Collembola cuticles and the three-phase line tension.

The cuticles of most springtails (Collembola) are superhydrophobic, but the mechanism has not been described in detail. Previous studies have suggested that overhanging surface structures play an important role, but such structures are not a universal trait among springtails with superhydrophobic cuticles. A novel wetting experiment with a fluorescent dye revealed the extent of wetting on exposed surface structures. Using simple wetting models to describe the composite wetting of the cuticular surface structures results in underestimating the contact angles of water. Including the three-phase line tension allows for a prediction of contact angles in the observed range. The discrepancy between the contact angle predicted by simple models and those observed is especially large in the springtail Cryptopygus clavatus which changes, seasonally, from superhydrophobic to wetting without a large change in surface structure; C. clavatus does not have overhanging surface structures. This large change in observed contact angles can be explained with a modest change of the three-phase line tension.

Seasonal change in the wetting characteristics of the cuticle of the Collembola Cryptopygus clavatus (Schött, 1893).

The littoral Collembola Cryptopygus clavatus spends the summer submerged, grazing on algae under water, and the winter on dry land. The cuticles of Collembola are, in general, highly water repellent, often superhydrophobic; the cuticle of C. clavatus has, in contrast, been described as not water repellent. Wetting properties are closely tied to surface structuring, and previous studies of Collembola cuticles have used the pattern of cuticular granules to explain the superhydrophobic properties of these cuticles. The wetting properties of the cuticles of C. clavatus were measured on animals acclimated to summer and winter. A significant difference in wetting performance was observed. Animals acclimated to winter conditions showed superhydrophobic non-wetting properties. Animals acclimated to summer conditions were not superhydrophobic, water droplets readily adhered to their cuticles. This large change in wetting behavior of the cuticle could not be explained by changes in the cuticular surface structure, which were very limited. Instead, we suggest a change in the epicuticular wax layer could explain the differences.

Surface structure and wetting characteristics of Collembola cuticles.

The cuticles of the arthropods Collembola (springtails) are known to be superhydrophobic, displaying such properties as water-repellence and plastron formation; overhanging surface structures have been suggested as the source of these properties. Superhydrophobicity is closely related to surface structuring and other surfaces with overhanging structures have been shown to possess robust superhydrophobic properties. In effort to correlate the wetting performance and surface structuring of the cuticles, from both a technical and evolutionary point of view, we investigated a selection of Collembola species including species from several families and covering habitats ranging from aquatic to very dry. The observed contact angles of wetting was in general larger than those predicted by the conventional models. Not all the studied Collembola were found to have superhydrophobic properties, indicating that superhydrophobicity is common, but not a universal trait in Collembola. Overhanging structures were found in some, but not all Collembola species with superhydrophobic cuticles; which leads to the conclusion that there is no direct link between overhanging surface structures and superhydrophobicity in Collembola.