Animals use signals in order to elicit reactions from conspecific or heterospecific individuals that will increase their own fitness. Signals involved in mating are perhaps the most important conspecific signals as they directly relate to an organisms fitness. How and why new mating signals arise and how they spread through a population remain open fields of research in evolutionary biology. However it has been difficult to study signaling traits from a genetic perspective since signals, and the receptors that enable their detection, can be complex morphological and/or behavioral traits based on many genes. Fireflies (Coleoptera: Lampyridae) offer an ideal system to study signal evolution because of their conspicuous and highly variable sexual signals. With over 2000 species worldwide, fireflies exhibit lighted signals ranging from simple glows to complex flashes, as well as unlighted long-distance pheromone signals. Aside from differing in pattern, lighted signals also differ in color and range from blue/green to orange. Genes are known that govern light emission color, luciferase, and visual receptor sensitivity, opsins. Here, I conduct foundational research on the variation and molecular evolution of the opsins of North American fireflies between and within species to better understand how signalers and receivers have evolved in this system. I find evidence for only two opsins, one longwavelength-detecting and one ultraviolet, across 38 firefly species. Both opsins show molecular changes associated with evolutionary transitions from nocturnal to diurnal behavior. In contrast, within one widespread species, Photinus pyralis, I find little variation in opsins and luciferase across populations. Finally, in using developing genomic resources to identify opsins and luciferases, I discovered substantial variation in genome size across the family. Investigation into proximate and ultimate causes of genome size variation showed a dynamic repeat landscape and little evidence for selective explanations of genome size evolution. These studies highlight the utility of fireflies as a system to study both the genetics underlying signal evolution and genome evolution.