Polarizing properties and structure of the cuticle of scarab beetles from the Chrysina genus

The optical properties of several scarab beetles have been previously studied but few attempts have been made to compare beetles in the same genus. To determine whether there is any relation between specimens of the same genus, we have studied and classified seven species from the Chrysina genus. The polarization properties were analyzed with Mueller-matrix spectroscopic ellipsometry and the structural characteristics with optical microscopy and scanning electron microscopy. Most of the Chrysina beetles are green colored or have a metallic look (gold or silver). The results show that the green-colored beetles polarize reflected light mainly at off-specular angles. The gold-colored beetles polarize light left-handed near circular at specular reflection. The structure of the exoskeleton is a stack of layers that form a cusplike structure in the green beetles whereas the layers are parallel to the surface in the case of the gold-colored beetles. The beetle C. gloriosa is green with gold-colored stripes along the elytras and exhibits both types of effects. The results indicate that Chrysina beetles can be classified according to these two major polarization properties.

Figure 1

Contour plots of MMSE data for C. woodi showing $m_{41}, P, e$, and $\left|\alpha \right|$ in the range $\lambda \in [245,1000]$ nm and $\theta \in [{20}^{\circ },{75}^{\circ }]$ (left and center). $m_{41}$ and $e$ shown rescaled to parameter maximum and minimum values (right).

Figure 2

Scattering contour plots of MMSE data for C. woodi showing $m_{41}, P, e$, and $\left|\alpha \right|$ in the range $\lambda \in [245,1000]$ nm and $\theta \in [{45}^{\circ }\pm {15}^{\circ }]$.

Figure 3

Contour plots of MMSE data for C. gloriosa measured on a green area showing $m_{41}, P, e$, and $\left|\alpha \right|$ in the range $\lambda \in [245,1000]$ nm and $\theta \in [{20}^{\circ },{75}^{\circ }]$ (left and center). $m_{41}$ and $e$ shown rescaled to parameter maximum and minimum values (right).

Figure 4

Contour plots of MMSE data for C. gloriosa measured on a gold-colored area showing $m_{41}, P, e$, and $\left|\alpha \right|$ in the range $\lambda \in [245,1000]$ nm and $\theta \in [{20}^{\circ },{75}^{\circ }]$.

Figure 5

Scattering contour plots of MMSE data for C. gloriosa showing $m_{41}, P, e$, and $\left|\alpha \right|$ in the range $\lambda \in [245,1000]$ nm and $\theta \in [{45}^{\circ }\pm {15}^{\circ }]$ measured on a green area (left) and on a gold area (right).

Figure 6

Contour plots of MMSE data for C. chrysargyrea showing $m_{41}, P, e$, and $\left|\alpha \right|$ in the range $\lambda \in [245,1000]$ nm and $\theta \in [{20}^{\circ },{75}^{\circ }]$.

Figure 7

Scattering contour plots of MMSE data for C. chrysargyrea showing $m_{41}, P, e$, and $\left|\alpha \right|$ in the range $\lambda \in [245,1000]$ nm and $\theta \in [{45}^{\circ }\pm {15}^{\circ }]$.

Figure 8

Optical microscopy images of (a, b) C. chrysargyrea, (c, d) C. woodi, and (e–g) C. gloriosa. The image in (f) is from a green-colored area and (g) from a metallic-colored area.

Figure 9

SEM images of the cross section of (a) C. chrysargyrea, (b) C. woodi, and C. gloriosa (c) on the metallic-colored area and (d) on the green area.