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[Vol. 91 
radiata. Seasonal changes in the size-composition of the popula- 
tions were similar to those found earlier in a Michigan population 
(Wise 1976). A field experiment and rearings of juveniles collected 
at different times of the season confirmed the presence of rapid and 
slow developers. Furthermore, these studies, in conjunction with the 
results of other research (Wise, submitted), established that varia- 
tion in the timing of reproduction by spring-maturing females is the 
major contributor to the variable developmental rates of young 
spiders during the summer. 
Progeny of early-spring maturing females develop rapidly, 
mature by August and deposit egg sacs before the end of the season. 
Hatchlings from these sacs over-winter as early instars and mature 
late the following spring. Progeny of these late-spring maturing 
individuals do not develop to maturity within the same season. 
Instead, these offspring over-winter as juveniles and mature early 
the next spring. On average, then, three generations are completed 
every two years in these populations. 
These two paths of development represent two alternatives, 
determined primarily by the seasonal timing of reproduction, but 
subject to the influences of other environmental variables. Thus the 
pattern as described is idealized. The actual fraction of spiderlings 
developing along each path varies between sites, and even between 
years at the same site, in response to differences in environmental 
factors such as web-site temperature, length of the growing season 
and possibly prey abundance. In different situations anywhere from 
a few to most of the progeny of late-spring maturing individuals 
may develop to maturity in the same season that they hatched. 
Some northern populations of N. radiata may be mainly univoltine, 
whereas populations at the southern limits of the species’ range may 
be entirely bivoltine. 
The phenology exhibited by Maryland and Michigan populations 
of N. radiata is intermediate between an annual and a bivoltine life 
cycle. This type of phenology has not been reported often for spid- 
ers. Whether or not this pattern is unusual, or is common but 
undocumented, remains to be established. 
Acknowledgements 
K. Cangialosi worked on the project from the beginning and 
made innumerable contributions to its successful completion. D. 
