No. 417.] NESTS OF AMERICAN ANTS. 703 
genus, and is naturally carnivorous, though it has been 
observed to attend aphides for the purpose of collecting their 
sugary excrement. Although this ant is, therefore, quite able 
to exist alone, it nevertheless has a very pronounced penchant 
for robbing the larvæ and cocoons of other species of Formica, 
eating great numbers of them but allowing others to develop 
and to function as its slaves, or auxiliaries. The latter feel 
themselves to be members of the colony in which they emerge 
from their cocoons, and direct all their activities to maintaining 
and defending their foster nest and its occupants. In Europe, 
as a general rule, the normal slaves of F. sanguinea are the 
workers of F. fusca, less frequently the workers of F. rufibarbis. 
Sometimes both species of auxiliaries may be found in the 
same mixed nest. In extremely rare instances the workers of 
F. rufa and F. pratensis may serve as slaves. The expeditions 
for robbing cocoons are usually carried out during July and 
August, but they seem to be rather infrequent or irregular and 
are not often observed. The tactics of F. sanguinea, like those 
of other dulotic ants, consist in surprising the colony they 
wish to rob and in carrying away the pupa as rapidly as pos- 
sible without engaging in unnecessary slaughter. Only the 
ants that offer active resistance are dispatched. 
F. fusca is most frequently enslaved because it is a weaker 
and more tractable species and forms smaller colonies than 
F. rufibarbis. The rare occurrence of F. rufa and pratensis in 
sanguinea nests is due to the more savage nature of these 
Species, which are enslaved only when they belong to small 
colonies or when they are of small size individually. 
The number of auxiliaries in nests of sanguinea varies 
greatly. In Holland, in more than 100 nests, Wasmann (91) 
found the ratio of sanguinea to slaves varying between 1:0 and 
1:3. Most frequently the sanguinea are from 2 to 5 times 
as numerous as their slaves. The number of the latter de- 
pends on various circumstances, such as the abundance or 
Scarcity of nests of the auxiliary species in the vicinity. It is 
à singular fact that the weakest colonies of sanguinea contain 
the greatest number of slaves, so that it would seem as if the 
dominant species tried to make good the deficiency in the 
