Adhesion of spider capture silks

Capture silk is a type of spider silk that makes the web gluey. It is an effective trap for the spider’s preys. To that purpose, spiders have found several ways to create strong glues, such as spinning a puffy ball of electrostatic nanofibers (dry adhesion) or drops of a glycoprotein mixture (wet adhesion). The proper understanding of spider silk’s glue may lead to more efficient and versatile adhesives for high standard applications. This work has been published in the Science of Nature, former Naturwissenchaften.

Related publications:

  • Adhesion of dry and wet cribellate silk

    H. Elettro, S. Neukirch, A.Antkowiak & F. Vollrath

    The Science Of Nature, 102(7):1–4 (2015)

    PDF File

  • Damage control through windlass mechanism in ecribellate webs

    H. Elettro, F. Vollrath, A. Antkowiak & S. Neukirch

    In preparation (2017)


Ecribellate_cribellate
Figure 1- Comparison of ecribellate (A, C, E) and cribellate (B, D, F) spider webs (A and B), capture silks (C and D) and spinnerets (E and F, G), from Blackledge et al. (2009).

Cribellate versus ecribellate

My PhD work focused on the ecribellate capture silk, that consists of gluey droplets on a silk thread (see elastocapillary windlass page). There is a second type of capture silk, evolutionnarily older: the cribellate silk.  From the macroscopic point of view, both webs have a similar architecture, see figure 1 insets A and B. However, the capture silk threads are very different: the ecribellate silk uses wet adhesion through its gluey liquid droplets, while cribellate silk uses the dry adhesion of highly puffed nanofibres, see insets C and D. Ecribellate capture threads are spun by a double spinneret, see inset E. The one connected to the flagelliform silk gland (FL) that spins the core silk thread and the one connected to the aggregate gland (AG) that produces the gluey liquid coating. Cribellate silk is very different from ecribellate silk. Its puffy morphology comes from the specific spinning process (Opell, 1993). During spinning, the large amount of produced silk nanofibres is combed by the spider with the use of its backlegs, see figure 1 inset G. The combing induces electrostatic charging of the fibres (Kronenberger and Vollrath, 2015), and subsequent self-repulsion leads to the observed periodic puffing. Additionnally, van der Waals interactions produce a strong adhesion force, that is analogous to the glue function of ecribellate silk.

Wet adhesion versus dry adhesion

Here we mention the existence of a confusion in the literature (Helmer, 2010). Zheng et al. (2010) wrote a paper in 2010 claiming that the bioinspired design of artificial fibres that mimic the structural features of the capture silk of the cribellate spider Uloborus and exhibit its directional water-collecting ability. At the origin of their study was the observation that wetted cribellate spider silk have a periodic structure with thin and aligned nanofibres zones combined with thick and randomly oriented nanofibres. Deposition of a water droplet on this structure allows control of the fluid and preferrable settlement on the thick zones. As mentioned above, cribellate capture silk uses dry adhesion, in contrast to the wet ecribellate spider silk. Consequently, wetting the (dry) cribellate capture silk annihilates the electrostatic interactions on which the adhesion relies. Furthermore, the hundreds of nanofibres – once forming a large puff through electrostatic repulsion – collapse into a much smaller area, which lowers dramatically the contact area. The main task of ecribellate capture silk, adhering strongly to incoming potential preys, is thus ruined by the combination of these two phenomena. The dramatic decrease in adhesion is measured in two ways: the frontal poking of a thread or the side poke followed by retraction. These two methods qualitatively yield similar results. For the frontal poke, the dry work of adhesion is 5 +/- 3 nJ while the wet work of adhesion is 0.02 +/- 0.03 nJ. For the side poke, the dry work of adhesion is 40 +/- 30 nJ and while the wet work of adhesion is 0.07 +/- 0.03 nJ. Wetting the cribellate thread is thus highly destructive in respect to its biological function, see figure 2.

cribellate_adhesion_sketch_test_higher_res
Figure 2- Comparison of the adhesion of the cribellate capture silk in its natural and collapsed states (see electron microscope micrographs insets). The adhesion work of collapsed cribellate silk is approximately 500 times lower on average than with cribellate silk in its natural state.
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