Caterpillar droppings are a fundamental
aspect of keeping these critters. Despite the sanitized scenes
depicted in most of these shots, the scene of caterpillar feasting
is, after a few hours, anything but sanitized. After all, they
are following the Lepidopteran Dietary Guidelines and are consuming
a Very-High-Fiber Diet, so they a have a rather substantial GI
Thus far, the most interesting
aspect of the "frass" (yet another imaginative scatological
euphemism!) is its cute dimple:
Anyone know what causes this?
It's only on one side, and the dimpled end emerges first. One possibility is that the muscle(s) that keeps the anus closed protrudes in the upstream direction when contracted and puts the dimple in each frass as it is pushed against the muscle prior to defecation.
Here's a better pic of a frass particle taken through the eyepiece of a stereo zoom microscope:
You can see that the frass is composed of chunks of fennel stem, probably each comprising 1 bite's-worth.
Animals, including insects, do not have the ability to digest "dietary fiber" (the material that makes up the cell walls of plant tissues), and because caterpillars do not chew as we do to grind food into smaller particles, each bite of stem stays more or less intact due to its indigestibility.
The same thing happens if we do not chew our plant foods--they emerge from the other end largely intact. What the caterpillar can digest would presumably be whatever contents spill from cells ruptured by the slicing mouth-parts (mandibles).
Do caterpillars have very active flora (bacteria) in their guts that could partially digest some of the cell walls and release more nutrients? Even if they do, the through-put is very fast (2-3 hours), and there wouldn't be time for much of this digestion to occur. It seems more likely that they are unable to scavenge nutrients from much of what they eat.