1974 ] 
Wilson — Camponotus fraxinioola 
1 8 3 
species might be recognizable, which would then take the name 
impressus. 
The colonies were transferred to glass tubes 15 cm in length and 
3-6 mm in inner diameter. The tubes were each plugged at one end 
with dry wads of cotton wool and placed in open plastic containers, 
the inner sides of which were lined with fluon to discourage climbing 
by the ants. Since the latter method does not always work with these 
highly arboreal ants, the containers were also supported by glass 
bottles set in petri dishes filled with heavy mineral oil. Water was 
made available in moistened cotton plugs at the bottom of test tubes 
placed next to the tubes housing the colonies. The ants were fed 
daily with honey and freshly killed insects. Entire colonies quickly 
habituated to strong light. They could be observed in toto with a 
swing-arm dissecting microscope without being disturbed in their 
new, highly simplified ( and fluon-lined ) universe. 
The Soldier as a Defensive Caste 
Since the time of Forel (1874) it has been known that soldiers of 
the subgenus Colobopsis of Camponotus use their oddly cylindrical 
heads to block the nest entrances. Minor workers returning from 
foraging trips identify themselves to the soldiers, presumably by 
colony odor. The soldiers then pull back to let them enter. C. frax- 
inicola, like most other Colobopsis , nest in the cavities of dead twigs. 
The nest entrances are neat, circular holes into which the heads of 
the soldiers fit snugly. By plugging glass tubes containing colonies 
with disks of cork, I was able to observe the excavation of the nest 
entrances on repeated occasions. The task was performed exclusively 
by the minor workers. This is somewhat surprising in view of the 
fact that the holes are cut to fit the specifications of the soldiers and 
not those of the minor workers. The soldiers also rested more con- 
sistently near the entrances of the glass tubes, with their heads point- 
ing outward in a high proportion of cases, even when they were not 
actively engaged in blocking the entrances. 
Undecane, stored in Dufour’s gland of the abdomen, is a general 
formicine alarm pheromone (Wilson and Regnier, 1971). When 
small quantities of this substance were allowed to evaporate near the 
nest entrance, all members of the fraxinicola colonies were thrown 
into the typical excited running movements of the fraxinicola alarm 
response. But some of the soldiers moved to the nest entrances, fill- 
ing those holes still unplugged at the start of the alarm reaction. 
