While having sex with a girl, you stick a finger in her asshole/poopshoot/brown eye etc.

The important part is getting a little bit of poopie on your finger tip.

Now, as if you were to perform a "Dirty Sanchez" (spreading the shit on her lip/nose) you stick your shit covered finger in her ear.

Proceed to yell "HOLY FUCK ITS A MEXICAN EARWIG!!!"