|European Cave Spider with egg sac. Francesco Tomasinelli.|
Spider silk is a remarkable material known for its strength and toughness. Most species secrete seven or eight types of silk for different purposes. Dragline silk, for instance, anchors spiders to a surface and is used as the backbone of most webs while other silks are used for web scaffolding or to wrap captured prey.
Female European cave spiders (Meta menardi), which live in dark, humid places ranging from northern Europe to Korea, produce tear-shaped egg sacs that hang from cave ceilings by a short stalk made of tubuliform silk. The fibers are relatively large, with an irregular surface unlike any other type of silk and are coated with a gluey secretion that fastens them together.
In the recent study, researchers collected the egg sacs from different caves in Piedmont, Italy and subjected the stalk silk to a variety of tests. A tensile testing machine pulled on the silk fibers until they broke.
The egg-sac silk of other spider species generally stretches by only 20 to 50 percent before snapping apart. The M. menardi stalks, on the other hand, stretched to two or three times their initial length, with some extraordinary examples drawn out to 7.5 times their initial length. The findings suggest the egg sac stalk is composed of densely and randomly packed fibers that can unroll a great deal in order to stretch.
“Evidence of the most Stretchable Egg Sac Silk Stalk, of the European Spider of the Year Meta menardi.” Emiliano Lepore, Andrea Marchioro, Marco Isaia, Markus J. Buehler, Nicola M. Pugno. PLoS ONE 7(2): e30500. doi:10.1371/journal.pone.0030500