Adhesion enhancement of cribellate capture threads by epicuticular waxes of the insect prey sheds new light on spider web evolution

Abstract

To survive, web-building spiders rely on their capture threads to restrain prey. Many species use special adhesives for this task, and again the majority of those species cover their threads with viscoelastic glue droplets. Cribellate spiders, by contrast, use a wool of nanofibres as adhesive. Previous studies hypothesized that prey is restrained by van der Waals' forces and entrapment in the nanofibres. A large discrepancy when comparing the adhesive force on artificial surfaces versus prey implied that the real mechanism was still elusive. We observed that insect prey's epicuticular waxes infiltrate the wool of nanofibres, probably induced by capillary forces. The fibre-reinforced composite thus formed led to an adhesion between prey and thread eight times stronger than that between thread and wax-free surfaces. Thus, cribellate spiders employ the originally protective coating of their insect prey as a fatal component of their adhesive and the insect promotes its own capture. We suggest an evolutionary arms race with prey changing the properties of their cuticular waxes to escape the cribellate capture threads that eventually favoured spider threads with viscous glue.

Evolution of aerial spider webs coincided with repeated structural optimization of silk anchorages

“…), resulting in cribellate spiders being largely outnumbered by ecribellate spiders (Bond and Opell ), and why only few cribellate spiders evolved aerial webs, although cribellate silk can be highly efficient in prey capture (Opell ; Opell and Schwend ; Bott et al. ). Our results indicate that the cribellum represents a significant physical constraint on the spinning of robust anchorages limiting the capability to build efficient suspended webs.…”

An anti-adhesive surface coating reduces adhesion during contact with cribellar threads in Pholcus phalangioides (Araneae, Pholcidae) but not in the web-owning spider Uloborus plumipes(Araneae, Uloboridae)

“…An anecdotal study by Fabre (1905) states that a lubricated straw fails to adhere to the gluey capture web of araneoid spiders, pointing towards a chemical protection. Under this assumption, we expected different results when testing spider legs with the same methodology on gluey and cribellar capture threads: a lipid surface coating preventing adhesion on araneoid capture threads supposedly increases adhesion when coming in contact with the cribellar capture thread, because such surface lipids seemingly are soaked up by the cribellar nano-fibrils, in this way causing strong adhesion (19). To check if P. phalangioides has an epicuticular layer capable of interacting with the cribellar fibrils we examined cribellar capture threads contacting hexane-washed and untreated legs under the electron microscope.…”

“…4 a). The cribellar fibrils obviously become embedded in some semi-liquid surface coating, changing the transparency of the thread (see (19)). In hexane-washed legs no such interaction was visible, indicating that the solvent actually washed off the semi-liquid waxes (Fig.…”

“…n-hexane), which should remove the wax layer. For the legs of U. plumipes we expect no difference in adhesion between the treatment groups because the adhesion mechanism of the cribellar thread as known yet does not favour a lipid surface coating as protection (19) but in contrast, this should even increase adhesion.…”

“…The adhesion is achieved by interlock/entanglement, van der Waals and hygroscopic forces (16–18). A more recent study found yet another adhesive mechanism of the cribellar capture thread: the fibrils become embedded in the epicuticular waxes of the prey’s cuticula, increasing the adhesion by a multiple (19). Cribellar spiders, however, never get entangled in their own webs (M. F., personal observation).…”