Pressure-induced circumferential and longitudinal deformations of tracheal tubes in the American cockroach.

Insects exchange gases through a complex internal network of tubes known as tracheae, which deliver oxygen directly to tissues and remove carbon dioxide. In some species, these tracheal tubes undergo active compression, periodically collapsing and reinflating to facilitate internal airflow. The mechanical behavior of the tracheal system is governed by its structural design, which in turn influences its physiological role in respiration. Despite the critical importance of tracheal material properties in insect respiratory function, there are relatively few published studies that characterize their uniaxial tensile behavior. In this study, we present new experimental methods for measuring the pressure-induced biaxial deformations of tracheal tubes isolated from the American cockroach (Periplaneta americana). To this end, an inflation-extension testing device was built to subject tracheae to increasing internal pressures (0-6 kPa) and axial displacements (0-0.2 mm). Local circumferential and longitudinal stretches were quantified using non-contact strain measurement techniques. In most cases, circumferential stretches increased nonlinearly with applied pressure at any axial displacements, whereas longitudinal stretches changed minimally. This behavior likely reflects the combined influence of structural anisotropy, mechanical coupling and geometric constraints. The observed deformations highlight the mechanical sophistication of insect tracheae. They underscore the importance of integrating geometry and microstructure to understand how these structures resist collapse, enable gas exchange and adapt to mechanical demands.