cool-critters:

Who knew some caterpillars can squeak?!
Does anybody have an explanation?

Spiracles, AKA the breathing holes that most insects have on their sides. Some insects have slightly modified spiracles, which, when air is forced through them, produce sound. Hissing cockroaches are the most famous example, but it’s also a tactic found in some beetles and, as seen here, caterpillars. 

It’s meant to startle a predator into dropping it, and is not actually an indicator of distress so much as a reaction to something that might be a predator. A more accurate translation of the squeaks would probably be “BUGGER OFF YOU FUCK”. 

And let me tell you, it might not seem very startling here, but picking up a caterpillar and having it scream at you will definitely make you drop it. Source for that: me picking up a hornworm and learning that they can yell.