That's how they roll: skeletal mechanics of enrolment and the importance of hydrostatic support in terrestrial isopods

Various arthropods can roll into a ball for protection. In terrestrial isopods (Oniscidea), this ability has likely evolved independently in different lineages. In this work, comparative imaging of extended and rolled isopods using micro-computed tomography of rapidly frozen individuals was coupled with live imaging. The results show that the points of rotation between consecutive body segments are positioned dorsolaterally and do not represent fixed joints, as previously assumed. Ridges at the anterior and posterior edges of consecutive dorsal plates (tergites) lean against each other at these points, allowing consecutive body segments to rotate relative to each other. Conglobating terrestrial isopods have shorter sternal plates than non-rollers. As a result, the sternal plates overlap only as an isopod rolls, forming furrows that accommodate the walking legs. The sternal plates are able to bend along a flexible suture at their midline, which is pulled inward during rolling. Both conglobating and non-conglobating terrestrial isopods are capable not only of ventral but also lateral flexion, which requires the separation of the surfaces that are in contact during rolling. The observed diversity of movements can be explained by the use of hydrostatic support provided by the hemocoel. The essential role of this hydrostatic support in the movement of isopods is demonstrated.