ANT-NESTS. 203 



shown in Fig. 165. In F. nifa and e.rscctoides, however, the mound 

 seems to have developed from multiple fused craters like those of F. 

 sanynnica, ncocincrca and subsericea, species which are also in the habit 

 of accumulating a certain amount of detritus about the openings. This 

 habit in the various mound-building ants is most easily observed when 

 their nests are constructed near railway tracks. In such situations the 

 Pogonomyr.mex and Formica workers bring together great quantities 

 of locomotive cinders and place them on their nests, so that the latter 

 stand out as black hillocks in striking contrast with the surrounding 

 soil and vegetation. In certain localities in Arizona, P. occidental-is 

 also covers its mounds with the dung-pellets of spermophiles, and Was- 

 mann has noticed that the European F. pratensis employs rabbit dung 

 and the dried flower-heads of Centaitrea in the same manner. 



Forel has shown that the mounds of F. rnfa serve the important 

 purpose of incubators for the brood. During the breeding season the 

 leaves and sticks of which they consist tend to acquire the high tem- 

 perature of a compost heap, and thereby accelerate the development of 

 the larvae and pupa. Escherich has found that the temperature of the 

 mounds is sometimes 10 C. higher than that of the surrounding air and 

 must be much higher than that of the galleries in the subjacent soil. 

 Undoubtedly the gravel mounds of Pogonomyrmex barbatus, mole- 

 fade us and occidcntalis are equally useful as incubators. 



Other mound nests, differing from the foregoing in their smaller 

 size and compact earthen structure, have been designated by Forel as 

 masonry domes (domes maqonnes). This authority, who in 1900 made 

 a hurried myrmecological excursion through the Atlantic States, was 

 surprised to find that many circumpolar ants (Lasius niger and flams. 

 Formica fitsca and sanguined) > which construct masonry domes in 

 Europe, fail to exhibit this peculiarity in the United States. He con- 

 cluded that these structures, which, like the large mounds, serve as incu- 

 bators, must be unnecessary in this country on account of its great 

 annual extremes of climate. This inference is certainly premature, for 

 although it is true that many of the circumpolar species do not make 

 domes in the Atlantic States, they have this habit in the Mississippi 

 Valley and Rocky Mountains, where the annual extremes of tempera- 

 ture are even greater. Formica subsericea and many species of Lasius 

 and Acanthomyops become dome builders in Illinois and Wisconsin, 

 although it must be admitted that the term " masonry domes " is not 

 always strictly applicable to their nests, since the earth of which they 

 consist is not firmly compacted but carried up rather loosely around 

 grass and plant stems. I have frequently seen such mounds of Lasius 

 aphidicola, Acanthomyops intcrjectiis and clariacr and F. subsericea 



