162
V. G. DETHIER
The mode of establishment of the trails by the army ant Eciton is
strikingly different from the methods just described for the collective
routes of other species. The collective trails established by army ants
are developed exceedingly rapidly in new raiding areas and followed
slavishly. As described by Schneirla (1944), "If the routes which spring
up and survive longest during a day of raiding are mapped in detail,
the result is a tree-like figure. The trunk line of this trail system leads
from the bivouac or home site, with more and more frequent branching
as the distance from the bivouac increases. At the very twig end of
the system, usually as much as 150-200 meters from headquarters at the
end of a given day, the newest trails grow outward into unraided ground
from which the insect booty is snatched. Each twig column ends in a
relatively small group of ants, a few dozen of them in an area not more
than 25 cm. wide, behaving in a rather peculiar manner as they push
onward. Hugging close to the ground with her antennae flitting close
to the surface in quick spasms of wasp-like vibratory movement, each
newcomer to the group meanders forward hesitantly a few centimeters
over fresh terrain, then quickly turns and rejoins the base column. In
this way the trails are extended onward, by a relay process in which no
given ant or ants have the lead' for very long but with many ants
participating successively in the trail-blazing process." In swarming out
from their nest the army ants apparently leave behind a chemical trail
several yards wide, but subsequent columns utilize only a portion of
this, and the trunk column nearest the nest follows a single narrow trail.
Schneirla (1953) devised tests to show that the central column utilized
the most saturated portion of the wide trail and that there was a concentration gradient from this portion to the periphery. Ants transported
from the center to various points distant from the center spiral wandered
till the center of the trail was regained. Those nearest the central stalk
generally turned directly toward it and regained it with greater ease than
those transported farther from the center. The greater the distance
from the center, the greater the difficulty experienced in regaining the
central column. From these results Schneirla concluded that there is a
concentration gradient of chemical from center to periphery.
The trails apparently persist for some time and withstand a certain
amount of washing by rain, because a long section of trail may drop
out of use for a period of two or three days and then be used again.
This is in some contrast to the situation described for A.
fuliginosus,
where returning workers begin to lose their way in a maze of interlocking loops after five to six minutes; and where after an hour or so the
trail definitely loses most of its canalizing effect. At the same time it
can no longer be developed by lycopodium powder and appears to have
V. G. DETHIER
The mode of establishment of the trails by the army ant Eciton is
strikingly different from the methods just described for the collective
routes of other species. The collective trails established by army ants
are developed exceedingly rapidly in new raiding areas and followed
slavishly. As described by Schneirla (1944), "If the routes which spring
up and survive longest during a day of raiding are mapped in detail,
the result is a tree-like figure. The trunk line of this trail system leads
from the bivouac or home site, with more and more frequent branching
as the distance from the bivouac increases. At the very twig end of
the system, usually as much as 150-200 meters from headquarters at the
end of a given day, the newest trails grow outward into unraided ground
from which the insect booty is snatched. Each twig column ends in a
relatively small group of ants, a few dozen of them in an area not more
than 25 cm. wide, behaving in a rather peculiar manner as they push
onward. Hugging close to the ground with her antennae flitting close
to the surface in quick spasms of wasp-like vibratory movement, each
newcomer to the group meanders forward hesitantly a few centimeters
over fresh terrain, then quickly turns and rejoins the base column. In
this way the trails are extended onward, by a relay process in which no
given ant or ants have the lead' for very long but with many ants
participating successively in the trail-blazing process." In swarming out
from their nest the army ants apparently leave behind a chemical trail
several yards wide, but subsequent columns utilize only a portion of
this, and the trunk column nearest the nest follows a single narrow trail.
Schneirla (1953) devised tests to show that the central column utilized
the most saturated portion of the wide trail and that there was a concentration gradient from this portion to the periphery. Ants transported
from the center to various points distant from the center spiral wandered
till the center of the trail was regained. Those nearest the central stalk
generally turned directly toward it and regained it with greater ease than
those transported farther from the center. The greater the distance
from the center, the greater the difficulty experienced in regaining the
central column. From these results Schneirla concluded that there is a
concentration gradient of chemical from center to periphery.
The trails apparently persist for some time and withstand a certain
amount of washing by rain, because a long section of trail may drop
out of use for a period of two or three days and then be used again.
This is in some contrast to the situation described for A.
fuliginosus,
where returning workers begin to lose their way in a maze of interlocking loops after five to six minutes; and where after an hour or so the
trail definitely loses most of its canalizing effect. At the same time it
can no longer be developed by lycopodium powder and appears to have