Adventures Among Ants - Mark W. Moffett [19]
I still needed to get a picture of how the raids related to each other. For a week, Paddy joined me at the Botanic Gardens to help me find out what the marauder ants were up to in the long term, in their choice of raiding locations. We mapped raids by marking each path with bamboo skewers emblazoned with neon-colored flags. Within days, the ground around the trunk trail resembled my back after my one session with a Singaporean acupuncturist.
Most often, the raids crisscrossed the belts of land flanking the trunk trail. That is where the pattern became clear. Marauder ant raids moved readily both over virgin soil and across or along the course of prior raids, even ones from a few hours earlier. When a raid passed over an abandoned path, the foragers at the front seldom showed a change in conduct, neither avoiding it nor turning to follow it. On occasion, a raid seemed to retrace an old path a short distance; presumably, there’s a latticework of residual scents from an old raid that must dissipate with time. But in general, each raid went its own way.
This differs from the activities of most ants, such as the seed-harvesting ants of the American Southwest. At first glance, the masses of harvester workers might be mistaken for an army ant raid as they pour out of their nest each day. Actually, though, they are less an advancing army than commuters caught in a traffic jam, reestablishing a trail to areas where they will then scatter to unearth seeds by foraging in the desert sand. Marauder raids resemble those of army ants in not being based on set courses; these mass foragers aren’t obliged to retrace their steps, and they easily cross unfamiliar terrain. The actions of the individual workers may be severely limited, but those of the raid as a whole are not. This is foraging in the pure sense, invoking the freedom to search unknown terrain, in this case moving as one.12
MAKING SENSE OF ANT SCENTS
A year and a half into my Asian sojourn, I made my way by train and bus from Singapore to the island of Penang, Malaysia, where I stayed for a month at a delightful research station on the beach. On several occasions, and with little warning, I was asked by the station manager to vacate my bungalow with its one small bed: a VIP from the American consulate required it for the weekend with his twenty-something “daughter.”
Thus evicted, I would take the opportunity to travel to another rainforest site on the island that was thick with marauder ants. Curious about how the ants communicate to stay in tight formation, on these excursions I studied the marauder ant’s ability to produce chemical trails. With a field microscope, I dissected workers to extract two organs associated with their rudimentary sting: the Dufour’s gland and the poison gland, both known sources of pheromone signals in other ant species. I used a fine forceps to tease free the ant’s infinitesimal glands: thin, translucent sacs small enough to be an amoeba’s luncheon treat. I then smeared the contents on the ground near processions of marauders, to lead the ants where I wanted them to go.
As I hoped they would, the workers dutifully followed the artificial runways. But their reactions indicated that the functions of the two glands differed. They followed the Dufour’s gland trails steadily and accurately and for a long time, which suggests that its secretion is critical in establishing trails, especially stable ones like a trunk trail. By contrast, their response to a crushed poison gland was to run like mad and sloppily follow the route for several seconds. Their brief excitement suggested the poison gland’s contents were reserved for inciting ants to capture prey or destroy an enemy.
This wasn’t enough evidence to produce a set of sound scientific conclusions, but workers engaged in raids were too sensitive to my presence to allow for experiments on them. From watching the workers follow the scent trails I had drawn, I hypothesized that a marauder ant raid is prompted by two trail