- Last updated on 2023.11.17 by Frans Janssens
Checklist of the Collembola: Note on the self-cleaning properties of the epicuticular surface of Collembola by removal of dirt by dew droplets flow

Frans Janssens, Department of Biology, University of Antwerp, Antwerp, B-2020, Belgium


In November 2006, Brian Valentine keenly observed and recorded several Collembola specimens that were completely covered by small dew droplets (Fig.1 and Fig.2). Also, Brian Kilford recorded in November 2007, a specimen of Allacma fusca covered in dew drops (Fig.2a). Since then, more and more such observations have been made, such as in December 2022, when Ed Phillips recorded a specimen of a new undescribed genus and species of Katiannidae covered in very fine dew droplets (Fig.2b).

Fig.1. Entomobrya sp.
Specimen with dew droplets, 2006.11.18, UK, Worthing.
2006 © Valentine, B.

Fig.2. Dicyrtomina sp.
Specimen with dew droplets, 2006.11.19, UK, Worthing.
2006 © Valentine, B.

Fig.2a. Allacma fusca
Specimen with dew droplets, 2007.11.04, UK, Dudley.
2007 © Kilford, B.

Fig.2b. Katiannidae Gen. nov. sp. nov.
Specimen with dew droplets, 2022.12.20, UK, Staffordshire.
2022 © Phillips, E.

In this short preliminary note, we will explore the possibility of the self-cleaning capabilities of the epicuticular ultrastructured surface by dew droplets flow.

Formation of dew droplets at the epicuticula

Fig.3. Basic hexagonal epicuticular ultrastructure.
2001 © Borensztajn, S.
Evaporation of water from the substrate soil surface, increases the moisture in the air. Dew is a form of dropwise condensation of evaporated water. Dew droplets form on a surface when the surface cools down to the dew point. This is the temperature at which the saturation of the evaporated water in the air occurs, resulting in condensation of water at nucleation sites of the cool surface.
The epicuticular ultrastructure forms a raster of open cells (Fig.3) in which the moistured air is trapped and held in place easily. The cells are typically 150 nm high and about 700 nm in diameter. Due to the abscence of air flow in these epicuticular cells, the moisture level of the air in the cells will rise more quickly than the moisture level in the air outside the cells. Therefore, the epicuticular cells will act as active nucleation sites for the formation of dew droplets. The dew droplets grow from these active nucleation sites. Due to the non-wettable properties of the epicuticular pillars and rims of the epicuticular ultrastructure, the larger dew droplets eventually are lifted out of the epicuticular cells.

To be completed.

Non-wettable properties of the ultrahydrophobic epicuticular regular rough surface

Fig.4. Entomobrya intermedia
Ball-shaped dew droplets, close-up.
2009.12.10 © Valentine, B.
Fig.5. Entomobrya intermedia
Ball-shaped dew droplets, close-up.
2009.12.10 © Valentine, B.
Notice the almost ball-shaped dew droplets on the non-wettable epicuticula of the Collembola specimens in Figs.1-5. These ball-shaped droplets are a clear indication that the epicuticula of Collembola is an ultrahydrophobic surface. Noble-Nesbitt (1963 cited from Robson, 1964:296) has shown that the hydrofuge properties of Podura cuticle depend on the wax capping of the large number of small surface tubercles about 0.4 micron apart. The intervening surface is wettable. So long as the wax on the tubercles is intact, only fluids whose contact angle is less than critical will penetrate. (Robson 1964:296). But the ultrahydrophobic properties of the cuticula are not only due to the wax depositions in the epicuticular layer, but also due to the characteristic epicuticular ultrastructure (Fig.3) (Janssens et al., 2004-). Non-wettability of the cuticle is ensured by the contours of microtubercles which cover the epicuticular surface. Similar structures arise in Onychophora and other terrestrial arthropods by convergence (modified after Robson 1964:281).

Note also that in many Poduromorpha, this epicuticular ultrastructure is superimposed on a secondary and even tertiary cuticular structure. Wenzel (1936) stated that wettability is improved by roughness for a hydrophilic surface, but gets worse for a hydrophobic surface. The non-wettable properties of ultrahydrophobic surfaces rely on the minuscule contact area of the droplets with these surfaces. The epicuticular ultrastructure reduces the contact area of the droplets with the epicuticular surface to a minimum.
Rough surfaces are, with the exception of mica and graphite, essentially all solid surfaces. They can be classified into three classes. A rough surface can be regular or irregular (random). Hierarchical rough surfaces are an intermediate case. The roughness structure of rough surfaces is on the submicrometer scale. The epicuticular ultrastructured surface of Collembola can be compared with a regular hierachical rough surface. The regular ultrastructure of the epicuticula is clearly at submicrometer scale (Fig.3). For such surfaces, Herminghaus derived an argument that a hierarchical structure of the roughness could render any surface non-wettable (modified after Blossey, 2003).

Fig.6. Omniphobic(*) silica surface microstructure
After Domingues & al 2017 Fig.B
(*) Omniphobic surfaces repel all known liquids
Fig.6a. Superomniphobic silica surface microstructure
After Arunachalam & al 2018 Fig.4
Inspired by the superhydrophobic epicuticular ultrastructure of Collembola, Domingues & al. (2017, 2018) and Arunachalam & al. (2018) experimented with omniphobic silica surface microtextures (Fig.6, Fig.6a) made of arrays of pillars and cavities of varying profiles (with simple, reentrant, and doubly reentrant edges) and shapes (circular, square, and hexagonal) with sharp and rounded corners to advance the development of coating-free liquid repellent surfaces.
This research might eventually also improve the development of self-cleaning products.

Self-cleaning properties of the epicuticular ultrastructure

Collembola take up oxygen by cutaneous respiration (in Symphypleona, in addition simple trachea are present). The gas exchange is driven by diffusion and occurs through the cuticula over the entire body surface. To maintain the cutaneous respiration, the epicuticular surface needs to be kept dry and clean. Otherwise the gas transfer rate would be substantially affected leading ultimately to suffocation. (after Hensel & al, 2015). Hence, a self-cleaning cuticula in Collembola is a mandatory survival feature.

The ball-like equilibrium shapes of dew droplets on ultrahydrophobic surfaces are only half the story of self-cleaning: to clean the surface, hydrophilic material has to be transported along it - and best, off it.

The Lotus-effect. Interestingly, non-wettable plant leaf surfaces, such as those of the famous Lotus plant, have a built-in elementary cleaning mechanism. This was noticed in the mid-nineties by botanists studying plant surfaces. They observed that droplets running off the leaves can carry dry contaminants along - the origin for the Lotus leaf's status as a sacred object of purity. (after Blossey, 2003).
The secret of the Lotus leaf can be found in numerous tiny pillars with a wax layer on top. Water drops are lifted by these pillars, get into a spherical shape and can simply not cover the surface. Dirt gets no chance to stick to the surface via water. The spherical drops roll off and take dirt particles with them. (Science Daily, January 15, 2007).
Microfluidics can also be based on droplets on ultrahydrophobic surfaces alone: because the droplets have very low contact areas with the substrate, they are easy to move by external forces, such as gravity, air flow, specimen movements or vibrations. (after Blossey, 2003).
The dew droplets continue to grow, and when they touch, they merge with one another until one is large enough to be pulled away from its position, e.g. by gravity. It runs off, transporting any contamination encountered on its path, and merging with the smaller droplets in his path. New droplets immediately begin to grow at the nucleation sites in the cleaned path. (after Lienhard & Lienhard, 2006).

Active cleaning - drinking from dew drops

Active grooming behaviour of the cuticula, in a characteristic tripod posture, has been observed in all kind of Neocollembola (= Neelipleona, Entomobryomorpha and Symphypleona). Specimens form a tripod with 3 legs of the same side of the body while using the opposite 3 legs (and collophore) to clean the body. (Janssens & Huskens, 2016-).
Fig.7. Sminthurus viridis
Drinking from dew drops on its body using the long eversed vesicle from its collophore.
From Holland, Drenthe
2011.11.20 © van Duinen, J.
Fig.8. Lipothrix lubbocki
Drinking from microscopic dew droplets on its body using the long eversed vesicle from its collophore.
From Belgium, Meeuwen-Gruitrode
2020.09.18 © Huskens, M.L.

The dew droplets on the body of Collembola do have an additional function in Symphypleona.

The eversible vesicles of the collophore are typically used to drink water from the substrate surface waterfilm (Brocklehurst & Janssens, 2006-) by pressing them against the waterfilm.
The long tube-like vesicles of Symphypleona apparently are used not only to drink from the substrate surface waterfilm but also from dew droplets on the body. As such the body surface is cleaned in active fashion. This behaviour has been observed in Sminthurus viridis from Holland, Drenthe (Fig.7) and in Lipothrix lubbocki from Belgium, Meeuwen-Gruitrode (Fig.8).