Spider and silk threads. Coloured environmental scanning electron micrograph (ESEM) of a spider spinning silk (blue threads). At the rear of its abdomen are its spinnerets (not seen), structures used to produce the silk which the spider uses to weave a web and trap its prey. Some spiders also use silk to spin cocoons for their eggs. Spider's silk is made of a protein polymer called fibroin. It is very elastic and can be stretched 30-40 percent of its length before it breaks.