Looking at tiny bugs, I have always wondered whether big ones, having much bigger brains or at least room for them, are "smarter" than small ones.
In answer to a question about the largest ant, you mention some that are 3 cm long, compared to the smallest that are <1mm. That is a length ratio of 30 times, which means mass and volume differ by 30^ 3 = 27,000 times. This is consonant with what you imagine, looking at a barely visible member of the phylum we kids called "piss ants" (which I'm sure is the Latin technical term!)
A 3 cm ant is like a roach or other bug and you don't expect it to be very smart, but it probably does have a large hard-wired repertoire of behaviors -- forage, fight, mate etc. But at 1 mm, brain shrunk 27,000 times, you have to wonder whether there are enough neurons to code for all that. Is there some limiting size beyond which bugs get dumber? I recall from high school biology fifty years ago that bugs probably don't have brains per se, but several ganglions (ganglia?), and most of the cells therein are used to drive muscles, so it's proportional and a small ant needs only a small ganglion for that. But for seeing and behavior and the like there must be a minimum number of neurons required.
So -- how tiny can they be before they start to get dumber?
To address your question about ant brains, we contacted one of your colleagues who is an expert on ant brains, Dr. James Traniello, and here is what he had to say:
"First of all, insects do have well-structured brains made of compartments of tissues with specialized functions in visual and odor information processing, motor control, and learning and memory! Only a small portion of the brain (and ganglia) appear to control muscles.
It's been a great challenge to understand the relationship between the structure and function of the brain and whether or not small size compromises processing ability and "intelligence." It's been assumed, particularly in vertebrates, that brain size and cognitive ability go hand-in-hand, but animals with larger brains are not necessarily "smarter." There is no question that tiny insect brains can generate complex behavior (In this respect, Charles Darwin thought the ant brain was more marvelous than the human brain). It's an open question as to how many neurons are needed to generate complex behavior. Some models suggest very few nerve cells are required for learning, for example.
Social life and ecology affect the evolution of brain size; body size alone does not appear to be a very good predictor of cognitive ability. Larger worker ants don't appear to be any "smarter" than small workers and any differences in behavior, especially learning and memory, are likely due to differences in their foraging ecology rather than their size per se. Larger ants may, for example, hunt for prey and have better vision, and this ability is reflected in the size of the optic lobes of the brain that process prey stimuli. Small ants may have relatively large brains, and different ant species have brain compartment proportions that fit their ecology and behavior.
It's also important to remember that ants are collectively intelligent: they solve problems as communicating groups. How this group intelligence affects brain evolution is not well understood. Ants with large colonies may have workers with relatively smaller brains. The smallest bodied ants may have fewer cognitive abilities as individual workers, but their colonies may be just as "smart" as colonies of ant species that have larger body size. "Intelligence" has to be considered in respect to ecology, social behavior and life history in all animals."
To see an image of an ant brain, please see the home page of Dr. Traniello's lab website: https://people.bu.edu/jftlab/Home.html
James Traniello (guest expert), Corrie Moreau, & the AntAsk Team