How do pupae inflate so fast?

[LarvaHunter]

I noticed my stag L3 was about to pupate and was still a grub waiting to pupate on his back, less than 12 hours later and I am looking at this thing that is 2x as big and totally different. Is it their juvenile growth hormone or what? I don't understand how it can grow so fast, almost like it has been inflated like a pool toy.

 

[Shade of Eclipse]

All insects inflate themselves with air as they molt in order to increase their size.

 

[LarvaHunter]

So they do inflate like a pool toy. Interesting. I never knew that. Thanks!