Spun silk can come from the leftover "waste" of mulberry-eating silkworm cocoons. "Spun" refers to the fact that the fibres are shorter and must be hand spun. Most silk you will come across is a mulberry silk - whether hand spun or not - because silkworms tend to prefer eating mulberry leaves, or when reared on farms (even organically-cultivated ones) they will be fed a diet of solely mulberry leaves. Only wild silkworms that are left to eat oak leaves will not technically be classed as mulberry silk. You will also come across mulberry silk as Bombyx mori from this specific mulberry-eating silkworm species.