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Psyche 
[Vol. 91 
Adaptive Significance of N. radiata’s Life History Plasticity 
Is the life history plasticity exhibited by individual N. radiata 
adaptive? In discussing the adaptive significance of developmental 
plasticity, Smith-Gill (1983) distinguished between developmental 
conversion and phenotypic modulation. Passive response of the 
organism to environmental variation constitutes modulation, which 
is not necessarily adaptive. Continuous phenotypic variation fre- 
quently characterizes this type of plasticity. In N. radiata, non- 
adaptive phenotypic modulation is expressed as the variation in size 
at maturity and fecundity within spring and summer populations 
attributable to different feeding histories or web-site temperatures. 
On the other hand, the filmy dome spider’s variable phenology 
apparently involves a developmental switch, and is likely an exam- 
ple of adaptive developmental conversion. 
Several features of N. radiata’s phenology argue against pheno- 
typic modulation as the only explanation for the variable develop- 
mental rates exhibited by this species. One major feature supporting 
the argument against modulation is the presence of two pronounced 
peaks in adult abundance. It appears that individuals do not 
develop at similar rates during the season, with cold weather or 
changing photoperiod simply arresting development of all stages 
equally. That is, while variables such as temperature affect devel- 
opmental rate, the effects apparently depend upon both the stage of 
the individual and the time of the season. Further support for devel- 
opmental conversion comes from the timing of the summer peak in 
adult abundance. Not only do no adults over-winter, but nearly all 
summer-maturing spiders have completed the adult molt by the end 
of August. Furthermore, some spiders continue to grow and molt 
for many weeks after the last maturation ecdysis has occurred, even- 
tually maturing the next season at a larger size than those that 
became adult in August. This pattern constitutes strong evidence 
that a developmental switch has occurred earlier in the season, 
either in the earlier instars or perhaps as a result of the type of eggs 
laid by late-maturing spring females. The developmental fates of 
juveniles collected at different times is also evidence that such a 
conversion from rapid to slower development occurs well in 
advance of the end of the season. Additional evidence comes from a 
rearing study that found marked differences in the developmental 
fates of hatchlings emerging at different times during June and July 
(Wise, submitted). 
