268 
Psyche 
[Vol. 91 
Methods 
The research, conducted at two sites 35 km apart, involved cen- 
susing undisturbed areas, manipulating the age structure of one 
population in a field experiment, and rearing field-collected juve- 
niles from both populations. 
Study Sites 
The terrain of the oak forest on the Liberty Watershed, 40 km 
northwest of Baltimore, Maryland, was hilly, with rocky outcrop- 
pings, fallen trees and small saplings offering a variety of substrates 
for N. radiata to attach its web. A population at the Liberty site, on 
a south-facing slope at 180 m elevation, was studied 1980-82. 
The Patuxent site was located south of Liberty on the Patuxent 
Wildlife Research Center near Laurel, Maryland, at an elevation of 
40 m. The oak forest at Patuxent differed from that at Liberty by 
having several beech and some pines. The terrain was flatter, with 
no rocky outcroppings and with a floristically less diverse under- 
story. Much of the Patuxent ground cover was Vacunium sp. Rela- 
tively rare at Patuxent before 1980, the filmy dome spider was more 
common 1 98 1 -82. During these years several studies were done with 
the Patuxent population. 
Determining the Phenological Pattern in Maryland Populations 
During 1980-82 I monitored seasonal changes in the size-class 
structure of the Liberty population by repeatedly censusing 13 areas 
marked with short, inconspicuous stakes. At Patuxent 8 similarly 
marked areas were censused, but in 1982 only. Mature N. radiata 
were collected at Liberty (spring, 1980-81; summer, 1980) and 
Patuxent (spring and summer, 1981-82) by capturing every adult 
encountered during a search of vegetation in the vicinity of the 
areas set aside for censusing. These spiders provided data on yearly 
and seasonal changes in size at maturity and fecundity. Collected 
adults were anesthetized briefly with CO2, measured and then 
paired in jars kept in the laboratory (1980-81) or on a covered, 
screened porch in the forest at Patuxent (1982). The spiders were fed 
fruit flies ad lib. Females deposited an egg sac within one to several 
days after being collected. Each sac was removed, eggs were counted 
and the diameters of 10 eggs were measured with an ocular 
micrometer. In 1982 all the eggs from a sac were then placed in a 7 X 
10 mm plastic vial and its open end was plugged with silk from the 
egg sac. The vial was placed in a humid jar, where the eggs were left 
