What do all of the over 45,000 described spider species on Earth have in common? Each makes at least one type of silk. And there are an awful lot of types out there.
An individual orb weaving spider – the kind that spins the classic two-dimensional aerial spiral webs that seem to always be suspended at human face-height – can produce seven different silks, each with unique material properties.
Dragline silk forms the frame of an orb web and is famous for its strength and toughness, comparable to that of steel. The capture spiral is made of a highly stretchy version called flagelliform silk. Orb weaving spiders use an additional type of silk to wrap prey and create web decorations.
But there’s another kind that, on the surface, doesn’t resemble silk at all: the sticky glue with which some spiders cover their silk capture threads. It doesn’t look like the classic threads that come to mind when thinking of spider silk, but the gluey substance from these webs is in fact a silk protein.
For many years, researchers have been uncovering the secrets of spider glue, which stays wet in its open air environment and sticky over many rounds of attachment and release. Its genetic blueprint has remained elusive, however, meaning scientists haven’t been able to think about setting up large-scale production of this potentially useful biomaterial.
Sarah Stellwagen, CC BY-ND
A silk that’s really a sticky glue
Under a microscope, orb weaver glue resembles beads on a string – little glistening spheres along a strand of stretchy support silk. Instead of being spun into a fiber as it leaves the spider’s body like other silks, the glue proteins are extruded as a jumbled mass. Their job is to stickily retain prey that get caught in the web.
Different spider species produce glue tailored to their habitat’s conditions and prey.
The glue of tropical orb weaving species is sticky in the spider’s wet habitat, but downgrades to just tacky in low humidity. The glue of orb weavers from dry regions becomes dilute and thin if the humidity is too high.
Bolas spiders forgo the orb web, and instead produce a large globule of glue at the end of a long strand of silk that they whirl rapidly through the air. The glue of this sticky snare is specialized for capturing moths covered with loose scales.
Widow spiders produce vertical, glue-covered trip lines that detach from the ground when encountered by an unsuspecting victim, springing the prey into the air where it hangs suspended. Unlike orb weaver glue, widow glue is resistant to fluctuating humidity.
These various specialized adhesive properties have intrigued biomaterials researchers who can dream up plenty of uses for artificial versions of spider glues. But without knowing the genes that code for these proteins, there hasn’t been a clear road map for how to produce synthetic spider glues.
Cracking a long, repetitive code
Surprisingly, researchers have only sequenced around 20 full-length spider silk genes despite the incredible diversity of spiders and decades-old interest in silk as a useful biomaterial.