adjacent sections of true pathway caused no noticeable disorientation. The conclusion is that at each
junction the ant has learned a chemical discrimination habit. Such a
learning could be the basis of MacGregor's spot system.
161
burdened ant returns in a straight line, MacGregor then investigated
other variations of oriented running, and found that as a nest remains
longer in a given territory looping decreases and direction improves. He
reasoned that, since a straight course is fashioned out of small segments,
the abolition of loops requires only intermittent stimulation at intersections. As ants continued to come home to a well-established nest they
always passed over one particular spot. The course was tortuous to the
spot, then straight. The spot was odorous, small, colony-specific, and
capable of giving the ant the proper sense of direction. The eventual
odor trail was made up of an accumulation of many of these spots. As
Carthy pointed out, however, the existence of a spot imparting direction
does not solve the problem of orientation in this species. After reaching
a spot the ant does not, according to Carthy, proceed in as straight a
line as would be expected if a kinesthetic sense were involved. Vowles'
(1950) experiments showing that this species can respond to polarized
light and Carthy's failure to demonstrate odor trails by dusting suggest
that visual cues are of considerably greater importance than MacGregor
believed, although MacGregor did demonstrate that at least in initial traillaying vision is of some use. On the other hand, he demonstrated conclusively that a blinded ant responded to the spot in the same manner as
a sighted one; therefore, the sense of direction imparted could not be
a visual one. He also showed that ants which missed the spot and
wandered in a looping path did not pick up the trails of ants returning
directly to the nests even though they crossed these trails. This observation suggests that there are no odor trails, at least at this particular stage.
Again the possibility cannot be overlooked that homing is different when
single ants are studied (as in Carthy's experiments with this species)
and when numbers of ants are employed (as in MacGregor's experiments ).
In any event this type of trail-laying is different from that of such
species as Acanthomyops.
Schneirla (1953), in reviewing the orientation of ants, offered a possible explanation of the use of directional odor
spots on the basis of maze studies with Formica incerta. He found that
this ant uses a learned dependence upon directionalized light, in particular, with a specific reliance upon olfactory cues at difficult critical
points. If a maze had been learned on the basis of light coming from one
direction and then at a junction between a correct turn and a blind
alley an adjacent piece of floor of true pathway and one of blind alley
were exchanged, the ant was greatly disturbed and tended to turn into
the blind alley. In contrast, an exchange of adjacent sections of true pathway caused no noticeable disorientation. The conclusion is that at each
junction the ant has learned a chemical discrimination habit. Such a
learning could be the basis of MacGregor's spot system.