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First published online August 8, 2008
Journal of Experimental Biology 211, 2576-2583 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.020164
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Micromechanical properties of consecutive layers in specialized insect cuticle: the gula of Pachnoda marginata (Coleoptera, Scarabaeidae) and the infrared sensilla of Melanophila acuminata (Coleoptera, Buprestidae)

Martin Müller1, Maciej Olek2, Michael Giersig2 and Helmut Schmitz1,*

1 Institute for Zoology, University of Bonn, Poppelsdorfer Schloss, D-53115 Bonn, Germany
2 Forschungszentrum caesar, Ludwig-Erhardt-Allee 2, D-53175 Bonn, Germany


Figure 1
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  		Fig. 1. (A) Applied trapezoidal loading pattern with holding time of 10 s (plateau of curve) to reduce influence of material creep. (B) Representative load–displacement curves for the exocuticular outer shell (IR exo) and the inner mesocuticular core (IR meso) of the Melanophila sensillum.

 

Figure 2
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  		Fig. 2. Cuticle structure of the infrared organ of Melanophila. (A) Light microscopical image of two eccentrically cut sensilla stained with Mallory trichrome stain. Yellowish colour indicates unpigmented (β-sclerotized) exocuticle (exo), red indicates mesocuticle (meso), blue indicates endocuticle (endo). (B) AFM topography of one centrally cut sensillum. The layered exocuticular shell surrounds an inner spongy mesocuticle with lacunae and a pressure chamber (pc). Scale bars, 5 µm.

 

Figure 3
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  		Fig. 3. Cuticle structure of the gula of Pachnoda marginata. (A–C) Light microscopical images of Mallory trichrome stained gula sections. Scale bars, 20 µm. (D–E) Topography images made with an atomic force microscope. Scale bars, 5 µm. (A) Rostral beginning of the gula plate (from arrowhead to the left). The unusual layering of the cuticle starts to develop in the centre of the image and extends to the left. Yellowish-stained exocuticle (exo) is superimposed by red-stained mesocuticle (meso) and blue-stained endocuticle (endo). (B) Image of the region probed with the nanoindenter. Only a very thin layer of endocuticle can be distinguished. (C) Magnification of the unusual surface region. (D) Topography of the sliced outer meso-/endocuticular layers, which appear grainy and differ from the smooth cut exocuticle and mesocuticle 3 (meso3) layers shown in E.

 

Figure 4
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  		Fig. 4. Means of (A) material hardness and (B) elastic modulus of the different types of cuticle of the Melanophila IR organ and the gula plate of Pachnoda marginata. Numbers of indents used for calculation are shown inside the bars. Error indicators show standard deviation. Abbreviations: exo, exocuticle; endo, endocuticle; meso, mesocuticle; IR, infrared.

 

Figure 5
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  		Fig. 5. AFM topography of sections through the Melanophila IR sensillum (A) and the Pachnoda gula (B) after nanoindentation testing. Some series of residual imprints of the Berkovich tip can be clearly discerned. Imprints on the elastic mesocuticle inside the sphere of the Melanophila IR sensillum can hardly be recognized. Scale bars, 1 µm (A); 3 µm (B).

 

Figure 6
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  		Fig. 6. (A) Structure and proposed function of the Melanophila IR sensillum. (i) Schematic demonstrating the functional principle. (ii) A more detailed morphological drawing [adapted from Schmitz et al. (Schmitz et al., 2008Go)]. Absorption of IR radiation by the cuticle and the fluid causes the mesocuticle and the fluid (blue) inside the lacunae (lac) to expand (indicated by red arrows). This results in an increase in pressure inside the inner pressure chamber (pc), which deforms the tip of the mechanosensory dendrite (d). The hard outer shell functions as a pressure vessel and, as suggested in the Discussion, may even shrink due to slight shortening of the embedded chitin fibres (broken arrows). (B) The structure of the Pachnoda gula as a frictional ball joint system. Red arrows indicate a possible direction of head movement.

 

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© The Company of Biologists Ltd 2008