THE TEMPORARY SOCIAL PARASITES. 44 z 



In the course of a year or two, the workers of these host species, being 

 unable to reproduce, gradual!)' die off and leave the parasitic queen 

 and her offspring in possession of their nest, as a pure colony, and 

 requiring no further assistance in its growth and development. The 

 colony becomes populous and aggressive, with nothing to indicate that 

 it began life as a parasitic community. This method of colony forma- 

 tion is adopted by some of the most powerful ants of temperate regions, 

 and some of the most remarkable temporary parasites are members of 

 the circumpolar genus Formica. Two groups of species in this genus, 

 one embracing F. rufa and its allies, the other F. exsecta and its 

 allies, are especially interesting in this connection. The workers of 

 these various species are very similar. They have red heads and 

 thoraces and black or brown gasters, but the cospecific females may 

 exhibit extraordinary differences in size, sculpture, pilosity and colora- 

 tion. According to size they may be divided into microgynous species, 

 in which the female is little larger and sometimes even smaller than the 

 worker, and macrogynous species, with the female considerably larger 

 than the worker as in the members of other Formica groups. It is a 

 singular fact that although the species in both of these subgroups are 

 widely distributed and often very common in certain localities, no one 

 has ever seen one of the females founding a colony independently. It 

 is known, however, that in some of the species the colonies are often 

 enlarged by adoption of females of the same species or even of a dif- 

 ferent subspecies. The microgynous species are most abundantly rep- 

 resented in North America. It was a study of these which first led 

 me to the discovery of temporary parasitism as a regular or normal 

 occurrence and to make the prediction (1904/0 that it would be found 

 to occur very generally in the rufa and c.vsccta groups on both conti- 

 nents. I will present the grounds for this prediction as succinctly as 

 possible. 



i. Microgynous Formicae of the Rufa Group. The commonest ants 

 with minute females in the mountains of the Atlantic States are F. 

 difficilis and its variety, consocians. The females of these are almost 

 as small as the large workers and are fulvous yellow in color. In the 

 Litchfield Hills of Connecticut I found that as a rule consocians lives 

 in populous, independent formicaries under stones which it banks with 

 plant debris. Like other members of the rufa group, it is a very pug- 

 nacious ant. During the course of several summers a number of small, 

 incipient colonies were found, containing a consocians queen associated 

 with workers of F. incerta, a variety of schaufussi, and sometimes also 

 with a few consocians workers. F. incerta is a cowardly ant which 

 forms numerous rather small formicaries in the same locality. The 



