Abstract
Air sacs are thought to be the bellows for insect respiration. However, their exact mechanism of action as a bellows remains unclear. A direct way to investigate this problem is in vivo observation of the changes in their three-dimensional structures. Therefore, four-dimensional X-ray phase contrast microtomography is employed to solve this puzzle. Quantitative analysis of three-dimensional image series reveals that the compression of the air sac during respiration in bell crickets exhibits obvious anisotropic characteristics both longitudinally and transversely. Volumetric changes of the tracheal trunks in the prothorax further strengthen the evidence of this finding. As a result, we conclude that the shrinkage and expansion of the insect air sac is anisotropic, contrary to the hypothesis of isotropy, thereby providing new knowledge for further research on the insect respiratory system.
Highlights
Insects are among the most diverse groups of animals on the earth, comprising approximately 5.5 million described species and representing more than half of all known living organisms[1]
Four main tracheal trunks connect the respiratory system in the head and the prothorax
Supplementary Movie 6 shows the compression of the air sac separately, in which different portions of the air sac show various compression situations
Summary
A typical insect species, the bell cricket (Meloimorpha japonica; Haan, 1842), was selected as the sample. At the 25th second, a short duration of the blue colour is next to a short duration of the red colour, followed by a longer duration of the green colour These changes in colours indicate that the portions around the 180th and 290th slice of the air sac experience short-lived compression every once in a while. According to the changing rate of the cross-sectional area, we selected blocks of slices 80–140, 150–210 and 260–320 as the segmented volume of the air sac According to the quantitative analysis of the three-dimensional structure of the air sac, the average segmented volumetric compression ratios near the 110th, 180th and 290th slices are 4.8%, 1.8% and 3.1%, respectively These results further strengthen the evidence for longitudinal anisotropic shrinkage. The arrow represents the direction of motion; the length represents the value of speed
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