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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 through-put.

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, presumably each comprising 1 bite's-worth.

Below is frass that was dispersed in water, then poured onto a paper towel. The chunks of fennel stem are cleanly sliced at various angles.

frass dispersed on paper towel

Animals, including insects other than termites, 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, as you can see above, 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), as well as water and nutrients (minerals, sugars, and other organic substances) from ruptured transport tissues: xylem and phloem.

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. That they grow, develop, pupate and then emerge as butterflies tells us that no matter how inefficient their digestive processes are, they do deliver sufficient nutrients.

Skin frass! There is one type of food, other than the usual food plant, that caterpillars consume from time to time: their own skin following shedding. While looking for shed head capsules among the frass of a few early-instar caterpillars, I ran across a similar-sized object that looked like neither regular frass nor capsule. It is shown below as a stereo pair (see instructions here for how to view these by crossing your eyes):

frass from ingested skin

There appear to be plates of material consistent with caterpillar skin, and the whitish material at upper middle right appears to display a couple of skin stripes. It seems that caterpillars don't do much of  a job digesting skin (at least the parts made of chitin), either.