Walnut ants

On March 20 I was walking in the woods of Florence, MA with some friends, and I found an unusual nut.  It was on the large side for a hickory nut, and had some ridges that sort of reminded me of a black walnut (Juglans nigra), but not really:

The mystery nut (left) and a typical black walnut.

So I put it in my pocket to see what I could figure out about it when I got home.  Ten days later, I was sitting at my computer and caught some movement out of the corner of my eye.  I turned and saw what appeared to be a very large booklouse walking across my auto insurance policy.  I looked more closely, and determined that it was actually a very small ant:

Temnothorax ambiguus, just over 2 mm long.

It didn’t look like a typical household ant.  My eyes scanned around for a possible source, and when they fell on the mystery nut sitting next to my insurance policy, it occurred to me that this might be a member of the genus Temnothorax, some of which are known as “acorn ants” because it is common for a whole colony to live inside a single acorn.  I posted the above image to BugGuide.Net, and James C. Trager confirmed my guess, further identifying it as T. ambiguus.  Since then I’ve seen several ants coming and going from the nut, and I’ve put it into a jar for the time being.

I should mention that these aren’t the only ants you might find inhabiting a nut.  Last spring I opened a hickory nut and found a colony of ants that Dr. Trager identified as Tapinoma sessile, sometimes called the Odorous House Ant.  He mentioned that Lasius alienus has also been known to inhabit acorns on occasion.

Colony of Odorous House Ants (Tapinoma sessile) in a hickory nut.

As for the identity of the mystery nut, I’m still not sure what to make of it.