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[Vol. 89 
substantial temporal heterogeneity in the environment and that via- 
bility of two-year forms is about the same as that of one-year forms. 
In addition, the data suggest that there may be differences among 
these species in the way a heterogeneous environment is expe- 
rienced. The percentage of two-year forms appears to be higher for 
O. montana and O. californica than for O. iridis (Fig. 1), and this 
suggests that variance in reproductive success is lower for O. iridis 
than for the other species. 
Although there are no data available to directly address these 
predictions, trap-nesting returns from northern Utah over the past 
10 years (Torchio, unpub.) suggest that O. montana and O. califor- 
nica populations are much more stable than are those of O. iridis. 
The latter species is only occasionally abundant and, more fre- 
quently, is totally absent from trap-nests. Conversely, trap-nest 
returns for O. montana and O. californica fluctuate within much 
narrower limits. Thus, the data available to us do not support 
Cohen’s ( 1966, 1968) predictions. 
Another characteristic expressed by these and other species that 
does not seem to conform to Cohen’s (1966, 1968) predictions is the 
relative viability of one- and two-year forms. For example, Sullivan 
and Wallace (1967) reported that mortality increased and fecundity 
decreased with prolonged diapause in the sawfly. Neodiprion ser- 
tifer (Geoff.). Although we were unable to compare the mortality 
rate of one- and two-year forms because it was impossible to assign 
immature deaths in the first year to either category, it seems clear 
that mortality of two-year forms must be higher than that for one- 
year forms because some of the former will be destroyed when the 
latter exit the nests in the first year (Table 3). Thus Cohen’s 
requirement that viability of one- and two-year forms be equal 
seems not to be satisfied. In this regard, the advantage of producing 
a greater proportion of two-year females than two-year males may 
simply be a mechanism to reduce mortality levels of two-year forms 
because females almost always occur in the inner cells of the nest 
where mortality due to emergence of one-year forms is minimal. 
Thus some degree of linkage between sex and developmental time in 
such a system would be selected for. Without linkage, sibling- 
effected mortality on two-year forms would be even higher. 
Despite the apparent lack of agreement between the data and the 
predictions of Cohen (1966, 1968), the between year variation in 
