80 Psyche [June 
The complexity of the genitalia will cause slight modi- 
fications to result in physiological isolation. 
In the case of the bee, Perdita opuntice, progressive excava- 
tion of the nest is probably continued year after year, by this 
means leading to the complicated galleries in the stone. In Fig. 
1, the nest at the extreme right had four blind pockets which 
were not constructed into chambers. In other places, similar 
extensions were present. These, along with the old chambers, 
undoubtedly would have been used by the next year’s brood. 
In places, as labelled in this same drawing, one could see a 
cocoon 9 or dead bee. So, by June 21, when the nest was chiseled 
out, the process of provisioning was near completion. By the 
first of July, the cactus flowers were beginning to disappear, and 
by the eighth not a bee of this species was to be seen at White 
Rocks. Just a very few of the cactus flowers remained. The 
bee had disappeared for the year. The larvae were beginning 
their long period of development, waiting quietly through the 
long months of the hot summer and bleak winter in their cells 
of stone, inpreparation for the emergence next spring as adult bees. 
Before this time, on June 20, I had covered the White 
Rocks from end to end mapping out the distribution of the bees 
as I found them in the cactus flowers. This covered a territory 
two and a half miles long by one quarter mile wide. Every- 
where that I found the cactus flowers, numbers of these bees 
were present. This plant is a native of this country. Beyond a 
doubt, when the Indians used the White Rocks as a look-out 
station, 10 in the pre-pioneer days, the cactus plants were quite 
generally distributed all around the territory. Perhaps a long 
time before that there were numerous rock formations all over 
the West, in which the bee made its nests. As time passed on, 
these disappeared, leaving the White Rocks as an erosive rem- 
nant on which Perdita opuntice and its favorite cactus flowers were 
stranded. Perhaps extensive collecting would reveal this bee at 
some other sandstone formation where it had been similarly 
isolated. Up to this time, September, 1927, no such proof is 
9 Probably of a parasite. 
10 Numerous implements used by the Indians have been recovered from 
the White Rocks. These include, especially, mortars, pestles, etc. 
