Light production in Photurius pennsylvanica (Photuris pennsylvanica) larvae occurs in the roughly 2,000 photocytes located in the heavily innervated light organ of the insect which is much simpler than that of the adult organism. The transparent photocytes of the larvae are clearly distinguishable from the opaque dorsal layer cells that cover them. Nervous and intracellular mechanisms contribute to light production in the photocytes. Nervous and intracellular mechanisms contribute to light production in the photocytes.It has been shown that fireflies can modify the amount of oxygen that travels through their trachea system to the light organ which plays a role in oxygen availability for light production. They do this by modifying the amount of fluid present within the trachea system. Because oxygen diffuses more slowly through water than in a gaseous form, this allows fireflies to effectively change the amount of oxygen reaching the photocytes. Spiracles can be opened and closed to control the amount of air that is able to pass through the tracheal system, but this control mechanism is only used as a response to a stressor.