Silk is a complex protein substance. It has an inner core of fibroin – a tough, flexible, long-chain polymer – and an outer, sticky coating of sericin that acts like glue; in the commercial production of moth silk the sericin is removed to enable the fibres to be unravelled and spun together.
Though fibroin has a very complicated molecular structure, it is still – like all proteins – a chain of simple building-blocks of amino acids, endlessly repeated.
Amino acids evolved in living organisms long before caterpillars and spiders diverged, and silk-like proteins are made by many other organisms such as velvet worms (squirted for prey capture), fungus- gnat maggots (web tubes to live in), lacewings (egg stalks) and weaver-ant grubs (used like glue-sticks to make leaf nests).
Spider silk is stronger, but otherwise so similar to insect silk that genetically modified silk-moth caterpillars (silkworms) have been bred to create it in a more easily harvestable form.