Andrena Nesting Biology 
143 
were slightly more numerous toward the rear of the amphitheater. This 
distribution is perhaps influenced by morning sunlight which reaches the 
rear area earliest. Early warmth after cool January nights may allow 
more foraging time. 
Conspicuous, circular tumuli were usually present at nest entrances. 
They ranged in diameter from 2.0 cm to 4.3 cm (x = 3.2 cm, N = 30). 
Tumuli height ranged from 0.9 cm to 2.0 cm (x = 1.6 cm). Most nests 
were clustered in groups of 2 to 5, with many tumuli overlapping. Solitary 
nests were seldom found. 
Nests were excavated with the aid of plaster of Paris poured into the 
entrances. Tunnels descended vertically except to circumvent roots. They 
were circular in cross-section, ca. 5.0 mm in diameter, and their walls 
consisted of unlined, slightly compacted, moist sand. The main shaft ex- 
tended from 28.5 cm to 47.0 cm deep (x = 37.8 cm, N = 46). At the bot- 
tom of the vertical tunnel a lateral tunnel ca. 1.5 cm long terminated in a 
single cell. Although additional cells may have existed, none was located. 
Thorp and Stage (1968) reported difficulty in finding laterals of Andrena 
placida which also nests in sand. 
Most cells were oriented horizontally to the main tunnel. They were 
slightly larger in diameter than the descending shaft (x = 5.7 mm, N = 
18) and were lined with a thin, transparent waterproofing material. Cells 
were shiny, smooth, and jet-black from soil carbon wetted as the cell was 
lined. Microscopic examination showed that individual sand grains were 
cemented together by the hardened lining material. 
Provisioning . — Pollen balls were spherical, dark yellow, and uniformly 
moist. Completed balls ranged in diameter from 2.5 mm to 3.2 mm (x = 
2.9 mm, N = 25). As previously noted for other Andrena (Michener and 
Rettenmeyer 1956, Linsley and MacSwain 1959, Davis and LaBerge 
1975), incomplete balls were smaller, less smooth, and drier than com- 
pleted balls. 
Slides were prepared of provisions taken from three different nests ca. 3 
m apart. Microscopic examination of several hundred pollen grains 
proved them to be of a single type, probably Acer rubrum, typically a wind 
pollinated species. We never observed bees upon the flowers as it was not 
possible to follow them outside the amphitheater and no A. rubrum were in 
the immediate area. The bees never flew through the hammock, but 
always left or entered the area from above the surrounding trees. As noted 
by Graenicher (1930), bees are seldom found in a shaded hammock 
interior. 
Adult bees were actively foraging when the study site was first visited 
on 31 December 1975. A steady progression of individuals left the 
