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[Vol. 94 
queenright colonies in the field, where each of the three species 
contained several workers of one of the other species and no 
evidence of slave-makers. Subsequent brood produced by these 
colonies was of the same species as the queen and the majority of 
workers in the colony. This suggests that facultative slave-making 
occurs among species which are normally enslaved by other species. 
For Formica, by contrast, no mixed colonies of these slave species 
have ever been reported in the absence of slave-makers. Laboratory 
manipulations with F. schaufussi and F. nitidiventris (in which two 
conspecific colonies are connected and no emigration is possible) 
have shown that colony fusions often occur, with relatively little 
violence, with the exception that at least one queen is usually elimi- 
nated (Goodloe & Topoff, unpublished data). Workers of the elimi- 
nated queen might be considered “enslaved” by the others, but there 
is no evidence that such fusions occur in nature. Admittedly, such 
conspecific mixes would be almost impossible to detect in the field. 
While the Leptothorax territorial encounters were marked by 
continuous violence (Alio way, 1980), Neoformica colony fusions 
were characterized by a curious lack of aggression. Although initial 
encounters with alien conspecifics resulted in fighting, this behavior 
often disappeared within a day, and subsequent encounters were 
impossible to distinguish from encounters with nestmates. This lack 
of aggression is not consistent with traditional concepts of territorial 
behavior. 
The lack of discrimination in the care of pupae from alien conspe- 
cific colonies in F. schaufussi and F. gnava is not surprising since 
extra conspecific workers can be absorbed into the service of the 
queen present in the nest and might provide an adaptive advantage. 
However, it is difficult to speculate on selection pressures that might 
exist when there are no known interactions between free-living col- 
onies of these species that would result in one colony coming into 
possession of a conspecific colony’s brood. 
Differential consumption of brood of another species is consistent 
with developmental studies (Jaisson, 1975; Le Moli & Passetti, 
1977; Le Moli & Mori, 1982) which show that some Formica species 
imprint, during the days following eclosion, to the brood present in 
the nest. After that time, they will care for the brood of the species 
to which they were exposed, and treat the brood of other species as 
food. In Raptiformica colonies, it is common to find more than one 
