The spherical form of air bubbles caught under a sheet-web or inside a new bell indicated that the value of the effective contact angle θe at the point of the three phase contact water/air/sheet-web was close to zero, confirming a physical argument according to which the surface of the wall must be hydrophilic (Woermann 2010).
The proteinaceous hydrogel, which is described here for the first time, corresponds in its properties to an ensemble of hydrophilic macromolecules forming a network in water. We assume that the solid fragments which can be seen in the SEM-pictures published by De Bakker et al. (2006) are remains of this hydrophilic hydrogel component. Several questions about the hydrogel remain open for further investigations, foremost concerning its chemical constitution. Is it some liquid silk, as primarily produced in the spinning apparatus glands, but without further structural differentiation? Or is it a glycoprotein secreted by a distinct type of spinneret gland? The pronounced hydrophilicity of both the silk threads and the hydrogel should also be the subject of further investigation. The new method of keeping the spiders offers the possibility of gaining sufficient amounts of the secreted products.
Argyroneta aquatica has three pairs of spinnerets with a large number of spigots (Foelix 1996). One has to assume that all components of the composite are products of the silk glands. However, the relationships between the components of the composite and distinct spigots or silk glands have not yet been analysed, nor has the mechanism by which the fine threads become twisted to a strong anchor thread been investigated yet (or vice versa).
The diving bell has to withstand a hydrostatic pressure difference corresponding to the pressure exerted by a vertical water column of up to several decimetres, and its wall must remain expandable. At the same time, it has the function of a physical gill. The observed combination of threads in the hydrogel which connects them like a flexible binder clearly meets this requirement. During the stepwise enlargement of the diving bell, Argyroneta aquatica reinforces it by further spinning on its inner wall. Since all components of the composite are hydrophilic, the adhesive forces between threads and hydrogel contribute to the stability of the bell wall, and the hydrogel can patch up areas of the wall and improve its stability. The adhesive forces between the components are documented by SEM-pictures taken during the drying process (Figure 2, middle). It is possible that the hydrogel also lowers the hydrodynamic permeability of the wall, thus suppressing a convective volume flow across the wall, and may retard the gas exchange between the air volume of the diving bell and the external water. Hence, the air-water interface at the open bottom of the diving bell might be important for the overall balance of the gas transfer, as recorded by Seymour and Hetz (2011). A retarded gas transfer via the bell wall may be an advantage for the resting spider when low pO2−values occur on warm summer nights and could reduce the number of the spiders’ trips to the surface to collect fresh air.