To further elucidate the possible biomechanical role of the outer helicoidal region, we performed qualitative fracture experiments: high load indentation was used to induce cracking and the sample surface was examined in SEM (Figure 6). Indents placed in the striated region (along the fiber axis, transverse section) require a fairly high load (\(\sim\)1000 mN) to induce widespread damage: cracks follow a rather straight path and spread radially in the matrix between fiber stacks (Figure 6A). A central remark is that all cracks going towards the highly mineralized region are stopped by the outer helicoidal arrangement (Figure 6B and Figure S10 ). This is consistent with the extensive delamination-based damage observed in transverse sections (Figure 2A) and confined within the striated region by the inner and outer twisted plywood. The damage observed in Figure 2A was not induced by indentation but, most likely, by sample preparation. Specifically, sample dehydration and related shrinkage are possible sources of local cracking. Since mineral is believed to replace water, the amount of dehydration (and shrinkage) should be higher in STR than in HMR (which is more mineralized and therefore should contain less water). At the same time, cracks were not observed in the OHR and IHR, which have a comparable degree of mineralization than STR but helicoidal arrangements preventing crack growth. A comparable damage behavior has been reported for the uropod back spike, which has a similar multilayered microstructure, but lacking the external highly mineralized region[42]. Indents performed in the highly mineralized region require smaller loads (i.e., 50 - 500 mN) to generate extensive cracking. In this heterogeneous region, a relevant observation is that damage is more likely observed far from the transition region, which is also the most compliant zone of the highly mineralized region. Indeed, indents placed in the vicinity of this transition region either do not cause cracks (Figure 6C) or induce damage running parallel to the transition region and its lamella (Figure 6D). Remarkably, no cracks are observed crossing the interface between the highly mineralized and the outer helicoidal region. Typical high-load indentation curves a for the highly mineralized region (HMR) and for the less mineralized but highly anisotropic region (STR) are shown in Figure S11 . HMR featured a characteristic pop-in event (highlighted by the arrow) associated with sudden damage beneath the contact surface. Conversely, the indentation curve of STR is free from pop-in events, indicating a more progressive damaging behavior. The tiny dimension of the OHR (less than 10 μm in width) precluded a quantitative assessment of the fracture toughness of this region as previously done for the impact surface of both spearer and smasher stomatopods [33,35]. Nevertheless, our indentation-based fracture study proofs the critical role of the OHR for decoupling damage mechanisms between HMR and STR regions. A closer examination of the surface of a fractured spike highlights the damage behavior of the regions from a different perspective (Figures 6E and F). There is a clear transition between the rather straight fractured surface of the highly mineralized region and the much rougher surface at its interface with the outer helicoidal region. Inside the latter, individual lamellae can be seen, suggesting a plausible role in crack deflection and a corresponding switching from a more brittle to a more ductile damage behavior. Furthermore, between the fiber beds of the outer helicoidal region, unbroken pore canals are present. As in decapods crustaceans [44], canals of the stomatopods also contain fibers [36] that may act as bridges between the chitin-protein fibers beds, possibly enhancing fracture resistance [47].