Many people fear spiders, because they have the reputation of being aggressive animals, only waiting for a chance to kill you. This is of course not true and on the contrary, once you start observing spiders you’ll find they have many fascinating qualities. Especially their highly adaptive silk with one of the toughest fibers not only found in nature but also in technical applications has fascinated humans for a long time now. According articles are frequently published and I don’t want to repeat the praise of the spider’s silk material. I want to direct your attention to the often overlooked fact that spiders are masters in fiber handling, especially when it comes to nanofibers!

Spiders can modify their silks’ properties during production, for example by changing their extraction speed. Some spiders use specialized combs for this extraction, like Theridiidae, Nesticidae, Pholcidae and Uloboridae [1-3]. Uloborids, however, bear another specialized comb, the calamistrum, processing nanofibers with a diameter of 10 to 30 nm [4]. As a reference: typical technical nanofibers have a diameter of 1000 nm. Already these “large” fibers bring along several problems during production and processing, limiting their area of application at the moment. Uloborids, however, seem to have no problem in producing and processing even smaller fibers. How do they master this task and what can we learn from these spiders?

Fig. 1: The cribellate spider Uloborus plumipes in its web.

The use of nanofibers in spiders

Uloborids, and about 9% of all other spiders, belong to the cribellate spiders. The majority of spiders, so-called ecribellate spiders, use viscous silk droplets as glue to capture prey [5, 6]. Cribellate spiders on the contrary use a wool of nanofibers [7]. It has long been suggested that prey is restrained only by entanglement in the cribellate fibers, by van der Waals forces (the ones used by the gecko: http://www.blogionik.org/gecko-feet-applications/) and in some species, also by hygroscopic forces [8, 9]. A recent study of mine showed that actually the predominant adhesive of this nanofibrous wool is the absorption of a viscous waxy layer covering all insects [7]. So cribellate spiders use a layer originally protecting their prey from desiccation as a component of their glue… I don’t think this tactic will favor the reputation of spiders.

 
Fig. 2: Cribellate capture thread adhered at the eye of a fruit fly.

How spiders produce and process nanofibers

Not only Uloborids but all cribellate spiders produce, process, and some even handle nanofibers to form complex capture threads. Some species process more than 40,000 single fibers and three different silk types into one functional capture thread. Breaking down the organization of this thread, one silk type is used to produce the basal, stabilizing thread. This supporting thread is surrounded by a sheet of nanofibers, which are connected to the stabilizing thread in periodically distances by the third type of silk [4].

Fig. 3: Internal structure of a capture thread of Uloborus plumipes.

Such a complex interweaving of fibers in the nanometer scale involves a highly coordinated movement of the spinnerets (spinnerets are digitate appendages at the belly of spiders, staffed with the spigots from which the fibers emerge). So the spider is comparable to a miniature sewing machine and uses its movable spinnerets to darn in the nanofibrous sheet to the stabilizing fibers (but using only one sewing thread). This is highly interesting for a biomimetic transfer, as the transfer of this process would enable humans to connect nanofibers permanently to supporting fibers. Such supporting fibers are already used in the fabrication of nanofibers to make their handling easier, but without a permanent connection, nanofibers often adhere (due to van der Waals forces) better to the processing tool than to the supporting fibers.

Fig. 4: Overview over the spinnerets of Uloborus plumipes during capture thread production.

But what function has the previously mentioned comb? Actually, the calamistrum has no vital function in the assembly of the thread, but in the processing of the nanofibers. In fact, these spiders are not only able to sew nanofibers to a stabilizing core fiber, but they process the nanofibers during thread production. Therefore, they brush the calamistrum repetitively over the extracted nanofibers. It is still elusive what exactly happens when the calamistrum comes in contact with the nanofibers. A recent study has excluded electrostatic charging of the fibers by triboelectrification [10]. Probably higher traction forces lead to a curling of fibers, either by a modification on protein level or by a mechanism similar to the one observed in curling ribbons after being pulled over scissors. It would be very interesting for a biomimetic transfer to understand how these spiders process nanofibers, as this is not possible with technical nanofibres to date.

This is just a brief outline of the abilities of cribellate spiders. Many more interesting facets have to be analyzed. In my opinion we can learn a lot from these animals and transfer this knowledge to technical nanofiber production and handling. At least, they have learnt to handle small scale fibers for more than 145 million years [11] and we’ve just recently started to understand the peculiarities and benefits when utilizing nano-materials.

References

1. Griswold C.E., Ramirez M.J., Coddington J.A., Platnick N.I. 2005 Atlas of phylogenetic data for entelegyne spiders (Araneae: Araneomorphae: Entelegynae) with comments on their phylogeny. Proceedings of the California Academy of Sciences 56(Suppl. 2), 1.
2. Huber B.A., Fleckenstein N. 2008 Comb-hairs on the fourth tarsi in pholcid spiders (Araneae, Pholcidae). Journal of Arachnology 36(2), 232-240. (doi:10.1636/CSh07-71.1).
3. Foelix R.F., Joel A.-C., Erb B. 2015 Wie die Kräuselradnetzspinne Uloborus ihre Beute einwickelt und verdaut. Arachne 2, 16-23.
4. Joel A.-C., Kappel P., Adamova H., Baumgartner W., Scholz I. 2015 Cribellate thread production in spiders: Complex processing of nano-fibres into a functional capture thread. Arthropod structure & development 44(6 Pt A), 568-573. (doi:10.1016/j.asd.2015.07.003).
5. Vollrath F., Fairbrother W.J., Williams R.J.P., Tillinghast E.K., Bernstein D.T., Gallagher K.S., Townley M.A. 1990 Compounds in the droplets of the orb spider’s viscid spiral. Nature 345(6275), 526-528.
6. Collin M.A., Clarke T.H., Ayoub N.A., Hayashi C.Y. 2016 Evidence from multiple species that spider silk glue component ASG2 is a spidroin. Scientific Reports 6, 21589. (doi:10.1038/srep21589).
7. Bott R.A., Baumgartner W., Bräunig P., Menzel F., Joel A.-C. 2017 Adhesion enhancement of cribellate capture threads by epicuticular waxes of the insect prey sheds new light on spider web evolution. Proceedings of the Royal Society B: Biological Sciences 284(1855). (doi:10.1098/rspb.2017.0363).
8. Hawthorn A.C., Opell B.D. 2003 van der Waals and hygroscopic forces of adhesion generated by spider capture threads. Journal of Experimental Biology 206(22), 3905-3911. (doi:10.1242/jeb.00618).
9. Opell B.D. 1994 The ability of spider cribellar prey capture thread to hold insects with different surface-features. Functional Ecology 8(2), 145-150.
10. Joel A.-C., Baumgartner W. 2017 Nanofibre production in spiders without electric charge. The Journal of experimental biology 220(12), 2243-2249. (doi:10.1242/jeb.157594).
11. Selden P.A., Ren D., Shih C.K. 2016 Mesozoic cribellate spiders (Araneae: Deinopoidea) from China. Journal of Systematic Palaeontology 14(1), 49-74. (doi:10.1080/14772019.2014.991906).