Why are the worms on the spider web empty shells?

In summer, it is very interesting to observe spiders preying on insects. When flies, mosquitoes and other small insects touch the spider web, the spider hiding in the corner of the web will quickly climb over, pull out long silk from its tail and tightly entangle the struggling prey. Later, it seemed that the spider was full, but there were a pair of insect shells on the spider web.

There are many empty shells of insects hanging on the spider web. This is because spiders catch trapped insects with pliers that grow at the front end of their bodies, secrete a poisonous juice from the pliers to paralyze the insects, and then spit a digestive juice called enzyme into the insects to dissolve protein, dilute the five internal organs, and turn the insects into juice and suck it into the stomach. But if the worm shell is not protein, it can't be dissolved by digestive juice, so the worm shell remains intact on the spider web.