Adventures Among Ants - Mark W. Moffett [70]
The workers of the subterranean army ant Dorylus laevigatus in peninsular Malaysia typically move in a dense mass.
The subs that I found myself observing at Gashaka, like the rubellus colony Caspar and I were following at the same time, lived on the grounds of the field station. This complex consisted of a couple of simple houses for the scientists and a concrete supply building with a veranda complete with two lounge chairs and a dining room table. Nearby was a dirt-floored, thatch-roofed kitchen. Most people would not have taken notice of a muddy basin a meter wide in front of this structure, where the cook discarded his dishwater, which was then slowly absorbed into the ground, leaving scraps of food. But for a biologist in a dry place, any source of nutrients and moisture is worth inspecting. And sure enough, here were the subs, smaller and a brighter orange-red than the rubellus workers. I spent hours watching them, pulling up a log for a comfortable seat. Each time I shifted a spaghetti strand to get a better view, the cook, keeping a safe distance, gave me an odd look—and who could blame him?
Watching the subs emerge from the earth made me wonder what it’s like down below for them. It must be one thing to excavate dirt incrementally, as most ants do while building a nest, and quite another to prowl through the soil for food day in and day out. Human beings rarely travel through the earth: cracks and crevices suited to our size and locomotion are scarce. But for creatures the size of an ant, the soil offers a number of travel options. Often, half of a soil matrix consists of pores, which result from the imperfect packing of different-sized particles, large to small, from sand to silt to clay. Larger organic matter can cement these particles into dirt clods, or peds, with crevices, or voids, between adjacent peds. All of these provide an infinite number of passageways for the subs, and if one of the pores or voids proves too cramped to pass through, they can expand it by raising and lowering their bodies.
Particularly beneficial to subterranean raiders are the cracks that form as soils dry, along with the macro pores that arise from the biological and physical impacts of dissolving minerals, tunneling worms, and decaying roots. Generally such conduits are more continuous than pores or voids, though the raiders still need to remove debris to keep them clear for transit. During such excavations, the soil particles are pushed from worker to worker under their bodies in a kind of bucket brigade.13 This was first observed in a South American army ant by the nineteenth-century naturalist Henry Walter Bates:
In digging the numerous mines to get at their prey, the little Ecitons seemed to be divided into parties, one set excavating, and another set carrying away the grains of earth. When the shafts became rather deep, the mining parties had to climb up the sides each time they wished to cast out a pellet of earth; but their work was lightened for them by comrades, who stationed themselves at the mouth of the shaft, and relieved them of their burdens, carrying the particles, with an appearance of foresight which quite staggered me, a sufficient distance from the edge of the hole to prevent them from rolling in again. All the work seemed thus to be performed by intelligent co-operation amongst the host of eager little creatures; but still there was not a rigid division of labour, for some of them, whose proceedings I watched, acted at one time as carriers of pellets, and at another as miners, and all shortly afterwards assumed the office of conveyors of the spoil.14
Sitting next to the washbasin, roasted by the midday heat, I pondered Bates’s observations. Stefanie’s Asian laevigatus nested in interconnected chambers underground, with one queen and more than three hundred thousand workers.