Abstract
A beetles’ first line of defense against environmental hazards is their mesothoracic elytra – rigid, protective forewings. In order to study the interaction of these wings with water, the surface microstructures of various beetles’ elytra were observed by Environment Scanning Electron Microscopy (ESEM) and Atomic Force Microscopy (AFM). Chemistry components were ascertained using X-ray photoelectron spectroscopy (XPS). All the beetles of various habitats (including desert, plant, dung, land and water) exhibited compound microstructures on their elytra. The wetting properties of these elytra were identified using an optical contact angle meter. In general the native elytra exhibited hydrophilic or weak hydrophobic properties with contact angles (CAs) ranging from 47.5° to 109.1°. After treatment with chloroform, the CAs all increased on the rougher elytral surfaces. The presence of wax is not the only determinant of hydrophobic properties, but rather a combination with microscopic structures found on the surfaces. Irregularities and the presence or absence of tiny cracks, hairs (or setae), pores and protrusions are important factors which influence the wetting properties. Rougher elytral surfaces tended to present a stronger hydrophobicity. Effects on hydrophobicity, such as surface microstructures, chemistry, environment and aging (referring to the time after emergence), are also included and discussed. Our results also provide insights into the motion of water droplets when in contact with beetle elytra.
Highlights
Hydrophobicity and hydrophilicity of solid surfaces have been researched extensively from a classical theory perspective [1,2,3], and in terms of potential applications [3,4]
The roughness average (Ra) value of 646102 nm, is not large enough to obtain a change from hydrophilic to hydrophobic
From the observation of microstructure, chemistry and wettabilities, we have demonstrated that the same groups of beetle elytra exhibit some consistency in their surface properties in order to exist in their selective environments
Summary
Hydrophobicity and hydrophilicity of solid surfaces have been researched extensively from a classical theory perspective [1,2,3], and in terms of potential applications [3,4]. Holdgate [15] has characterized four major groups of insects in relation to their water wetting properties These include terrestrial and aquatic species which can comprise of smooth and rough surface cuticles. One of the interesting groups includes the terrestrial and semi-aquatic species whose surfaces are very rough or covered with hair piles. They have very high advancing and receding contact angles (CAs), often over 150u, which generally indicates low adhesion to water. These adaptations are more often structural rather than chemical since many insects already have chemistry which is at the near upper limit for smooth surfaces
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