41 8 Bulletin American Museum of Natural History. [Vol. XXII.] 



plausible in this instance, will not, however, apply to the preceding 

 cases, for the Polytrichum. and Form,ica exsectoides are both subboreal 

 organisms, and the pitcher-plant and Cremastogaster pilosa are both 

 of subtropical origin, and there is every reason to believe that these 

 organisms have been associated with each other for long periods of 

 time. It seems natural to suppose that in all the cases described in 

 this article the havoc wrought by the plants is not sufficiently great 

 seriously to impair the vitality of the respective species of ants. In 

 other words, the drain of a heavy annual destruction of individuals 

 or even colonies may be easily borne by organisms capable of repro- 

 ducing so rapidly and abundantly as these insects. 



EXPLANATION OF PLATES. 



Photographs of Formica exsectoides mounds found near Scotch Plains, New 

 Jersey. The dimensions of the nests are given on p. 404 under their respective 

 numbers. 



Plate LXIII. Nest No. 11. of unusually large size and still retaining the 

 broad basal zone of sparse grass so characteristic of younger nests. 



Plate LXIV. Four nests (not included in the table on p, 404) from a group 

 of several in a clearing. These nests had been denuded of all grass by their ant 

 inhabitants. 



Plate LXV. Nest No. 7, the largest of the colony. It was surrounded by 

 a zone of moss (Ditrichum pallidum) which, however, does not show in the 

 photograph. 



Plate LXVI, Twin nests invaded by moss {Polytrichum commune) . The 

 one in the foreground is No. 6 of the table on p. 404. 



Plate LXVII. Nest No. 10, showing the zone of Polytrichum advancing 

 and extinguishing the grass zone of an earlier stage (conf. Plate LXIII). 



Plate LXVIII. Nest No. 8, showing the Polytrichum zone far advanced 

 toward the summit, or umbilicus, which it will eventually cover. 



