A chain of men, standing in a circle are having simultanious anal sex with each other. To accomodate this, the uneven members of the chain provide the movement, while the rest remains stationairy.
The phenomenon is said to look like a caterpillar.
Caterpillar-sex is said to be extremely pleasureable, but it takes a lot of practice.
"At a kinky party, Matthijs meets the people he found on the internet to have caterpillar-sex with."
"I want to caterpillar you at the gay-bar"