The unique ribbon morphology of the major ampullate silk of spiders from the genus Loxosceles (recluse spiders).

The morphology of silk produced by recluse spiders (Loxosceles arizonica) was investigated by scanning electron microscopy, atomic force microscopy, and transmission electron microscopy. This silk consisted entirely of very long, thin ribbons of width 2-4 microm and thicknesses of no more than 40 nm. The correspondence in shape and dimension between the silk ribbons and the elongated aperture of the major ampullate spigot indicated that these ribbons were major ampullate silk. Selected area electron diffraction patterns from single ribbons were indexed with an orthorhombic unit cell (a = 9.43(2) A, b = 8.96(3) A, c = 6.96(1) A). This unit cell is in good agreement with that previously reported for synthetic poly(L-alanylglycine). Thus it is likely that the crystalline regions of the major ampullate silk of L. arizonica consist of an alternating glycine-L-alanine motif that has adopted a beta-sheet structure. The amino acid composition achieved with the silk of L. arizonica as well as that of L. laeta confirmed that the major amino acid constituents of this silk were glycine and L-alanine in nearly equal amounts. As it was noticed that the dry ribbons were highly electrostatic, it is suggested that the electrostatic interaction plays an important role in prey capture for Loxoseles.