Pseudomyrmex sp. (gracilis ?) (Elongate Twig Ant).
Size: approx. 10 mm. Photo: 08/16/11, 10 am.

Since the 1960s the Elongate (or Mexican) Twig Ant (Pseudomyrmex gracilis) has become widespread in Florida. It has a painful sting. Nests are small (few ants), have a single queen, and one small entrance. Unlike other species in this genus, P. gracilis is not an acacia-specialist, but nests in a wide range of vegetation. This specimen was darting swiftly over my lantana leaves, which made it difficult to take this photo. There are other Pseudomyrmex species in Florida. 
bug guide (this photo):
J. K. Wetterer, Fl. Entomol. 93, 535, 2010: