Apterostigma collare

4mm | January 19, 2011 | Armonia Nature Preserve, Limon Province, Costa Rica

I spotted the structure below on the underside of a large leaf. I really didn’t know what it was, and I gently poked at it. It was quite fragile as it turns out, and it fell open to reveal an ant nest. I then immediately regretted not having taken a photo beforehand. The next day I was lucky enough to find another one, also pictured.

6cm x 4cm | Unexpected ant nest

4cm x 2cm | Nest with an ant entering

These nests are the work of an ant in the genus Apterostigma. Ants of Costa Rica has an info page for this genus in Costa Rica. I tried to use the key there to identify these, but it was a bit technical for me. I’m basing the species identification on the statements from the site that seem to indicate that only Apterostigma collare builds these nests under leaves. There are some more photos of nests at that same site.

Like the well-known leaf-cutting ants, Apterostigma ants grow fungus for food. They make their nests out of the fungul hyphae.  Other Apterostigma species in Costa Rica build their nests underground, in leaf litter, inside rotting logs, and in other protected locations. At least one other species, Apterostigma robustum, builds its nests on exposed tree trunks, but it takes care to camouflage them with bark fragments. Apterostigma collare nests, in contrast, visibly stand out on the undersides of leaves.

Ants inside nest, fungal hyphae visible on leaf surface

I counted around 20 ants in that nest that I unintentionally opened. It didn’t occur to me at the time to try and identify the queen. In any case, none stood out as any different than the others at the time or in the images after review.

It’s not clear to me what exactly the ants provide for the fungi to grow on. I did notice what looked like some bits of insect exoskeletons mixed in.

Ant worker

There’s definitely all sorts of odd bits visible inside the nest.

Frontal view

These long-legged ants move about pretty slowly, so I took advantage of the opportunity to take quite a few photos.

On the move, albeit slowly

The ants themselves were varying shades of orange, some darker than others.

Individual coloration was variable

After a while, I could see some of the workers were picking up small globular items. I’m still not sure if those are larvae or pupae, but I assume it’s one or the other.

Worker carrying larva?

Here’s a closeup view of one, where at least a few structures are visible.

Larva?

Here are just a few more images of these interesting ants.

Worker

All Apterostigma ants, by the way, are hairy like the ones shown here.

Worker, preening

Maybe one of the ant people who read this blog can share a few more interesting tidbits about these ants. Hopefully I won’t get corrected on my identification.