Insect wax coatings destroy stickiness of spider web glue Free

Halloween cliché or scourge of the houseproud, spider webs are notorious for their stickiness. Often coated with glue, the strands of many webs ensnare passing victims or accumulate dust. But Anna-Christin Joel from RWTH Aachen University, Germany, explains that the fluffy silk produced by one family of spiders – cribellate spiders – is glue free, hooking onto their victims instead, in addition to absorbing the waxy coating on an insect's surface and becoming embedded in the protective layer. Which raised the question, how much of an impact do insects’ waxy coatings have on the adhesiveness of spider webs, dry and sticky alike? Few researchers had checked how well spider silks adhere to real insect shells, mainly testing the stickiness on smooth uncoated surfaces – such as glass or metal – which are not the silk's usual target. Joel and her colleagues, Lucas Baumgart and Sascha Schlüter, set about investigating how well spider silks adhere to the wax-coated shells of insects, including house crickets, green bottle flies and cowpea weevils.

First, the team encouraged lace-webbed spiders (Amaurobius similis), Australian house spiders (Badumna longinqua) and garden centre spiders (Uloborus plumipes) with their glueless fluffy silk, gluey silk European garden spiders (Araneus diadematus) and silver-sided sector spiders (Zygiella x-notata), and dry web Trinidad dwarf tarantulas (Cyriocosmus elegans) to spin their webs. Then, they collected strands of the silk that ensnares victims, attaching the threads to a paper clip before dabbing them onto an insect and filming the insects as they struggled to break free. Surprisingly, all of the weevils escaped the smooth dry tarantula silk and the gluey silver-sided sector spider's silk within a matter of seconds, although both silks clung on slightly better to the crickets. However, the fluffy silks held on much tighter, with some of the weevils remaining ensnared for more than a minute, and the crickets rarely broke loose from the lace-webbed spider's fluffy silk. Despite being legendary for their stickiness, it seems that gluey spider webs are nowhere near as sticky for wax-coated insects as we thought.

Just how much of an influence do the insects’ waxy coatings have on the adhesive powers of the different spider silks? Baumgart and Schlüter bathed 187 house crickets, 300 cowpea weevils and 249 green bottle flies in solvent to collect sufficient waxy coating to find out how the wax mixtures affect the adhesiveness of each spider silk. This time, the team compared how strongly the spider silks attached to naked foil, foil smeared with the insects’ waxy coatings and the insects themselves; and it was clear just how disruptive the waxy coatings are for the sticky spider webs. The gluey European garden spider's adhesion plummet by up to 50% when the foil was smeared with the insects’ waxy coatings. In contrast, the stickiness of the foil increased dramatically for the fluffy lace-webbed spider, Australian house spider and garden centre spider silks when the waxy coat was applied. Even the smooth glueless tarantula web, which is probably most similar to the first spiders’ webs, clung on significantly better when the foil was coated in the insect waxes. So, insect wax coatings seem to be an intrinsic component of web stickiness for spiders that produce fluffy silks.

But why do spiders persist in producing sticky silks when their glue is less effective on the waxy shells of insects? Joel suspects that spiders evolved their low-cost alternative sticky silk because of the high cost of producing dry fluffy cribellate silk. Instead, spiders that produce gluey silks depend on their speedy reactions, rushing to incapacitate victims before they can liberate themselves from their less sticky bonds.