The best part of traveling with a group of biologists in a place like the Cheringoma Plateau is the impossibility of ever being bored. Not only can you witness hilarious and exotic injuries (where else do people get bitten by bats?), but every day brings new discoveries of things you never thought existed. At our first camp, which was located on the rim of a deep, stunningly beautiful limestone gorge called Nhagutua, our team’s myrmecologist Leeanne Alonso found a colony of what is arguably the most amazing ant species on the planet. Leeanne was collecting ants running on the trunk of a large Knobthorn (Acacia nigrescens), and on a whim pulled off a piece of bark from the tree. To her surprise she found underneath a nest of tiny, yellow ants, which she immediately recognized as members of the genus Melissotarsus.
A few things make Melissotarsus stand out among other ants. Unlike other species, their adult workers retain the ability to spin silk (a characteristic typical of many ant larvae), which is produced from a gland on the underside of the head and spun with special brushes on the tarsus. The brushes pull the viscous fibers from the silk spigots, stretch, shear it, and line the wood corridors with it. The silk lining presumably helps prevent the invasion of the colony by predators, and may help keep the corridors free from fungi and other pathogens.
Melissotarsus workers never leave the nest to forage outside, for two important reasons. For one, they have a strongly reduced sting, which makes them highly vulnerable to attacks by other ants. In fact, minutes after Leeanne had opened their nest it was invaded by Crematogaster and Pheidole ants, which quickly wiped out all workers and larvae present in the exposed corridors. But even more importantly the ants could not forage outside their narrow corridors even if the world outside was safe and friendly, for the simple fact that they cannot walk. Yes, these six-legged insects are incapable of walking or even standing on flat surfaces outside the narrow confines of their nest. Inside the narrow wood corridors they move in a way reminiscent of a rock climber squeezing between two vertical walls by pressing the back and knees against the wall (a move known as stemming). Their first and third pair of legs points down, like in all other insects, but the second pair of legs is usually held up, with the feet pressing against the ceiling. I placed a few individuals on the flat surface of my portable photo studio, and they immediately fell on their back and flailed their legs helplessly.
Since these insects cannot leave their nest inside the tree, they are forced to obtain all their food there. They do so by raising scale insects, mostly of the family Diaspididae. Symbiotic relationships between ants and scale insects are not uncommon – many ant species tend and protect them in exchange for honeydew produced by these floem-feeding insects. Melissotarsus also collect honeydew from some species of scale insects and groom them constantly, removing wax from the insects’ bodies and preventing the formation of the protective shield that scale insects are typically covered with. But the bulk of their food comes not from the sugary water exuded by the scales (in fact, most of the species they raise don’t produce honeydew), but rather from slaughtering the scale insects for meat. This, as far as I know, is the only example outside of human societies of an organism raising another species for its flesh, and adds animal husbandry to the already impressive list of seemingly human skills (e.g., architecture, farming, solar navigation) that ants had evolved long before our species first appeared on Earth.
(You can see photos of another species of Melissotarsus on Alex Wild’s blog.)