Often, larger frass looks similar to miniature ears of corn. The shape has to do with the intestine being able to absorb as many nutrients as they can from a very low nutrient food base. So, there is a large surface area on the frass which looks like corn on the cob.
JRsbugs said:Aha! The thought comes to mind that in order for the caterpillars not to be noticed, it has to camouflage it's droppings. They might resemble something else which is not palatable!