430 WILLIAM M. MANN 



biologically, from the fusca group, that is, from a fusca-Yike 

 ancestor. In fusca, the habit of two or more queens jointly 

 forming a colony— "pleometrosis" of Wasmann— sometimes 

 occurs, but with this ant it is merely an occasional method of 

 colony-formation. The typical method, according to which the 

 individual female after the marriage flight starts her colony unaided, 

 is most general. In F. pratensis and F. rufa pleometrosis is more 

 fully developed, and is of use in splitting up the colony into 

 branches, and by means of these extending the colony in the 

 near vicinity. Biun considers that the great success of rufa as 

 a species is accounted for by this habit of colony splitting. The 

 origin of this bianch-forming habit has been explained by Was- 

 mann as an adaptation to special methods of life. Rufa and 

 pratensis have become adapted to life in certain ecological situa- 

 tions, in which they are sporadically very abundant, in contiast 

 to other more widely adaptable ants such as Lasius, Tetramor- 

 ium, etc., which occur in nearly all kinds of localities. The 

 special vegetative conditions to which rufa and pratensis are 

 adapted are exhausted after the long residence of an ant colony 

 in one place, and then it is beneficial to be able to split the 

 colony, and enable it to spread in the immediate vicinity where 

 the conditions are the same, rather than to send off swarms to 

 less favorable localities. This branching can be accomplished 

 by the raising of reserve queens, which produce branch nests 

 for the excess workers. Each season, during the time of flight, 

 large numbers of sexual forms are held back in the nest. This 

 habit has gradually modified the normal instinct of the female, 

 the mneme of which has thus been weakened (" buszt an Frische 

 ein"). After the marriage flight the normal instinct of an ant 

 queen is to dig a hidden chamber, but in rufa, whose ancestors 

 were continually surrounded by workers, because of the inher- 

 ited engrammes, there is developed a strong "social desire," 

 which drives it to seek worker society. Here there are three 

 possibilities. The female may leturn to one of the peripheral 

 nests of the mother colony, becoming in reality parasitic on the 

 members of her own colony, which is the first stage in social 

 parasitism. Many do not reach their own colonies, but find 

 other nests of the same species, or of another race and take up 

 with them, while a comparatively small number, reaching rufa- 

 free ground, enter the nests of strange species. This latter is the 



