Introducing Tetramorium immigrans: a better name for the long-established pavement ant

No longer fighting over a name.

Meet Tetramorium immigrans. 

I have never been more pleased to report a taxonomic name change than this one. Long called “Tetramorium caespitum”, then “Tetramorium species E” once it became clear the Eurasian T. caespitum was a complex of cryptic forms, the pavement ant has spread across the world and is now among most common urban ants in North America. After decades of confusion, Herbert Wagner has published a fine monograph on the taxonomy of the species complex. Among Wagner’s many discoveries was that Santschi’s 1927 “immigrans” was valid for this world-traveller. An apt change, and a fine resolution of a long-standing problem.


How To Tell The Difference Between the Japanese Pavement Ant And The Common Pavement Ant

Urban ant collectors across temperate North America are undoubtedly familiar with the pavement ant Tetramorium caespitum*. This small brown insect is as common as dirt along sidewalks.

The pavement ant is not native here. Rather, it is a European species that proliferates in the novel habitats where Americans added cement and paving stone to previously uncapped, pavement-free soils. Since we love our sidewalks and our asphalt, we have created a lot of ant habitat and a lot of pavement ants.


Identification of the pavement ant in North America was straightforward until recently. Tetramorium caespitum is a small, blocky, brown ant with a squareish head, a two-segmented waist, a series of lengthwise ridges on the head, two nubbin-like spines on the propodeum, and an antennal socket with a distinct ridge as described here.

This diagnosis failed in the 1980s when an extremely similar species was introduced to St. Louis. The newcomer, the Japanese pavement ant Tetramorium tsushimae, is so similar in appearance to its European congener that correct identification even under high magnification involves measuring several body parts on a sample of workers and performing statistical analyses. On average, the new introduction is slightly smaller and with slightly larger propodeal spines. Your chances of nailing the ID based on a single worker aren’t great.

Tetramorium tsushimae
There is one easy identification trick that works pretty well at low magnification in the field, though. The trick is worth learning, because Tetramorium tsushimae appears to be more aggressively invasive than the common pavement ant and may become more common as it spreads from Missouri and Illinois.

Here’s the trick:

Colonies of the Japanese pavement ant usually host a great deal more color variation in the workers.

While older T. tsushimae are uniformly dark, the same as their European counterparts, younger workers are strongly bicolored, with a light thorax, giving colonies a more varied appearance. This difference should be visible in the photograph above.

Now that you can spot the difference, keep an eye out for T. tsushimae. It could show up many places where T. caespitum is currently king.

*sometimes called “Tetramorium sp. E.”, for reasons too lengthy to discuss here.

source: Steiner, Florian M., Birgit C. Schlick-Steiner, James C. Trager, Karl Moder, Matthias Sanetra, Erhard Christian, and Christian Stauffer. 2006. Tetramorium tsushimae, a New Invasive Ant in North America. Biological Invasions 8(2):117-123.


The sidewalk battles have begun

Tetramorium pavement ants from neighboring colonies engaged in battle.

Spring has arrived in full! The birds are singing, the tulips blooming, and the trees are greening.

But, whatever. As an ant guy my favorite indication of the season is much more chitinous:

massive urban ant warfare

The Tetramorium pavement ants that live under every sidewalk in town have begun their spring expansion. When colonies meet, each dispatches as many workers to the front as they can muster. The larger colonies push out harder, while the weaker one are forced back into smaller territories more appropriate to their numbers. The battles can last for days.

I’ve seen several of these sprawling conflicts this week, within a few blocks of my house, and I imagine elsewhere in their range the pavement ant action is also heating up. If you live in the midwest, atlantic coast, or northeast, keep an eye out along urban walkways for what seems like a shimmering oil slick. The spectacle is well worth watching.

The Jedi Ant

Tetramorium jedi Hita Garcia & Fisher 2012

Those of us who enjoy the creative side of taxonomic nomenclature received an early Christmas present this week with the publication of a Zootaxa paper by Paco Hita Garcia & Brian Fisher. Their new revision of Malagasy Tetramorium kelleri and tortuosum species groups contains a slate of gems such as Tetramorium nazgul and Tetramorium jedi, above. From the etymology of the Jedi ant:

This new species is named after the fictional, noble, and wise guardians of peace from the “Star Wars” universe created by George Lucas.

This Malagasy myrmicine is not the first ant with a Star Wars heritage. Ten years ago Fernando Fernández named a South American species Adelomyrmex vaderi.

source: Hita Garcia, F, Fisher, B. L. 2012. The ant genus Tetramorium Mayr (Hymenoptera: Formicidae) in the Malagasy region—taxonomic revision of the T. kelleri and T. tortuosum species groups. Zootaxa 3592: 1-85.

Answer to the Monday Mystery: An Unexpected Tetramorium

Tetramorium sp. (aculeatum-group) – Kibale forest, Uganda

When I first saw the trail of leggy brown myrmicines splayed along a tree trunk, I thought the ants must be Pheidole. After all, they moved like predatory Pheidole I know from South American forests. But their ranks contained none of the telltale big-headed soldiers, and when I viewed the shape of the mesosoma in my photos I knew the identification had to be something else. But what?

Marek gets 8 points for his correct answer, a species in the Tetramorium aculeatum group. Additional points go to Guillaume D. (2 pts) for being first to the subfamily, and to Josh King (1 pt) for making me laugh. Thanks also to Brian Taylor and to Pangapaco for additional information.

The trouble with the mystery ants is that we Americans are accustomed to Tetramorium as drab, chunky, monolithic little insects. Most of our species are small, blocky things imported accidentally from the old world. Here’s the ubiquitous pavement ant, for example:

Tetramorium sp. nr. caespitum (Illinois)

North America’s most extravagant Tetramorium is like a slightly longer, slightly lighter pavement ant:

Tetramorium bicarinatum (Florida)

Yet Africa is different.

There, for reasons we can only guess, Tetramorium flowers into an astounding array of shapes and hues. Consider:

Tetramorium pulcherrimum (Kibale forest, Africa)
Tetramorium sericeiventre (South Africa)
Tetramorium sp. (Kibale forest, Africa)

For more, see Antweb’s Tetramorium of Africa.

Visiting other regions with open eyes forces us to challenge what we think we know of particular lineages. Evolutionary processes have played out differently among the continents, and Africa has indeed been kind to Tetramorium.

Oh, and our monthly mystery winner, with 10 points from early October, is Warren. Contact me for your loot, Warren!

Tetramorium bicarinatum

All this talk about copyright infringement is a real downer. It’s time to perk things up with pretty ants:

Tetramorium bicarinatum workers gather nectar from glands of an invasive mallow. Some plants use nectar to attract ants as a defense against herbivorous insects, as ants also eat insect eggs and caterpillars. (Orlando, Florida, USA)
A more field-guidey shot of T. bicarinatum. This ant is presumably native to Asia, but thanks to global trade is now found in warmer climates worldwide.

New Species: Tetramorium mahafaly

Tetramorium mahafaly Hita Garcia & Fisher 2011

Francisco Hita Garcia and Brian Fisher have just published an open-access paper on the Tetramorium species groups of Madagascar. Tetramorium is a very large genus and this work, sizeable though it is, really only lays the groundwork for a coming monograph by delineating clusters of similar species and providing a key to the resulting groups.

The paper contains one new species, though: the lovely Tetramorium mahafaly, pictured above.

source: Hita Garcia, F., Fisher, B. L. 2011. The ant genus Tetramorium Mayr (Hymenoptera: Formicidae) in the Malagasy region—introduction, definition of species groups, and revision of the T. bicarinatum, T. obesum, T. sericeiventre and T. tosii species groups. Zootaxa 3039: 1–72.

The Battle for Clinton Lake

Two pavement ant colonies fight for territory along the shores of Clinton Lake, Illinois.

Even the most epic moments of ant warfare can seem inconspicuous from the towering height of our human eyes. The fisherman above, for instance, didn’t even flinch at the hostilities at his feet, even after I pointed out the boiling mass of angry ants. “Someone must’ve spilled something there,” he grunted as he moved on.

But nothing was spilled. This was a territorial boundary between two large colonies of Tetramorium pavement ants. I happened across it while hiking along the shores of Clinton Lake last week, just in time to watch the ants’ numbers escalate along the front. Both colonies were pouring all their fighting-age workers into the fray, hoping to push the borders back and claim a larger swath of prime lakeside real estate.

Granted, from six feet up pavement ants don’t look like much: (more…)

Battle of the Pavement Ants

Battle of the Pavement Ants, definitely not Tetramorium caespitum

While walking through the park yesterday, I happened across a sidewalk boundary dispute between two colonies of Pavement Ants.  As is their habit, these little brown ants opted to dispense with diplomacy in favor of all-out warfare.

Incidentally, if I had to pick one thing that annoys me about the purely molecular systematists, it is their tendency to avoid dealing with the taxonomic consequences of their work.  A recent paper by Schlick-Steiner et al (2006) gave a detailed picture of the genetic structure within the Tetramorium caespitum species group.  Among their results was that the common Pavement Ant, a widespread insect introduced from Eurasia, was definitely not Tetramorium caespitum, the name everyone has used for this insect for a century or so.

But instead of figuring out what latin name should apply to our common Pavement Ant, or even devoting a paragraph to giving it a new valid name, they labelled it an unceremonius “Tetramorium Species E”.    So there it is: Species E.

photo details: Canon MP-E 65mm 1-5x macro lens on a Canon 20D
ISO 100, f/13, 1/250 sec, flash diffused through tracing paper