Research Article

The Stomatopod Dactyl Club: A Formidable Damage-Tolerant Biological Hammer

Science  08 Jun 2012:
Vol. 336, Issue 6086, pp. 1275-1280
DOI: 10.1126/science.1218764

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  1. Fig. 1

    Morphological features of the stomatopod dactyl club. (A) A generalized stomatopod body plan and (B) a magnified view of the anterior end of O. scyllarus. The arrows denote the location of the dactyl club’s impact surface. (C) Backscattered scanning electron micrograph of the club’s external morphology and (D) a microcomputed tomographic longitudinal section through the anterior half of a complete specimen showing the constituent dactyl (D) and propodus (P) segments, revealing their differences in electron density (the second thoracic appendage with its terminal dactyl club modification is highlighted in red). (E) Cross-sectional analysis of the club illustrates the three distinct structural domains: (i) The impact region (blue), (ii) the periodic region [further subdivided into two discrete zones: medial (red) and lateral (yellow)], and (iii) the striated region (green). The periodic region of the propodus is shown in orange. (F) Optical micrographs, revealing the buckled rotated plywood structural motif of the impact region, the pseudo-laminations of the periodic region, and the thickened circumferential band with parallel chitin fibers in the striated region [(A) adapted from (37), (B) courtesy of S. Baron, and (D) courtesy of].

  2. Fig. 2

    Micromechanical and compositional variability in the stomatopod dactyl club. All of the images and plots in the right-hand column of the figure correspond to parallel analyses through the same region of a dry specimen, facilitating direct comparisons between ultrastructure, micromechanics, and elemental composition. (A) Diagrammatic backscattered electron micrograph through the dactyl club indicating the locations of the impact region (IR) and the periodic region (PR) and the corresponding optical [darkfield (DF), brightfield (BF), and differential interference contrast (DIC)], and backscattered scanning electron (BSE) micrographs of the area boxed in red. (B) Large-area nanoindentation [elastic modulus (E) and hardness (H)] map of the dactyl club and a corresponding line scan, including a high-resolution plot through five superlayers; periodicity: ~75 μm overlayed on a corresponding DIC micrograph. (C) EDS maps and line scans showing the nonuniform elemental distributions in the periodic and impact regions (the Mg concentration EDS data have been expanded by a factor of 5 relative to the Ca and P concentrations).

  3. Fig. 3

    Synchrotron x-ray diffraction (XRD) analysis and distribution of various mineral phases in the dactyl club. (A) A single diffractogram taken from the impact region (IR) of a transverse cross section. The preferred orientation of the hydroxyapatite (HA) crystal’s c axis (green arrows) is placed at the peak intensity of the HA (002) reflection. (B) Representative XRD patterns from the impact (IR) and periodic regions (PR) compared against standards. The colored areas of the impact and periodic region diffraction patterns were used to estimate the mineral concentrations shown in (C). (C) Mineral concentration maps for the HA and the amorphous phases (each of the four synchrotron maps measures ~2.5 mm across). The sloped black lines denote the preferred orientation of the HA c axis (002). A composite mineral concentration map (lower left) confirms that both maps measured the same boundary between phases. An x-ray transmission map (lower right) correlates inversely with mineral concentration.

  4. Fig. 4

    Chitin fibril helicoidal structural motif within the periodic region (with periodicity: ~75 μm). Comparisons between a generalized three-dimensional model of a helicoid (A) with an SEM fractograph (B) and a polished surface from a transverse cross section (C). (D) A visualization of the chitin fiber orientations from the x-ray scattering analysis of 92 separate diffractograms obtained through two superlayers. (E) Three representative χ plots of the α-chitin (110) reflection used to calculate fiber angles. The plots show changes in χ across the range of angles between each chitin fiber bundle and the x-ray beam. A charge contrast scanning electron micrograph from a damaged coronal cross section (F) with false color (G) and a model of a helicoidal slice (H), which accurately reproduces the fracture patterns.

  5. Fig. 5

    Dynamic finite element analysis (DFEA) and micromechanical modeling. (A) Geometry of the dactyl club/propodus system striking a target at 20 m/s. Color-coding corresponds to the different elastic properties and mass densities used for DFEA simulations (data obtained from nanomenchanical characterization of hydrated specimens and synchrotron x-ray transmission studies). (B) Evolution of the maximum principal stress σmax during the impact event until the propagating pressure wave reaches the end of the propodus. (C) Maximal principal stresses within the dactyl club at ~2 μs after impact. (D) Toughening strategies of the dactyl club: (i) hard outer layer for maximum impact force; (ii) modulus transitional domain for crack deflection between the impact surface and the bulk of the impact region; (iii) periodic region with helicoidal pattern and modulus oscillation for crack shielding. a, crack length; x, coordinate perpendicular to the crack front propagation; ξ, relative coordinate ahead of the crack tip in the periodic region (ξ = x a); E(x), elastic modulus oscillation.