1968] 
Leuthold — Crematogaster 
239 
most efficient and probably relying to some extent on visual orientation. 
With each run the trail is improved, so that after three return passages 
on the average (6 experiments in which 4, 3, 3, 3, 2 and 1 return 
passages were needed) at standard conditions (see Methods) it was 
efficient enough to guide newcomers accurately to the goal (fig. 5). 
All new ants from the food source contribute in trail-laying. With 
an increasing number of ants, curved paths in the trail become 
gradually flattened out into straight lines. This is an expected result 
since the ants have a tendency to follow each other as well as the 
trail, and they start cutting the curves as soon as they become 
crowded on the trail. The fact that a new shape of trail is estab- 
lished regardless of the original one means that, in this phase, the 
concentration of the trail is still increasing. After V2 to 1 hour no 
further typical trail-laying behavior can be seen, even though there 
is still a high foraging activity. Whenever a well established trail 
is covered by a band of aluminum foil, the ants first hesitate in cross- 
ing over, but very soon they walk over the strip, scattered but holding 
their main direction. Within a few minutes a new trail on the 
obstacle is established by the crowd of crossing ants. However, the 
typical active trail-setting behavior with their hindlegs cannot be 
observed in this case. Hence we have to conclude that footprints 
of lower efficiency are set as well during regular (or at least not 
obviously modified) walking. This “passive” trail-laying, when hun- 
dreds of ants walk over the same place, is efficient enough to main- 
tain, replace or change successively an old trail, whereas the “active” 
trail-laying enables a single ant to establish an efficient new trail, a 
communicative device used primarily during the recruitment phase 
to a new food source. 
LOCALIZATION OF THE PHEROMONE SOURCE 
By selective bioassaying the petroleum ether standard extracts of 
different body parts (see methods) the hind tibiae of C. ctshmeadi 
turned out to contain the trail substance. Dissections of the hindleg 
showed the tendon of the claw to be swollen into a spindle-shaped 
reservoir located in the tibia (pi. 19). As is common in insects the 
claw tendon originates in the femur, with muscle insertions in the 
femur and tibia, and runs through the whole leg down to the terminal 
joint. The actual reservoir is a specially adapted structure seemingly 
adapted to a role in trail-laying. It contains an oil-like liquid. One 
single droplet of approximately 0.03 mm in diameter transferred with 
a pipette into 20 /A of petroleum ether yields 100% positive trail- 
following using 2.4 /xl for a 12 cm long trail, compared with a 
control made up out of haemolymph and muscle tissue from the same 
