2O2 



ANTS. 



pebbles and vegetable detritus such as straws, twigs, pine-needles, 

 leaves, etc. The propnrti' m> of these various constituents differ greatly 

 in the different spirit -. In our eastern I'onnica c.vsccloiclcs ( Fig. 109) 

 which constructs conical mounds sometimes a metre in height and two 

 to three in diameter at the ba>e. earth greatly predominates, whereas in 

 the Furopean /". nt/a ( Fig. 110) and our western subsp. obscnripcs 







FIG. iii. Mound of thatching ant (Formica obscuripes) of Colorado, made of coarse 



twigs and grasses. (Original.) 



i Fig. ill) the dome-shaped nest consists of a mass of sticks or 

 pine-needles resting on a large crateriform earthen base. In 

 Pogonomyrmex molefaciens and occidentalis ( Fig. 108) the mound 

 consists very largely of pebbles. The number and position of the 

 nest openings is also highly variable. In F. nifa the numerous open- 

 ings are scattered over the whole surface of the mound, in F. 

 c.vsectoidcs they are mostly aggregated in a broad belt around the 

 base, in inolcfacicns there is a single opening at or near the summit, 

 whereas in P. occidentalis the single entrance is situated at the base, and 

 almost invariably on the southern or eastern side (Fig. 108). There can 

 be little doubt that the mound nests of the species of Pogonomyrmex 

 mentioned above have arisen from the large crater, which is the only 

 form of nest in most species of the genus, through stages like those 



