How do worms automatically appear in rotten fruits?

It is usually the case that flies have laid eggs ON the fruit just before it ripened, and then when the fruit ripens the eggs hatch and the larvae feast on the ripe/rotten fruit to sustain themselves. read more

2) They are born inside the fruit and grow eating the fruit, finally when the fruit falls down from the tree they come out and hide inside the ground. 3) they create a hard shell (cacoon) like body for them self and stay inside that hard shell through out the winter. read more

Worms don't really automatically appreat in rotten fruit. If you are talking about long pinkish worms, it probably crawled into it. If you're talking about little white ones or maggots, those are the larval stage of flies and other insects that probably laid their eggs in it. As the egg developed, maggots appear. read more