Abstract
Most insect tissues acquire ambient oxygen through tracheal system, which originates from the exoskeleton, subdivides into bronchial conduits and spreads in whole insect body. Although diffusion has been proposed to be the primary mechanism for tracheal system to convey oxygen, difficulties in looking through the opaque exoskeletons and tissues impede quantitative analysis of tracheal capacity for oxygen diffusion. Here, we demonstrate three dimensional tracheal images in the lantern of fireflies with 3 μm spatial resolution by using synchrotron phase-contrast micro-tomography. Terminal branches of tracheal system (tracheoles) with a diameter of 0.2 μm were viewed by Transmission X-ray Microscopy (TXM) with spatial resolution of 40 nm. Oxygen contained in lantern tracheal system plays important role in triggering bioluminescent reaction in light emission cells (photocytes). It was previously held that during lantern is at quenching state, oxygen going to the photocytes from tracheal system was presumed to be impeded by fluids contained in tracheoles. Bioluminescence was triggered by the withdrawal of tracheolar fluids by tracheolar and tracheal end cells, facilitating oxygen diffuse into the photocytes. However, quantitative analysis of lantern tracheal system shows that diffusion though, is extremely efficient in air-filled tracheas, oxygen may be used up by mitochondria adjacent to the tracheoles. Though tracheal compression in lantern implies forced convection as a mean of respiration, the limited compressing area and the inconsistence tracheal branches to Murray’s law make it unlikely for forced convection as primary mechanism. Energy expenditure of firefly light emission was reported with distinct values. Analysis of tracheal surface density in male’s lantern shows that the gas exchange capacity is close to insect jumping muscles. Measurement of bioluminescent intensity in three species of fireflies at close distance shows that maximum bioluminescent intensity range from 0.75 ~ 4.6 x 1012 photons per second, which indicates volume-specific oxygen consumption rate of 5 ~ 40 nmol per cm3 tissue per second. The range is partially overlapped with that of jumping behavior of juvenile locusts (~ 40 nmol/cm3/sec). We thus suggest that lantern is a moderately active organ, varying with species and genders.