Abstract
The dorsal surface of the woodlouse Porcellionides pruinosus is covered with minute spheres, providing its characteristic powdered appearance. Little has been known about their composition and formation. A previously suggested function of these structures was to increase the hydrophobicity of the cuticular surface. We studied the ultrastructure, composition and formation of the spheres as well as tested whether they affect the hydrophobicity of the cuticle. We determined the composition of the spheres with histochemistry and scanning electron microscopy after applying various chemicals. We studied the process of their formation with transmission electron microscopy and assessed the hydrophobicity of the cuticle by measuring contact angles of water droplets with its surface. Our results show that the spheres are largely organic. They contain proteins and glycoproteins or possibly polysaccharides without detectable amounts of lipids. By studying the formation of the spheres we established that they are epicuticular structures. They are deposited early in the premolt stage of the molt cycle around branching extensions of epidermal cells. The sphere-covered cuticle of P. pruinosus is more hydrophobic than the cuticle with experimentally removed spheres as well as the scale-covered cuticle in a related species.
Highlights
Terrestrial isopods or woodlice (Oniscidea) are a group of crustaceans that have successfully colonized dry land, diversifying into more than 3700 known terrestrial and amphibious species (Sfenthourakis and Taiti, 2015)
We studied the process of their formation with transmission electron microscopy and assessed the hydrophobicity of the cuticle by measuring contact angles of water droplets with its surface
The isopod body is covered by a cuticle which represents an exoskeleton
Summary
Terrestrial isopods or woodlice (Oniscidea) are a group of crustaceans that have successfully colonized dry land, diversifying into more than 3700 known terrestrial and amphibious species (Sfenthourakis and Taiti, 2015). The spheres were suggested to be waxy secretions (Schmalfuss, 1978) This was later disputed due to the inability of nonpolar solvents, such as hexane, to remove the spheres from the surface of the exoskeleton. Their composition remained unknown, as they were not removed by any other treatment either to which they were subjected. These included aqueous acetic acid and potassium hydroxide solutions (Hadley and Hendricks, 1985). It has recently been shown that the spheres can be retained in paraffin sections and that they stain with the periodic acid-Schiff histological stain, indicating that they contain polysaccharides (Csonka et al, 2018)
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