344 
Psyche 
[December 
exceptional even though significantly higher in number than on a 
side turn without a trail. 
There are other factors in addition to increased colony activity 
and trail-laying that enhance the efficiency of the recruitment to 
mass-foraging. Some evidence for their existence is supplied by the 
fact that the number of newcomers on a trail side bridge during 
recruitment increases at a rate higher than would be expected from 
a simple proportionate increase to the number of bypassing ants 
(Fig. 7C. Two additional identical experiments gave the same re- 
sult). The recruitment phase had been induced by offering food at 
the end of the blank side bridge in the above described arrangement. 
All ants were allowed to go back and forth on both bridges. Con- 
sidering the actual number of ants going onto both, the trail bridge 
and the food bridge (on which a trail is also established very quickly), 
it is obvious that factors additional to the ones previously described 
are involved in the recruiting mechanism (Fig. 7). They were 
analyzed by behavioral observations. The alerting stimulus of finder 
ants does not only induce increased colony activity, but also increases, 
for a short time at least, the tendency of contacted nestmates to 
deviate from the old pathway (Fig. 3). Hence all ants which had 
been alerted in the vicinity of a side turn have an increased inclination 
to explore onto it. Many times an alerted ant was observed to run 
a fast zigzag course on the old bridge, then come upon the turn 
and head straight to the extremity of the new path. This behavior 
is especially beneficial at the beginning of the recruitment phase since 
it increases the chance of bringing new pioneers to the food while 
the activity in the colony is still low and the trail on the side bridge 
still incomplete. Other properties which improve the efficiency of 
mass-foraging consist of the tendencies of all ants, but predominantly 
of the alerted ones, to contact actively encountered nestmates, to join 
gathering aggregations of ants and to follow closely other nest- 
mates. These mechanisms gain importance as soon as a few foragers 
return from the food bridge to the main bridge where they usually 
go very slowly and accumulate, so that newcomers are easily attracted 
to the area. The influence of the attractive effect of returning ants 
has also been observed on a dead-end trail on a side bridge without 
food. In the experiment all newcomers were removed from the side 
bridge during the first 30 minutes, whereas in a following period of 
36 minutes the scouts were allowed to return from the excursion. 
The activity of the colony stayed at a constant basic level during the 
whole experiment. The rate of newcomers, however, was seen to 
increase during the second period because of the overall attractive 
