282 
Psyche 
[Vol. 91 
The result is a life history in which the timing of reproduction is the 
major variable determining the developmental patterns of the 
offspring. 
In Maryland, spring-maturing N. radiata mate and lay eggs for 
8-10 weeks, from May through early July. Offspring of these adults 
thus emerge from egg sacs over a considerable span of time. Spider- 
lings that hatch from sacs laid by early-maturing adults have time to 
complete development and reproduce before winter. Many of those 
that hatch from egg sacs laid at the end of the spring reproductive 
period have a different fate. While some may mature by the end of 
the season, not all develop that rapidly. Those that emerge from the 
later egg sacs, or that capture insufficient prey or construct webs at 
sites with low temperatures, over-winter as juveniles and become the 
early-maturing adults of the following spring. Populations exhibit- 
ing this life history pattern complete approximately three genera- 
tions every two years. 
The degree of polymorphism exhibited by a population appears 
to vary with the length and average temperature of the growing 
season. Almost all N. radiata at the Patuxent site completed two 
generations in 1982. However, at the higher, cooler Liberty site, and 
in the Michigan population studied previously (Wise 1976), a signifi- 
cant number of the offspring of spring females over-wintered as 
immatures and did not become adult until the next year. The 
summer peak of adults is reduced in N. radiata populations in the 
Catskill Mountains of New York (Martyniuk, pers. comm.). These 
patterns lead to the prediction that extreme southern populations 
of the filmy dome spider are consistently bivoltine, whereas popula- 
tions at higher latitudes and elevations are univoltine. A substantial 
number of populations, perhaps most, likely show a life cycle that is 
intermediate between an annual and a biannual, or bivoltine, 
pattern. 
The extent of the life history polymorphism may vary yearly at 
the same site, judging from experiences at Patuxent 1981-82. In the 
1981 field experiment a substantial fraction of the spiders was still 
immature at the end of the season, yet in the following year very few 
progeny of spring adults failed to reach adulthood by August. In 
1982 almost an entire day of searching at Patuxent was required to 
locate the 41 juveniles for the rearing experiment, yet in the previous 
year the earlier developmental stages had been much more abun- 
dant the same time of the season. Possibly this difference in age 
