ANT STRUCTURES 



475 



unfavorable conditions are not always sharply defined, but merge into 

 each other. 



In some cases it is quite evident what constitute unfavorable condi- 

 tions. Ground that is constantly wet or liable to inundation can not be 

 occupied; hard, rocky surfaces, or even very thin soils, are not available; 

 soils so sandy or friable that underground tunnels dug in them will not 

 stand are evidently not available for the establishment of colonies. 



Between soils most favorable and unfavorable ones there are all sorts 

 of gradations, so that one is prepared, for this reason alone, to find the 

 ant-hills bigger and more abundant in some places than in others. It is 

 evident that it is all a question of adaptability, however, rather than a 

 matter of choice on the part of the ants. 



Just what kind of soil is most favorable for the ants I can not state 

 positively. My general impression is that the mounds are most abun- 

 dant on clayey soils, whether the clay comes directly from the decomposi- 

 tion of feldspathic rocks or from the disintegration of shales and slates. 



This preference for the clayey soils is well shown at many places 

 through the diamond-bearing highlands of the interior of Bahia, where 

 the diamond-bearing quartzites, known as the Lavras series, are under- 

 lain by a thick series of shales called the Caboclo (or Paraguassii ?) 

 series. The Lavras beds being quartzites, or sandstones, break down 

 into a very sandy soil, while the Caboclo shales form a stiff, clayey soil, 

 and as they are adjacent to each other the line of demarcation between 

 the two soils is usually well defined. While traveling through that dis- 

 trict in 1907, I was frequently able to locate myself geologically by the 

 abundance or absence of the ant-hills. Not infrequently the line of 

 parting between the two series was concealed by a thick soil and over- 

 grown with forests, but the distribution of the mounds would often show 

 the line of parting within 20 or 25 meters. 



My former assistant, Mr. Eoderic Crandall, who has traveled exten- 

 sively in Bahia, Pernambuco, Piauhy, Minas, and Goyaz, writes, in reply 

 to my inquiries, as follows regarding the preference of the ants for cer- 

 tain soils: "In Bahia the ants of all kinds show a preference for the 

 Estancia and Caboclo shales ; next to these the granites seem to have the 

 biggest nests." (Private letter; Eio, June 23, 1909.) 



I infer that the smaller number of the mounds on the sandy soil is 

 due to the fact that during the rainy season water soaks through into the 

 burrows, and the excavations do not stand up where the soil is wet. 



Thinldng it possible that the exposure of the mounds or of the ground 

 on which they stand to the sun might infiuence location and distribution, 

 an outlook has been kept with these questions in mind. It does not 



