2 
Psyche 
[March 
hundred minors. These figures are based on totals secured by pro- 
longed baiting of two of the colonies. Since neither nest was put 
out of action by the baiting it seems certain that the estimate is on 
the conservative side, yet there would be justification for the view 
that a much smaller population was present. There is rarely a con- 
spicuous accumulation of excavated soil or of chaff around the nest 
entrance, for both are brought to the surface gradually and in small 
quantities. Even when a crater is built its diameter seldom exceeds 
three or four centimeters. Moreover, the crater is a transient struc- 
ture for, since ridicula mixes the chaff with the excavated soil 
particles, the texture of the crater is loose and light and it is easily 
scattered by wind or rain. Hence much of the time the only indica- 
tion of a ridicula nest is the nest entrance itself. This is never more 
than five millimeters in diameter and, more often, its diameter is 
about two millimeters. In addition, the nest entrance is frequently 
blocked up and drifted over with windblown dust and detritus. 
Early in this study the writer found it necessary to mark the nest 
entrances in order to be sure of their exact position. 
There is a simple explanation for most of the above features. While 
ridicula will sometimes bring in other seeds, it is mainly interested in 
those of the careless weed, Amaranthus palmeri. These seeds are 
matured throughout the year, hence there is an ample supply of them 
at all times and large numbers are not garnered seasonally. More- 
over, a great many of the palmeri seeds are free of any covering when 
they are brought to the nest. As a result there is no occasion for 
the production of a large chaff pile or an extensive crater since, in 
the genus Pheidole, both these features usually result from a seasonal 
excess of grass seeds which must be stripped and stored 2 . 
The soil in which ridicula nests is the Victoria loam, a fine-tex- 
tured, compact soil which is virtually stone free. As the walls of 
excavations made in it are slow to crumble, there was every reason to 
expect that a ridicula nest could be fully exposed. Actually this 
proved to be impossible. It was easy to trace the main nest passage, 
which consists of an unbranched shaft of remarkably uniform diam- 
eter (about 2 mm.) that descends vertically through the soil to a 
depth of about thirty-two inches. It was not difficult to demonstrate 
the existence of lateral passages leading away from the main shaft, 
for the workers would open up the transected ends of these passages 
2 Similar considerations apply to Ph. cerebrosior Wh. which mainly garners 
the seeds of desert portulacas ( P . oleracea Linne and P. retusa Engl.). This 
crop is seasonal but, since the seeds are bare when brought to the nest, no 
chaff pile results. 
