Semaphorins (Semas) are an evolutionarily conserved family of proteins that have been implicated in the development of a wide range of tissues in a variety of organisms ranging from arthropods to mammals. Understanding the function of Semas in a developmental context has led to the elucidation of their roles in various disease states. With this in mind, it is critical to understand the basic function and conservation of this family of proteins across different species to better appreciate their importance from a developmental and evolutionary perspective. In order to better understand the broad impacts of Sema1a signaling, my graduate work focused on two dipteran species – Drosophila melanogaster and Aedes aegypti. In my work, novel roles of Sema1a in the well-studied model organism D. melanogaster were identified. I showed that Sema1a signaling regulates cellular growth within the wing imaginal disc tissue. This work suggested Sema1a signaling could modulate growth in developing neurons in addition to its well-studied function in axon guidance and highlighted similarities between axonogenesis and tumorigenesis (Chapter 2). In addition, our comparative, multiple species approach has also allowed us to fill the fundamental gap in understanding the development of disease vector mosquitoes. Specifically, we characterized the role of Sema1a in the central nervous system (CNS) and olfactory system development of the non-model species and deadly disease vector, Ae. aegypti (Chapters 3 and 4). Not much is known about the development of these arthropods, including the mechanisms underlying the development of the olfactory system. The olfactory system is of particular interest because it plays a critical role in mosquito behavior (e.g. host-seeking and mate selection) and, therefore, is a necessary component of disease transmission. The results of this work showed that sema1a plays a critical role in Ae. aegypti CNS and olfactory system development and demonstrates the importance of studying the development of disease vectors.  The work presented in this dissertation underscores the importance and utility of a comparative approach in developmental biology. It also advances the vector biology field by making strides in understanding the developmental biology of a species that is responsible for the transmission of deadly diseases.