Meat ants (Iridomyrmex purpureus) make large, bare nest mounds in sunny places, often with radiating tracks, like this one in the Riverina Grasslands. But what do their nests look like under the ground?

Go for a wander in the grasslands of the Riverina and you might notice an abundance of holes in the ground. If you see critters scurrying in and out of the holes (like the meat ants in the picture above) at least you know what type of beast lives in them. But often you see just holes, and the inhabitant is either hidden below, or perhaps long gone.

Who made this hole? And is it still hiding in the burrow below?

Sometimes the shape or other features of the burrow entrance can give clues about what made it. For example, someone once told me that burrows with crescent-shaped entrances are made by scorpions:

Entrance to a scorpion burrow?

While this silk-lined burrow entrance, complete with lid, was almost certainly made by a trapdoor spider:

Entrance to a trapdoor spider burrow.

I really don’t know what made this burrow, with an entrance hidden under a pile of frass (that’s the fancy term for plant bits that have been chewed up and spat out again):

A frassy crater

Crater with the frass removed – showing the entrance to a burrow.

Perhaps some sort of Polyrhachis ant? The nest entrance of the mulga ant Polyrhachis macropa is a large mound decorated with dried leaves.

But even though I could guess at what beasts made the burrows, I was still left wondering what the burrows were like inside. Big, small, deep, shallow? A number of chambers or just one? And wouldn’t it be great to illustrate this hidden world in the Riverina Grasslands colouring book?

So when I returned to Brisbane after the field trip to southern New South Wales, I started looking for information about these burrows. My main question was ‘what sort of burrows do creatures make in cracking clay soils?’. These are the soils beneath the grasslands, so-called because cracks are formed when the soils dry out. But what is the configuration of the cracks, and do the critters use these, or dig their own?

Interestingly, most people I asked (who are generally very knowledgeable ecologists and zoologists) could not answer this question. I didn’t have time to do an exhaustive literature review, but it seemed that not much was known about burrows in clay soils. Which perhaps is not very surprising, as they’d be hard work to dig up and investigate.

But a few people provided some very interesting clues as to what’s going on down there, and these inspired the final ‘burrows’ picture below, which will appear in the Riverina Grassland Ramblings colouring book:

An underground view of the Riverina Grasslands. See if you can find: Broughton Pea (Swainsona procumbens), Small Vanilla Lily (Arthropodium minus), Meat ants (Iridomyrmex purpureus), Ink Pot fungus (?Podaxis pistillaris), Grasses, Cottonbush (Maireana aphylla), Prickly Saltbush (Salsola australis), Fuzzweed (Vittadinia cuneata), Fat-tailed Dunnart, Curl snake, Cow, Wolf spider, Brown snake, Scorpion (Urodacus armatus), Australian Plague Locust and Trapdoor spider

So how much is the picture evidence-based, and how much have I invented?

Melissa Bruton who’s dug pitfall traps in the cracking clay soils around Dalby confirmed that most of the cracks would probably be vertical, with some horizontal too. Most animals use what’s available, with minor modification. This information is the ‘backbone’ of the picture, so I was thrilled to hear from Melissa.

Arachnologist Tracey Churchill told me that trapdoor spiders (mygalomorphs) tend to use deeper burrows than wolf spiders (araneomorphs). This is because mygalomorphs, with two pairs of book lungs, tend to dry out more quickly than araneomorphs (one set of book lungs and the other pair reduced to tracheae/spiracles which lessens water loss). Wow, that took me straight back to 2nd year Invertebrate zoology at Monash Uni,a mere 27 years ago! Do zoology students still learn these anatomical details which often have a crucial bearing on the ecology of an organism?

Robert Raven, the spider guru from the Queensland Museum, mentioned that wolf spiders don’t make much of a burrow, and tend just to follow the existing cracks. Also something about cane toad murals which left me perplexed. But since cane toads haven’t reached southern NSW I left that mystery for another day.

Many years ago, I was a holiday babysitter for Fiona Clissold’s colony of Australian Plague locusts, when we were both doing our PhD’s. So I dredged up my memories of the female locusts laying their eggs in the little pots of sand that we provided in their hot-and-steamy controlled temperature room in the bowels of the Zoology building.

A scorpion enthusiast called Mark who I found via Google suggested that the scorpions were likely to be Urodacus armatus and they made a simple and shallow burrow. Thanks Mark!

But maybe best of all, just before I started the above drawing, I attended a ‘Walk with the Ants’ at The Planting Festival at Woodfordia, which was conducted by ant guru Kirsti Abbott, accompanied by some folks from Australian Ant Art. The Australian Ant Art people were displaying an aluminum cast of an Iridomymex purpureus ant nest, which I could use as a model for the nest in my drawing. It was a beautiful case of serendipity.

So there you have it, my current best guess of what those grassland burrows might look like. However, it’s just a working hypothesis. I have no doubt that we still have a lot to learn about what really lies beneath. PhD anyone?

This is the third post in a series about how I created the Riverina Grassland Ramblings colouring book. Previous posts are How to draw a grassland Part One and How to draw a grassland Part two: Ecology in pictures.