.IXT-XESTS. 20 5 



the advantage of an eastern exposure which lengthens their daily 

 activity. On a western slope, on the contrary, they lose the early 

 morning hours, are too warm in the afternoon and are unable to do 

 much after nightfall." Those nests, therefore, have the most favor- 

 able position which are exposed to the sun in the morning and shaded 

 in the afternoon. Not only is this advantage apparent in the greater 

 abundance of nests on southern and eastern slopes, but the nests them- 

 selves may show structural adaptation to the position of the sun. I 

 have already called attention to the constant position of the nest open- 

 ing at the base of the southern or eastern slope of the mounds of 

 Pogononiynnc.r occidcntalis. Huber says that the yellow ants (Lasins 

 flams ) of Switzerland " serve as compasses to the mountaineers when 

 they are enveloped in dense fogs or have lost their way at night ; for 

 the reason that the nests, which in the mountains are much more 

 numerous and higher than elsewhere, take on an elongated, almost 

 regular form. Their direction is constantly from east to west. Their 

 summits and more precipitous slopes are turned towards the winter 

 sunrise, their longer slopes in the opposite direction." These remarks 

 of Huber have been recently confirmed by Tissot (Wasmann, 19070) 

 and Linder (1908). The latter has shown that the elongate shape of 

 the mounds is due to the fact that the ants keep extending them in an 

 easterly direction in such a manner that only the extreme easterly, 

 highest and most precipitous portions are inhabited by the insects. I 

 have observed a similar and equally striking orientation of the mounds 

 of Formica argcntata in the subalpine meadows of Colorado. 



By far the greatest number of ant-nests, at least in many parts of 

 the world are excavated in the soil under stones, logs, boards, etc. 

 Most of our ants, including even those that construct large mounds, are 

 very fond of nesting in such places during the younger colonial stages. 

 In fact only two of our terricolous species Doryinyrnie.r pyrainicus in 

 the Southern, and Prcnolcpis imparis in the Northern States are so 

 rarely found under stones as to indicate that they have a pronounced 

 aversion for such sites. The advantage of nesting under stones is con- 

 siderable, for these not only protect the entrances, galleries and cham- 

 bers from rain and wind and enable the ants to dispense with the labor 

 of roofing over their surface excavations, but they are of even greater 

 service in conserving the moisture in the underlying soil while rapidly 

 taking up the sun's heat and thus accelerating the incubation of the 

 hrood. Nearly all ants prefer flat stones of moderate dimensions .and 

 not too deeply buried in the soil. Many Formica of the rufa and 

 sanguinea groups (nepticula, difficilis, consocians, microgyna, obscnri- 

 rcntris, orcas, ciliata, dakotensis, Integra, rubicunda, etc.) bank the 



