What is the function and origin of silk glands and spinnerets in spiders?
The development of spinnerets and silk represents a major evolutionary shift that has defined the biological and ecological uniqueness of spiders within the arachnids.Silk glands produce the silk that the spider uses for a variety of purposes.The spinnerets are the special organs that the spider uses to extract and manipulate the silk as is it is produced from the silk glands.
Spiders evolved from ancestors that had limbs on the abdomen,as did arthropods like crustaceans such as crayfish.In fact,one of their few living marine relatives,Limulus,the so-called "king crabs",has retained abdominal limbs,which have been lost or greatly modified in terrestrial spiders and other arachnids.The spiders' spinnerets are almost certainly derived from these ancestral abdominal limbs.In the basal (lowest) segments of spiders' limbs are small excretory glands - the coxal glands - that secrete and excrete waste body fluids.It seems that the silk glands may represent highly modified excretory glands that now manufacture silk instead of waste products,just as the spinnerets represent highly modified limbs.
It is possible that an intermediate stage in this process could have been the production of a secretion that included pheromone (scent) chemicals put out by the spider as a primitive "signal line" by which a spider could find its way back to its retreat burrow.This role was then taken over by the production of silk.The silk then became useful not only as a safety line,but also for prey capture,manufacturing egg sacs and a host of other activities.
[Modified from text by Dr Mike Gray - Principal Research Scientist (Spiders)]
Reference:Foelix,R.F.1996.Biology of Spiders.Oxford Thieme.