This landscape holds more than we see on the surface!
You may know the classic story about how ants and aphids live together in an ecological partnership. Aphids feed ants their excess sugars in the form of honeydew, and in return ants protect the aphids against predators and carry them to new host plants. The relationship forms a sort of miniature version of humans and their domesticated livestock.
What you might not know is that the ant-aphid partnership extends, and even flourishes, well below the soil surface. A great deal of ant ranching takes place underground, sometimes in vast, sprawling networks spanning the root systems of several trees. These hidden interactions are less amenable to study than their above-ground counterparts and are consequently much less understood. Yet they are ubiquitously underfoot, noticed only when winged reproductives of the ranching ant species emerge in numbers to take flight, or when a plant disease vectored by these underground insects begins cutting into crop production.
Below, I share a selection of photos of these mysterious subterranean systems. They span many species on several continents, but look closely and you will see the ants and their livestock show convergently similar adaptations to cramped soil interspaces: short legs, pale coloration, reduced eyes.
A cross-section of an urban lawn reveals plump pink ground pearls (Margarodidae) feeding on grass, and tiny Solenopsis molesta thief ants tending them for honeydew. Urbana, Illinois, USA
A few centimeters underground, Solenopsis molesta thief ants tend to ground pearls tapped into grass roots. Illinois, USA
Lasius californicus citronella ants with their mealybugs. Citronella ants are among the most abundant ants in North America, but as these insects are strictly subterranean most people never see them. California, USA
A Lasius arizonicus citronella ant tends to a mealybug. Arizona, USA
Citronella ants only emerge briefly above ground for their annual mating flights. Illinois, USA.
Honeydew from root aphids (Aphidae: Eriosomatinae) is an important part of the diet of the subterranean ant Lasius nearcticus. New York, USA
Linepithema micans astride a ground pearl. The "pearl" is a honeydew-producing scale insect that feeds on the sap of roots. Rio Grande do Sul, Brazil
Lifting a large stone reveals a nest of Pheidole big-headed ants with their ground pearls. Tolima, Colombia.
Acropyga goeldii is so dependent on its mealybug associates that young queens carry a bug with them on their flights to start a new colony. Minas Gerais, Brazil
An Acropyga acutiventris worker ant grabs one of her "cattle" and runs for cover when the photographer uncovers her nest. Queensland, Australia.
Mealybugs tended by Lasius umbratus are covered in thick, waxy filaments. Here, a worker relocates a small bug. Illinois, USA
The views expressed are those of the author(s) and are not necessarily those of Scientific American.