Molecular organization of the nanoscale surface structures of the dragonfly Hemianax papuensis wing epicuticle

Abstract

The molecular organization of the epicuticle (the outermost layer) of insect wings is vital in the formation of the nanoscale surface patterns that are responsible for bestowing remarkable functional properties. Using a combination of spectroscopic and chromatographic techniques, including Synchrotron-sourced Fourier-transform infrared microspectroscopy (FTIR), x-ray photoelectron spectroscopy (XPS) depth profiling and gas chromatography-mass spectrometry (GCMS), we have identified the chemical components that constitute the nanoscale structures on the surface of the wings of the dragonfly, Hemianax papuensis. The major components were identified to be fatty acids, predominantly hexadecanoic acid and octadecanoic acid, and n-alkanes with even numbered carbon chains ranging from C14 to C30. The data obtained from XPS depth profiling, in conjunction with that obtained from GCMS analyses, enabled the location of particular classes of compounds to different regions within the epicuticle. Hexadecanoic acid was found to be a major component of the outer region of the epicuticle, which forms the surface nanostructures, and was also detected in deeper layers along with octadecanoic acid. Aliphatic compounds were detected throughout the epicuticle, and these appeared to form a third discrete layer that was separate from both the inner and outer epicuticles, which has never previously been reported.

Figure 1. Scanning electron micrographs of Hemianax papuensis wing membranes.

Cross-sectional (a, c, e) and surface view (b, d, f) images were taken of each wing before chloroform extraction (a, b), after 10 s extraction (c, d), and 1 hr extraction (e, f). Loss of surface structure is visible on the wings subjected to chloroform extraction; no structure is evident after extraction for 1 hr, whereas the internal wing structure appears unchanged. Scale bars = 400 nm.

Figure 2. Representative infrared spectra of untreated and chloroform extracted wing membranes of Hermianax papuensis.

The intensity of the CH (as indicated) stretching bands decrease successively with extended extraction time. Spectra were acquired in transmission mode.

Figure 3. Atomic proportions of oxygen and nitrogen in the wings of Hemianax papuensis.

Figure 4. Relative proportions of the major compound classes and chain length of dragonfly wing epicuticle components.

Figure 5. Proposed model of the epicuticle of Hemianax papuensis wing membranes.

Three layers are contained within the epicuticle: the outer epicuticle, the meso epicuticle and the inner epicuticle.