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Bulletin Aynerican Museum of Natural History^ [Vol. XXIII, 



the first is the depth or vertical, the second the breadth or transverse diameter. 

 The chambers are either spherical, or if one diameter exceeds the other, it 

 is most frequently the transverse, so that the chambers are often oblately 

 spheroidal. As the galleries enter and leave the chambers at opposite 

 points on their roofs and floors, the globular cavities have the appearance 

 of being strung on the galleries like beads on a string. The most frequent 

 nests are those of the form A-C, comprising only two galleries and two 

 chambers, and these are the only ones described by previous observers 

 (Morris, McCook, Swingle, Forel). The entrance gallery is commonly a 



Fig. 20. Nest of Atta {Trachymijrmex) septentrionalis in pine barren near Lakehurst, New 

 Jersey, about ^ natural size. Tlie circular entrance is in the middle of the figure; the excavated 

 sand is dumped out in a heap in front of it (below). (Photograph by the author.) 



few cm. in length and the first chamber is very small (2.8 X 3.2 cm. on an 

 average). These represent the whole of the nest dug by the mother queen 

 while establishing her colony, the other chambers and galleries being added 

 subsequently by the workers. The table and the figures show very clearly 

 that the length of Gal. II and the size of Ch. II, greatly exceed the queen's 

 excavations and are in turn surpassed by subsequent excavations (Gals. 

 III-IV and Chs. III-V). Nests with three, four and five chambers, like 

 D, E, and F, are rarely encountered. Of the last I have seen only a single 

 example and this was peculiar in having Chs. Ill and IV deeper than broad. 



