Borax, the '20 mule team' brand. Yeah, that stuff is great. I mixed up a big batch when I had a cat (roaches). It's a 1:1 mixture of Borax and Powdered Sugar, with just enough water to make a thin paste. I kept it hydrated for a while, but after no longer finding wallcrawlers... I dried out, so I ground the residue into a fine powder and left it in the cat food cans I had used and put them back in hiding. Guess it's still working, been 4 years or so, and no live roaches, but then again the cat died 3 years ago, so no pet food lying around.
It works by interfering with their ability to absorb water. The arthropod muscles work on a hydrolic principal, so low on fluid means no mobility.
I research the web alot, and it seems that fire ants live on a liquid diet. They regurgitate (puke up) digestive enzymes, and suck up the resulting fluids, which the share with their friends (kind of gross). They do break off chunks of food items and carried them back to the colony, so other workers can do this as well. Kind of fill up on the fuids on site, and bring back a piece in the same trip, pretty efficient. Mostly they are interested in proteins and carbs, anything else is garbage. Cat feces is full of proteins, so probably why they like yard (f...ing neighbor's cats). Cat's diet is mainly protein.