xii, 109 p. : ill. (some col.) A print copy of this thesis is available through the UO Libraries. Search the library catalog for the location and call number. / There are two fundamental environmental rhythms that organisms in nature encounter: (1) the daily rhythm of light and dark that is due to the rotation of the earth about its axis and (2) the yearly seasonal rhythm due to the angle of the earth's rotation relative to the plane of its orbit around the sun. All eukaryotes have an endogenous circadian (daily) clock that allows for the timing of biological events within the context of the daily light:dark cycle. A wide diversity of plants and animals in temperate regions use photoperiodic (daylength) cues to time life history events, such as reproduction and diapause (insect dormancy) within the context of the yearly seasonal cycles. This dissertation focuses on the relationship between the circadian clock, photoperiodic time measurement and diapause. Chapter I serves as an introduction to biological timing and briefly summarizes the chapters that follow Chapter II outlines why Drosophila melanogaster , the workhorse of modern insect genetics, is not an appropriate system for the study of photoperiodism. Chapter III defines the Nanda-Hamner response, the circadian phenotype used in this dissertation, and proposes that the NH response is due to a rhythmic level of circadian disorganization in response to environmental cycle length. Chapters IV and V deal primarily with the long-held proposition that the circadian clock forms the causal basis of photoperiodic time measurement. I show that variation in the circadian clock does not covary with photoperiodic phenotypes among natural populations of Wyeomyia smithii , and thus these two processes are evolutionarily independent. Chapter VI describes the first forward genetic screen for candidate genes involved in photoperiodism and diapause termination in any animal. Chapter VII is a discussion of the complexity involved in studies of the genetics of photoperiodism and diapause and how historical inertia of scientific hypothesis acts to confound, rather than clarify, the relationship between genotypes and phenotypes. Chapter VIII is a concluding discussion of the implications of the work presented. This dissertation includes both previously published and co-authored material. / Committee in charge: William Cresko, Chairperson, Biology; William Bradshaw, Advisor, Biology; Patrick Phillips, Member, Biology; Eric Johnson, Member, Biology; Stephen Frost, Outside Member, Anthropology