76 
Psyche 
[April 
points out, built up at the rate of 1/i cu. ft. per annum, the 
second at the rate of 3, and the other two at the rate of 1. The 
second hill arrived in about one year to above the average size 
of the mounds of that region, and its very great rapidity of 
growth may have been due, McCook supposes, to the fact that 
it was not a new colony but was reconstruction work built up 
on the ruins of an old mound. 
McCook inferred that mounds might require five to seven 
years to be built up to average size and that once full grown they 
do but hold their own. He says there were good reasons to sup- 
pose that some of the large mounds might be thirty years old but 
no evidence that any of them last through great periods of time. 
McCook states that the two small mounds in the corn field were 
probably new communities and it is evident that only such new 
communities should be considered in reckoning the initial rate of 
construction. Each mound is the communal work of a family 
and grows as the family prospers and multiplies year by year 
since its individuals escape death in the winter by withdrawing 
deep under ground in subterranean tunnels and are known to 
be able to live in captivity as much as seven years. The very 
rapidly rebuilt mounds on the other hand are the reconstruc- 
tion work of prosperous communities that are able to repair 
even great amounts of destruction and removal of old mound 
materials. 
As these estimates of McCook on the rate of building 
mounds by Formica exsectoides seem to be the only ones printed, 
the following data collected near Baltimore, Md., may be worthy 
of record. 
Near Lutherville and Timonium in Baltimore County there 
is a settlement of these ants embracing some two hundred mounds 
and measurements made at irregular intervals from 1905-1924 
furnish added means for judging of the rate of growth and the 
age of the mounds of these ants. During this period some of the 
mounds have remained in existence; either growing or remaining 
stationary in size; others have disintegrated; and others have 
sprung up anew. Parts of the entire area have been largely 
abandoned and other parts have been invaded by new mounds. 
In one of these newly populated areas measurements have been 
