Abstract
Oxygen is an important and often limiting reagent of a firefly's bioluminescent chemical reaction. Therefore, the development of the tracheal system and its subsequent modification to support the function of firefly light organs are key to understanding this process. We employ micro-CT scanning, 3D rendering, and confocal microscopy to assess the abdominal tracheal system in Photinus pyralis from the external spiracles to the light organ's internal tracheal brush, a feature named here for the first time. The abdominal spiracles in firefly larvae and pupae are of the biforous type, with a filter apparatus and appear to have an occlusor muscle to restrict airflow. The first abdominal spiracle in the adult firefly is enlarged and bears an occlusor muscle, and abdominal spiracles two through eight are small, with a small atrium and bilobed closing apparatus. Internal tracheal system features, including various branches, trunks, and viscerals, were homologized across life stages. In adults, the sexually dimorphic elaboration and increase in volume associated with tracheal features of luminous segments emphasizes the importance of gas exchange during the bioluminescent process.