Essays

Wild On The Prairie: Invertebrates

The ecology of ants as discovered by children
November 12, 2000

Everyone has seen pictures of jungle army ants marching, marching, marching, carrying pieces of a leaf, back to their fungus farms deep underground. Occasionally I find a mesquite stripped of its leaves. We have a fungus farming ant here in Midland, but I have only once found the chimney turret of their nest under a mesquite stripped leafless. Later, I perused a scientific article on Trachemyrmex, which stated that they cultivate their fungus on litter (dead insects, pieces of skin from carrion, dung, owl pellets, and so on, but not leaves). Oh, well.

There is another possible answer. Imagine this: a thick-skinned, drab little caterpillar, which has just hatched from an egg laid on a mesquite leaf, eats busily away. In a couple of days, it sheds its skin in preparation for its second instar. Attired in nice fresh skin, it begins to protrude two pairs of tentacles -- tentacles with two hard structures. The hard structures vibrate over the head and two things occur -- first, a pheromone is released, and second, a vibration is sent through the leaves and branches.

Humans cannot hear or smell either phenomenon. My ol' buddy Crematogaster comes running. When the ant arrives, it finds droplets of clear liquid oozing from two openings near the tail of the caterpillar. The ant greedily licks the liquid while sitting atop the caterpillar. If there is not enough liquid to satisfy the ant, it stomps on the back of the caterpillar, drumming to the rhythm of the caterpillar's vibration.

That is not the whole story, though. I must first explain why I call a small red and black ant my buddy. One year during summer camp at the Sibley Nature Center, I took a group of eight- and nine-year-olds out to find insects in a twenty-five mile-per-hour wind. We swept our nets through the bunchgrass and found almost nothing. (Not even a crab spider can find purchase on a dancing and whipping plant stalk). In desperation, I led the group to a dense mesquite thicket where, again, we found nothing. At first.

"Mr. Williams! Look on the green beans. Ants!"

"Mr. Williams! They are red and black, and are sticking their butts in the air!"

"Oh, so it is Crematogaster. That is the way you identify them. Every animal has a behavioral trait that gives its identification away. Birders call it their "jizz."

"Mr. Williams -- they are eating little bugs!" Well, not quite. I mentioned Crematogaster in a story about a Purple Thistle. The ants were licking the rear ends of leafhoppers, collecting honeydew. This "gross" fact attracted the kids into a thorough study of the mesquite beans. We found thirty species of insects on or near the beans -- a complete food web, with six different trophic levels (including parasites and two levels of predators). Crematogaster saved the day, so I have always been grateful to my little buddies.

Oh yeah, back to the little caterpillar. I believe something like the following probably happens, but I admit that I have never observed it. My speculation is based on similar behavior that takes place between ants and caterpillars of other species. Hopefully, some kid will turn off the television next summer and wander outside to go daydream under a sprawling mesquite. That kid might just discover something that no one else has seen (or at least documented in scientific literature.

Certain cousins of our local ants are symbiotic protectors of the caterpillars. During the growth cycle of the caterpillars, the ants accompany them and even seem to herd them about in response to the actions of the caterpillars’ predators. The nocturnal or crepuscular defoliation of the mesquite may represent a way to avoid predators.

In this related species, the ants construct tiny chambers just under the soil surface where the caterpillars are hidden when they are not feeding. I have seen long-legged ants known as “spider ants” carry the drab little caterpillars, but they might have been wishing to eat them. Other predators are likely to be parasitical wasps seeking to deposit eggs under the surface of the caterpillar’s skin, as well as birds that wish to eat them. Paper wasps also love the little caterpillars, which they take back to the nest to tear apart and feed to their larvae.

Among the allied species, ants come out of the chamber first and run all over the caterpillar's host plant. If no predators are present to chase them, they come back for the caterpillar and lead it to a branch with plenty of leaves. Before daybreak it is herded back to the chamber to hide once again.

There is a changing of the guard every day or two -- another small group of ants takes over and guards the caterpillar, while the others return to the nest. This makes me wonder if they are going home to "sleep off the honeydew?" Is honeydew like a drug or alcohol to the ants? Can ants get drunk?

When the first generation of caterpillars is ready to pupate, the chrysalides have similar glandular openings to continue releasing pheromones so that the ant will keep protecting it in the chamber. Sometimes several caterpillars and chrysalides can be found in one chamber. The ants help the tiny blue butterfly scrape its way out of the dirt, and off it goes.

In the fall the caterpillar that is being tended by the ants is led to their nest. In the allied species, the caterpillar eats the larval ants. Why do the ants put up with such behavior? Besides sugar, honeydew contains amino acids which are very rare elsewhere in the ant's diet. The over-wintering generation of caterpillars pupates inside the ant nest.

In west Texas, there are four species of butterflies known to have symbiotic relationships with ten different species of ants. All are Blues. Reakirt's Blues are known as the Mesquite Blue to some lepidopterists. Will a local naturalist be able to document this incredibly wonderful behavior? A kid and a camera are all it takes.