A common sight along the roadsides and logging gaps of the lowland forests of Borneo is the slender gray trunk of a sun-loving tree with long, up-curving branches naked except for a few large leaves at the tip. These trees belong to the genus Macaranga, in the Euphorbiaceae family.

They seem quite unremarkable, but working with them produces a feeling that they are not quite so simple. Most days we would cut down at least a few branches of Macaranga, preparing wood samples, then scanning and pressing leaf samples. In these moments I always felt unsettled, like something was crawling on my skin. And often I had to then flick off a large number of small black ants from my arms and legs and chest, or forcefully tear away the ones that had attached themselves to my skin with their jaws.

The mystery of where all these ants were coming from was solved when we sawed through a large branch of Macaranga pearsonii.

It turns out that the stems of this species are hollow. These interior galleries play home to a colony of small black ants. Presumably the ants defend the plant against its enemies, and receive shelter and resources in exchange. I can certainly verify their willingness to defend their home after cutting it in half.

This is a classical ant-plant mutualism. I was later to learn that the ants belong to the genus Crematogaster, and the species here is almost certainly Crematogaster borneensis (see Fiala et al. 1999 for a full treatment of the coevolutionary story). But two mysteries remained to me. First, how did the ants ever leave their hidden fortress worlds? I saw no exits from the thick wood of the branch. And second, what exactly did the ants receive in return for their soldiering?

The first question was resolved after a bit more exploration. I found that the thick wood of the main branches transitions to thinner branches comprising a set of segments each joined to another where a leaf attached to the stem. In each of these segments was one small hole – just big enough for a single ant’s head to poke through. It is hard for me to imagine the intricate network of paths through the plant that every ant walks, but perhaps a tree is not so different from a set of underground tunnels.

And the second question was resolved after a closer look at the leaves. At the base of each leaf lamina near the attachment point of the petiole, there were two small circular forms. These were extrafloral nectaries – plant structures that the ants would visit to obtain sugars and so feed themselves. I later learned that the number of extrafloral nectaries per leaf helps to determine the identify of the species.

A formidable and well-fed set of defenses. But some species in Macaranga do more to protect themselves. You may have noticed a red tint around the ants’ gallery in an earlier photograph. This species also exudes a copious bright-red latex. This is a toxic and sticky set of defense compounds meant to mechanically or chemically stop an insect attacker. In only a few seconds a thick and sticky red mess welled up from the stem, almost like blood.

In most of Euphorbiaceae, this latex produces a strong painful rash for humans – luckily in this species the ants were the worst of it for me. I imagine the situation would be different for a passing caterpillar or beetle.

These Macaranga species are a marvelous example of the coevolution of insects with plants – some positive interactions, as with the ants, and some negative, as with the latex. These interactions have led to the marvelous adaptations of this species’ branches and leaves. It is a dance between species’ forms, played out over millions of years. And it only takes a small amount of careful observing to appreciate.