1972] 
Alcock — Bembix americana 
163 
favor individuals that build multi-celled nests and thereby avoid 
having to dig through a stony compacted surface repeatedly. How- 
ever the population studied by Parker nested in sand dunes where 
this argument cannot apply. Parker was deeply impressed by the 
powerful winds that regularly swept the area often blowing fe- 
males away from their nest entrances. Perhaps with these condi- 
tions, selecting a new site and attempting to initiate digging would 
be both extremely time consuming and energetically demanding. 
This remains conjecture. 
It should be noted moreover that this hypothesis is weakened by 
the fact that those Bembix that dig very long (up to 1 m) burrows 
deep into sand dunes (e.g. occidentalis and pruinoso ) do not con- 
struct multiple celled nests. Although these are large and powerful 
species, nonetheless the costs of nest building must be great. It is 
difficult to understand why one long burrow could not accommo- 
date several cells. 
Nevertheless, assuming that there is some imperfect relationship 
between the expense of nest building and multiple cells, one would 
expect intraspecific variation in nest design to occur in species which 
are most flexible in the selection of nesting areas. Most North 
American Bembix are relatively restricted to one soil type (Evans 
1966, p. 352). However, amoena and americana are quite variable 
in nesting ecology while the requirements of the other species that 
make both single and multi-celled nests are uncertain. 
As a final question, why should the females I watched have tended 
to build a single celled nest first and then a multi-celled one? Per- 
haps such a strategy insures that if a wasp is able to provision only 
2 cells at least her offspring will be in 2 separate locations. But if 
she is able to provision more, it may then be to her advantage to 
reduce building costs by constructing side burrows and additional 
cells and to expend her time and energy in foraging and provision- 
ing. Probably the middle of the nesting season coincides with the 
peak abundance of prey and the best weather conditions. Thus it 
may be especially important not to spend this time hunting for and 
digging a new nest, time that could be spent provisioning cells be- 
fore the onset of the fall rains in Seattle (which often begin by 
mid-August). Thus the nest building behavior of a female ameri- 
cana in Seattle may reflect conflicting pressures to avoid placing all 
her eggs in one nest while at the same time making the most of the 
short time available to her to collect prey. 
