Here's something reported on another discussion group concerning boric acid
and ants:

I've heard two explanations: (1) The boric acid gets onto their feet,
they
groom themselves, and it causes scratches in their exoskeleton, and they
dehydrate, and (2) it acts as a stomach poison.

Roaches breathe through their feet. The boric acid is aspirated, and does
its damage by suffocation and destruction of pulmonary tissue. Or, so said
the Ortho rep when I worked at a local hardware store.

I sure as heck don't know! But I FINALLY got rid of a huge anthill in my
front yard- (After they killed a Red Delicious apple tree) by mixing borax
and sugar with a bit of water. Put it in cottage cheese containers with
holes cut along the top of the sides (boy, is that confusing?!) and partly
buried them in the anthill.

For about 10 days, I didn't think it had worked either. I'd tried
pyrethrum, rotenone and even Sevin over the past 1 1/2 years, without any
results. 3 weeks after setting those "traps" the anthill is empty and dead!
Finally- something that works, and relatively non toxic.

I also had my usual yearly invasion of ants in the kitchen, and setting
cottage cheese container tops with that mixture in them on the counters
completely eliminated them within 3 days.

They supposedly take the stuff back to the nest and it kills the queen as
well.

Olushola