The researchers suggest that the segments of the protein with the repeating blocks form long, springlike shapes. Spiders make as many as seven different types of silk, says coauthor Cheryl Y. Hayashi. Insects get entangled in the sticky web, she explains, because the stretchiness of capture silk lets the web oscillate back and forth after the insect hits it. If the web were stiff, the insect might just bounce off. Researchers have cloned several genes for dragline silk, the type that the nursery rhyme spider must have spun to lower itself down beside Miss Muffet.
Spiders use dragline silk to form the guylines and framework for wheel-shaped orb webs.
In fact, Lewis says, dragline silk is only one-fifth as elastic as capture silk. Dragline silk proteins and capture silk proteins have similar turn-forming blocks of amino acids. However, the researchers found that these blocks repeat an average of 43 times in the capture silk, compared to only 9 times in the dragline silk. That fivefold difference in length corresponds to the difference in elasticity between the two proteins, Lewis says. The stretchy section of the protein may not spiral in the way Lewis describes, cautions John Gosline, a biomechanic at the University of British Columbia in Vancouver.