The genus Formicium includes at this moment 5 species and is known only from queens and males. Workers were never found. The wingspan of the sexuals is the biggest among ants, both extant and extinct types combined, that have ever been found. The queens have a maximum wingspan of 13 to 15 cm. They were real giants (one of the species is called F. giganteum). Two of the species are known from queens and males, both from an Eocene deposit in Messel, Germany. The others are only known from isolated wings (Britain and USA). It is suspected that more species can be found in Messel and nearby deposits.