Why do Oak Processionary caterpillars form processions?

Firstly ,I don’t live in Europe. I live in South India, and my garden has a Night Flowering Jasmine tree.

And these trees are quite popular, but notorious for their caterpillars. In our region ,there are no specific names or identification these caterpillars and the thick mat they form on the tree trunks.

And one day, I was looking through images of caterpillars online ,when one seemed similar. It was called Oak Processionary.I am not sure if the caterpillars found in my area are the same species or something else entirely, but their behavior is similar . As per as Wikipedia and some other websites, they are usually found head to tail moving together.

My question is

• Do they have some chemical signaling agent like pheromones ,or like ants when they follow each other?
• Why do they form such processions in the first place? Wouldn’t having such a large ,conspicuous gathering make it easier for predators to spot them from afar?