Most spiders are content to sit and wait for unsuspecting victims to blunder into their silken traps, but webs are only ever as good as the structures that secure them to the surfaces that support them. ‘This [attachment] is done with a special type of silk, called piriform’, says Jonas Wolff, from Macquarie University, Australia, adding that most spiders only produce this silk when they need to securely anchor a structural thread. However, when Wolff and Milan Řezáč, from the Czech University of Life Sciences, realised that the piriform silk-producing glands of ground spiders (Gnaphosidae) were different from those of other species – they had fewer and the glands were significantly larger than those of web-spinning spiders – the colleagues were intrigued. ‘Why should a lineage of spider deviate from a pattern that is so widespread and hence a long-term success story?’, wondered Wolff. That, coupled with the ground spiders’ novel hunting strategy – they actively ambush prey, snaring them with piriform silk, and even pick fights with insects and arachnids that are larger than themselves – led Wolff, Řezáč and Stanislav Gorb, from the University of Kiel, Germany, to find out more about the ground spider's extraordinary silk and the glands that produce it.
However, ground spiders are not very common in northern Germany. It was only while collecting animals in the Southern Alps that Wolff stumbled across Drassodex heeri concealed beneath stones. Returning to Kiel, Wolff pitted the predators against giant house spiders (Eratigena atrica), lace webbed spiders (Amaurobius fenestralis) and silver-sided sector spiders (Zygiella x-notata) while filming the encounters from beneath. ‘The attacks can be very quick; it can be hard to distinguish what is going on’, says Wolff, who slowed the movies down to see Drassodex attach silk to the floor of the enclosure before running quickly around its prey, producing a trail of sticky thread that dried quickly, ensnaring the victim's legs until it was neatly trussed up.
But how did the Drassodex piriform silk differ from the silk produced by web spinners? This time, Wolff staged the spider ambushes on a sheet of plastic from which he could cautiously collect the silk. Although isolating individual strands was extremely difficult, Wolff eventually collected 17 straight specimens and measured the silk's strength. Comparing the piriform silk with other silks produced by web-spinning spiders, he realised that Drassodex piriform silk combines the toughness of dragline silk with the remarkable stretchiness of spiral capture threads from webs. Also, the glue coating the thread is extraordinarily deformable and tough, withstanding shear stresses that are more than 750 times the stresses that artificial glues can endure.
After the silk's material properties had been defined, Tomáš Krejči and Řezáč investigated the silk-producing glands and saw that the spigots through which piriform silk is expelled are much larger than those of other species; probably to eject a thick layer of glue at high speed in order to overwhelm victims that may lash out and injure them. The team also realised that the enlarged spigots might become clogged by dried silk components, until they noticed that the spinnerets have a unique anti-clogging mechanism. They remain closed until just before they deploy the silk, when the fluid pressure in the arachnid's body rises and the spigot pops open.
Wolff adds that Drassodex spiders are unable to spin the densely packed piriform silk disks used by web-spinning spiders to anchor their webs because of their enlarged piriform silk glands, so they are unable to anchor structural threads. ‘This is a textbook example of a trade-off, where a highly efficient prey capture mechanism has evolved at the cost of reduced thread attachment, which is a basic function in all spiders’, he concludes.
