Spinning an elastic ribbon of spider silk

The Sicarid spider Loxosceles laeta spins broad but very thin ribbons of elastic silk that it uses to form a retreat and to capture prey. A structural investigation into this spider's silk and spinning apparatus shows that these ribbons are spun from a gland homologous to the major ampullate gland of orb web spiders. The Loxosceles gland is constructed from the same basic parts (separate transverse zones in the gland, a duct and spigot) as other spider silk glands but construction details are highly specialized. These differences are thought to relate to different ways of spinning silk in the two groups of spiders. Loxosceles uses conventional die extrusion, feeding a liquid dope (spinning solution) to the slit-like die to form a flat ribbon, while orb web spiders use an extrusion process in which the silk dope is processed in an elongated duct to produce a cylindrical thread. This is achieved by the combination of an initial internal draw down, well inside the duct, and a final draw down, after the silk has left the spigot. The spinning mechanism in Loxosceles may be more ancestral.