SOCHA, J.J.; LEE, W.K.; WESTNEAT, M.W.; University of Chicago; Argonne National Laboratory; Field Museum: Beetle respiration mechanics analyzed with synchrotron x-ray imaging

Many insects actively compress their tracheae to convectively move air through the body for respiration. Tracheae in the head of ground beetles are highly compressible, providing substantial tidal volume. Here we provide detailed kinematics of this recently described novel mode of insect respiration. Synchrotron x-rays (25 keV) were used to observe changes in volume in the tracheal system of the ground beetle, Platynus decentis. We recorded phase-enhanced x-ray videos of 46 restrained beetles using optical magnifications of 2X and 5X and a field of view of about 1×2 or 2x3mm. Tracheal compression was observed in 80% of the beetles, with a frequency of 0.50�0.16 Hz (mean � SD). Tracheal compression either occurred synchronously throughout all major tracheae of the body and legs or occurred locally in the head and prothorax only. In many cases abdominal tracheae were not compressed. The primary, secondary, and tertiary tubes of the head and prothorax of 5 beetles (19 total respiratory cycles) were analyzed in detail. Compression occurred primarily in the lateral plane for the primary and secondary tubes, and in the dorsoventral plane for the tertiary tubes. Volume change was calculated by measuring change in tracheal width and length and calculating volume for an elliptical cylinder. Compression duration was 1.12�0.52 s and total tidal volume was 38�15%. The tidal volume of primary tracheal tubes was 51�23%, secondary was 34�15%, and tertiary was 26�18%. We suggest that convective air transport by tracheal compression is a primary means of respiration in ground beetles for key tissues such as brain, eyes, and muscles of the head. Tracheal compression in the head and thorax is widespread and appears to be a major component of insect respiratory physiology.