1907.] 



Wheeler, Fungus-growing Ants of North America. 



757 



alteniating galleries and chambers and that turrifex continues its excava- 

 tions according to this pattern, Avhereas flourishing colonies of septenirionalis 

 change to the racemose type which bears an unmistakable resemblance to 

 the nests of Atta s. str. 



The greater length of the turrifex galleries in pure sand is undoubtedly 



^"rv^' 





Fig. 25. Section of nest of Atta (Trachymyrmex) turrifex showing four chambers exposed 

 (at points of ijaper triangles numbered 1 to 4). About i natural size. (Photograph by Mr. 

 C. G. Hartmann.) 



due to the need of reaching a stratum of greater dampness. In the dry Trans 

 Pecos deserts the same tendency is observable. In that region I repeatedly 

 endeavored to excavate nests, but was never able to reach the chambers on 

 account of the extreme hardness of the stony .soil. I am convinced, hov,- 



