Adventures Among Ants - Mark W. Moffett [29]
With blows from her mandibles, an Acanthognathus trapjaw worker in Costa Rica repels a pseudoscorpion from the tiny hollow twig occupied by her colony. Behind her, a larva feeds on a springtail.
Acanthognathus displays the special skills required for solitary-foraging species to snare these speed demons. Success among springtail-hunting virtuosos depends on stealth and the use of mandibles as an unusual tool. Devices such as trap jaws and stingers are especially common among species with small colonies with only one kind of worker, such as Acanthognathus, whose workers so often need to act alone. Unlike with the antlers of moose or the tusks of elephants, their function is not to impress but to kill and butcher.
“Trapjaw ants” like Acanthognathus have evolved repeatedly among lone-foraging species. Typically, their mandibles are long, with pitchfork-like teeth only at their far ends, and they can open 180 degrees or more. In many cases, the jaws come equipped with trigger hairs. While the ants can be slow, their “bear-trap” jaws are not: the fastest muscular-driven action for any animal is achieved by the jaws of one group of these ants, Odontomachus.10 These speed-biters nab insects and also ply their mandibles as defensive tools, striking them against the ground when harassed; the resulting recoil sends them flying head over heels to safety. In Surinam, I’ve seen schoolchildren, betting over candy, make a game of encouraging the Odontomachus ants’ bouncing behavior while trying to avoid their searing stings.
Long jaws are great for catching prey but impractical at mealtime. Asian Myrmoteras, another group of creeping trapjaw ants that nest in any dark corner of the leaf litter, chew their prey from afar using the spiked tips of absurdly thin mandible blades that they can open an extraordinary 280 degrees. After chewing, they walk forward to place their mouths on the victim and feed at the oozing wound, then circle back to chew some more—the most awkward and labor-intensive approach to dining I have witnessed in all my travels.11
Acanthognathus have a partial solution to this logistical problem. While they use their long jaws to seize skittish springtails, they avoid the arduous dining experience of Myrmoteras by having a face like a Swiss army knife, with an entire arsenal of utensils at their disposal. To eat, they open their jaws wide, revealing a pair of what look like normal mandibles but are actually curved teeth, sprouting near the base of the longer bear-trap blades. The workers masticate their springtail meals to a pulp with these minijaws. As the small jaws are of a piece with the rest of the mandible, chewing with them sets the bear-trap blades waving to such a degree that feeding ants often knock over their neighbors.
Marauder ants have no elaborate built-in tools with which to seize springtails. Instead they must rely on commonplace, workaday mandibles (which have several small teeth along their forward margins, as do those of most ants). Furthermore, the marauder’s massive, frenetic societies are at the opposite extreme from those of the slow, stealthy Myrmoteras and Acanthognathus. The tempo of an ant species tends to relate to its colony size.12 Workers in small societies tend to be slow and cautious—a sensible way to approach elusive prey like the springtail on a low-energy budget. (Is the per capita energy quota of a small colony indeed likely to be smaller than that of a large one? Picturing a colony as a superorganism, a physiologist might predict that this would be the case. Since larger creatures are relatively efficient, burning fewer calories per unit of weight—or when measured microscopically, per cell—this gives them energy to spare.13 We can extrapolate that the same would be true for superorganisms, resulting, for example, in decreased labor demands for each individual in a large nest.14 If so, life must be precarious for Acanthognathus colonies—which, in my experience, are very rare, with no more than eighteen workers