Hydnophytum and Myrmecodia are Ant inhabited plants . They may provide a food source or shelter for the ants, and are often protected in return.
For information about non-fern ant plants, check out an updated article by Frank Omilan, Franklin Omilan article revision 9-08 form vs moseley.pdf.
The plants below are representative of what I ship. They are, from left to right, Hydnophytum formicarum, in a 4" square basket, Myrmecodia tuberosa var. salomonensis, in a 4" octagonal basket, Hydnophytum moseleyanum, in a 4" octagonal basket, and Myrmecodia platytyrea ssp. antoinii, in a 2 3/4" round basket.
The plant below is Hydnophytum moseleyanum, in a 6" orchid pot. Note the holes, which allow ants access to spaces in the caudex.