236 
Psyche 
[Vol. 89 
forms in unpatterned linear arrangements. All three species appear 
to be genetically polymorphic for the time necessary to complete 
development. 
There was an association between sex and time required to com- 
plete development. Two-year forms were more frequently female 
than male and one-year forms were more frequently male than 
female. Female offspring are also typically placed in the innermost 
cells of the nest. In these linear nests, if a two-year form occurs 
between the nest exit and a one-year form, then the latter will fre- 
quently destroy the former to gain egress from the nest. Thus, this 
association between sex and developmental time may act to lower 
sib caused mortality. 
In addition, the association between sex and developmental time 
may constrain an approach to the equilibrium sex ratio because of 
selection for a particular developmental form. 
Although spatiotemporal heterogeneity of weather and/or re- 
sources has frequently been offered as an explanation for such 
developmental polymorphisms, it is by no means clear that this is 
the case for these species of Osmia. The high proportion of two-year 
forms, and the differences between developmental forms in mortal- 
ity, and perhaps fecundity as well, do not fit the profile which is 
typically offered for parsivoltine species. 
Acknowledgments 
We thank Glen Trostle, Mary Klomps, Pauline Anderson, Char- 
lene Roth, and Barbara Becker for their unselfish efforts in prepar- 
ing nest blocks, dissections of nests, and weighing bees; and Drs. S. 
D. Beck (Univ. of Wisconsin), Jerome Rozen, Jr. (Amer. Mus. Nat. 
Hist.), and C. A. and M. J. Tauber (Cornell Univ.) for their helpful 
comments on the manuscript. 
Literature Cited 
Beck, S. D. 
1980. Insect Photoperiodism. Academic Press, Second Ed., New York. 387 pp. 
Chippendale, G. M. 
1977. Hormonal regulation of larval diapause. Annu. Rev. Entomol. 
22:121-138. 
Cohen, D. 
1966. Optimizing reproduction in a randomly varying environment. J. Theor. 
Biol. 12:119-129. 
