Nevermind- found it on the Googles!  Diatomaceous earth is made of sharp particles and evidently it cuts the larvae to shreds before they can mature and form their little exoskeletons.  I also used those mosquito dunks when I had gnats in my hydro systems years ago.  They contain a bacteria or small critter of some sort that feeds on the gnat eggs or larvae.  I can't remember if that actually helped or if I just learned to live with them.  I guess my hydro setup caused temporary memory loss for some reason!