1906.] Wheeler, Relations of Ants to Plants. 4^5 



face." On the contrary, it is probable that much of the earthen 

 material of the mounds, like all the vegetable detritus, is collected 

 by the foraging workers from the surface of the area surrounding the 

 nest. This material is brought together and built into the mound in 

 the very same manner as the harvesting ants Pogonomyrniex harbatus 

 and P. occidentalis collect their pebbles, and F. ruja its vegetable 

 detritus. The interior of the mound of all these ants is riddled with 

 galleries in which the larvae and pupae can be kept during the warm 

 hours of the day for the purpose of accelerating their development. 



It is not only an easy matter to observe any of our mound-making 

 ants in the act of picking up and carrying materials to its nest, but 

 the nature of the materials in certain localities is indisputable proof of 

 such activity. Along the railroad tracks through Indiana, Ohio, and 

 western New York, may be seen dozens of mounds of F. fusca var. 

 suhsericea covered with a thick layer of locomotive cinders that have 

 been carefully collected by the worker ants. Even more striking 

 is the same habit in Pogonomyrniex barhatus var. molefaciens along 

 the railways of Texas, and in P. occidentalis in similar situations in 

 Colorado and New Mexico. The large nest cones are often so black 

 with cinders as to stand out in very sharp contrast with the pale 

 surrounding soil. Along the Chicago, Burlington & Quincy R. R. 

 for many miles east of Denver, Colorado, the nest cones of occiden- 

 talis close to the track are deep black, while those further away, to a 

 distance of about thirty or forty meters, show a gradual fading in 

 color, with the decrease in available cinders, to the light-colored 

 nests of the open plains beyond. Recently, while collecting ants near 

 Florissant, Colorado, I came upon a colony of three nests of F. ex- 

 sectoides var. opaciventris. One of these, near the Colorado and 

 Midland R. R. track, was heavily covered with cinders like the nest 

 cones of P. occidentalis above described. Cinders in such situations 

 are, of course, very suitable material for retaining the sun's heat, but 

 it would be going too far to attribute to the ants any knowledge of 

 their value in incubating the brood. Any particles of such a size and 

 shape as to be conveniently carried to the nest, will be collected by 

 these indefatigable insects. At Ash Fork, Arizona, I found some 

 of the cones of P. occidentalis covered with the dung-pellets of sper- 

 mophiles, and Wasmann has recently observed a similar habit on the 

 part of the European F. pratensis, which, in certain localities, covers 

 ts nest with the dung-pellets of rabbits and the dried flower-heads 

 of Centaurea. 



Even more interesting than the dimensions and composition of 



