VARIATION IN CELL NUMBER IN 
A POPULATION OF BEMBIX AMERICANA 
(HYMENOPTERA: SPHECIDAE)* 
By John Alcock 
Department of Zoology, Arizona State University, 
Tempe, Arizona 85281 
This paper reports the results of observations made during the 
summers of 1971 and 1972 on a small group of Bembix americana 
Fabricius nesting in a vacant lot in Seattle, Washington. This pop- 
ulation was judged to be intermediate to comata Parker and spinolae 
Lepeletier at the subspecific level (H. E. Evans, pers. comm.). 
Probably no North American digger wasp has been more thor- 
oughly studied than B. americana (Evans 1957, 1966; Evans and 
Matthews 1968). However there has only been one report of this 
species making nests with more than a single cell (Parker 1925). 
Evans (1966) notes the uniqueness of this account and the difficulty 
of accurately digging out nests in dense colonies. Nevertheless in 
the course of my work I discovered that some of the wasps in Seattle 
were making multi-celled nests. The evidence for this is presented 
here as well as data on the nest building behavior of marked females 
over a period of time. 
The Nesting Area 
Eleven species of sphecids and pompilids nested in a 13 m section 
of an old path running through a grassy area in a vacant lot in 
Seattle. The lot, which was largely covered with blackberry bushes 
(Rubus laciniatum) , bordered Lake Washington and was within 
sight of the University of Washington. The nesting area was rel- 
atively open and somewhat less pebbly than other parts of the path. 
However, only a few places were truly sandy and most nests trav- 
elled in part or in whole through hard-packed gravelly earth with 
pebbles up to 5 cm in diameter present. 
The 2 most abundant species in the aggregation were Philanthus 
crabroniformis Smith and B. americana ; at the peak of the nesting 
season there may have been as many as 40 americana nests. In the 
summer of 1971 a group of nests were marked and closely watched 
for periods of 1-3 hr almost every day between July 24 and August: 
* Manuscript received by the editor September 30, 1972 
158 
