No. 415.] NESTS OF AMERICAN ANTS. 517 
Although of considerable interest, the artificial mixed col- 
onies (Wasmann's Z II, r and 2) are omitted in this scheme, 
because they are mere beginnings in a field of experiment that 
has been little cultivated in Europe and is still untouched in 
America. It might be possible to include them with the cases 
of synclerobiosis. The natural abnormally mixed nests (Was- 
mann's Z II, 3 a) are best treated in connection with the nor- 
mal cases of dulosis. Combinations of regular and irregular 
mixed and double nests (Wasmann's category C) are rather 
rare and exceptional, and may be placed as compounds of the 
simpler relations under some one of the seven headings above 
enumerated. 
I. PLESIOBIOSIS. 
As restricted in the present paper, plesiobiotic, or double 
nésts comprise only those cases in which two, or rarely more, 
colonies of ants of different species excavate their galleries in 
close contact with one another. They are usually established 
under stones or logs, but a peculiar group of such nests is 
formed by several species that live within the precincts of 
the huge, exposed, mound-like nests of the agricultural ants 
(species of Pogonomyrmex). The colonies inhabiting double 
nests are usually inimical, or at best indifferent to one another. 
Hence, when living under stones or in old logs, they very care- 
fuly wall up the intervening space, so that the galleries 
belonging to the two households cannot inosculate. 
Two classes of double nests may be distinguished. One of 
these embraces a vast series of merely accidental associations 
of two (or, more rarely, more) species. The associations of the 
other class are claimed to occur with a certain regularity and 
frequency, as if one or both of the species concerned were set- 
tling into definite and constant symbiotic relations. The cases 
of the former class are of comparatively little interest, except 
in so far as they represent what must have been the very first 
step in the development of the more specialized unions (xeno- 
biosis, dulosis, colacobiosis, etc.). Any attempt at cataloguing 
these various associations would be unprofitable, if not impos- 
sible. As an illustration of such cases, it may suffice to 
