Tiny reddish-brown ants sharing a Pavement Ant burrow

2012 January 11

On June 26, 2011 I turned over a concrete stepping-stone to see what was underneath. There were ants. The ones I saw first looked to be Pavement Ants, which are a species that has been here before. Here’s one trying to lug an ant cocoon to safety after I exposed their nest:

But then, looking more carefully at the galleries the ants had dug under the concrete, I noticed some of these very tiny (less than 2 mm long) reddish-brown ants crawling along. They seemed to be peacefully coexisting in the same tunnels as the pavement ants, and pretty much ignored each other.

At first I thought that they might be “Thief Ants”, Solenopsis molesta, based on their size and the way that they were cohabiting with another ant species. But, the waist was all wrong, so I put them on BugGuide to see what the experts thought. And James Trager promptly identified them as Brachymyrmex depilis, based on their size, color, and the fact that their antennae had nine segments (this is fewer than most other similar ants, which usually have 12 antenna segments).

They had their own clutches of pupae and cocoons, a lot of which were much, much bigger than they were. In this clutch, you can see a few small cocoons mixed in with a bunch that are easily twice as long (and therefore about eight times as massive).

Here’s the clearest picture of the antennae that I have, with what I think are the nine antenna segments pointed out by arrows. And the head is transparent enough that you can see the nerves from the antennae continuing down into the brain.

Sorry about the picture quality, but it is pretty tiny. If the ant is 2 mm long, then those antennae are only about 0.5 mm long, and roughly 0.01 – 0.025 mm wide (There are single cells in your blood that are almost that wide). It looks like the segment at the very tip is significantly larger than the others (it probably has most of the sensory nerves in it), and the one at the very base is much longer and narrower than the others (it mainly exists just to give reach, not flexibility).

There are a bunch of much clearer pictures of this ant species here, some of which show the number of antenna segments much better.

Those bigger larvae and pupae were going to be the winged queens. Once they hatched out and matured fully, they were going to fly off sometime in August or September along with the much smaller males, and mate. They would then land, tear off their own wings, and then find someplace to start new nests.

These little ants aren’t found above ground very much, they evidently mainly live on honeydew that they get from “subterranean, sap-feeding hemipterans”. Basically, underground aphids. Up until this moment, I had not been aware that there were underground aphids, but there you go. So these ants are basically farming other insects underground that feed on plant roots, and milking them for their sugar. And, I suppose, defending their subterranean herds from predators like centipedes and rove beetles.

9 Responses
  1. JennyW permalink
    January 11, 2012

    I think I’ve seen some of these when pulling up weeds. It looks like there are small aphids on the roots and teeny ants. Pretty cool! Now, if I could just figure out what kind of ants we have that are reddish & black and move tons of dirt when making a nest. The top of the anthill is easily the size of a cup saucer (flat tho, not rounded) and the ants get REALLY agitated when disturbed. I call them the ‘angry ants’.

  2. Carole permalink
    January 11, 2012

    Fascinating, thanks for taking me on your journey.

  3. January 12, 2012

    Thanks, Carole

    JennyW: These little guys are evidently very common, and are about the tiniest ants that you are likely to encounter, so you are probably correct that they are the ones that you are finding while pulling weeds.

    As for the big black-and-red ones, if they have red heads and bodies with a black abdomen, I think they are most likely one of the Mound Ants in the Formica rufa group. We have them all over the place too, and they are pretty savage. We often see them farming aphids on our trees and shrubs.

  4. January 12, 2012

    I very much enjoy your blog, and this post was one of the best. My primary interest is in odonates, although I piddle around with beetles and lepidopterans a little. I really don’t care to get into ants, despite reading a lot from E. O. Wilson, and spending quite a bit on several of his “ant books”. Your approach to blogging was a prime motivator for my own effort, in which I’m trying to document the fauna, but primarily “the bugs of Booger County”.

  5. January 13, 2012

    Thanks, George!

    Personally, I think it would be really cool if every state (or, ideally, every county) had somebody running an on-line photo inventory of the insects that they can find around their yard. Then we’d really have a good idea of exactly what lives where.

  6. January 17, 2012

    No need to apologize for the photos, they were terrific. I wonder if the host ants ever feel the need to move the visiting queen for construction or gallery cleanout tasks and if there are depilis warriors who would get wound up about the whole affair and start a brawl.

  7. January 17, 2012

    Thanks, KT

    I was really surprised to see the two different kinds of ants sharing galleries like that. I actually saw the pavement ants walk over the depilis workers (which were only half their size) as if they weren’t there. The actual nursery galleries for the two ant species seemed to be separate, though: I didn’t see any depilis workers on the pavement ant cocoons, or vice versa. It may well be that there are “common areas” where they are willing to coexist, and “off limits” areas where they will actually fight off intruders, and the individual ants respect those boundaries.

  8. Fred permalink
    January 19, 2012

    How big are the subterranean aphids?

  9. January 19, 2012

    Don’t know yet. Now that I know they are down there, I’ll have to find some and measure them. They’re probably about the same size as the ants, though (around 1-2 mm)

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