Caterpillars: Mixed Media Artists

The Art of Excrement

The Found Objects of this essay lay in scattered groups of dozens to hundreds on a sandy road in the longleaf pine forests of northern Florida.  It’s likely that only an entomologist would have noticed them, let alone have stooped to collect a couple of dozen.  The size of small blueberries, these objects resemble fancy little beads that might look good on an Earth Child’s necklace.  Six deep grooves divide them into six equal segments, like the segments of a miniature orange, but their composition is rough and brown, and they can be readily crushed into a loose powder between the fingers.  With magnification, they resemble small, sculptured eggs.

Caterpillar droppings collected on a sandy road in the pine forests of north Florida

These artfully sculptured objects are caterpillar droppings, the final product of the digestion of oak leaves by the caterpillar of a silkworm moth, or perhaps a hickory horned devil.  The beads measure seven to eight millimeters long and five to six in diameter, so the caterpillar was clearly a very large one.  But why do the caterpillars defecate such elaborately sculptured poop?  Are they nature’s multimedia artists?  Are they expressing some aesthetic opinion of beauty for a discriminating public?  Is there a desperate competition among caterpillars to produce meaningful poop, as there is among some contemporary artists?  Or is the fancy poop an unintended consequence, a side-effect of something important?

It seems rather marvelous that an animal would go to so much trouble to beautify something that it then commits to the greater world, to lie on sandy roads waiting for an entomologist to come along, and that that entomologist would then try to understand how and why the caterpillar made excrement with such complex geometry and regularity. 

Actually, this complexity is a response to a physiological problem created by the rules of physics.  Not so romantic or mystic as an aesthetic impulse in a caterpillar. The problem arises from the universal laws of geometry that apply whenever a life-process takes place across, out of, into, through or from a surface. 

Imagine you are a tiny caterpillar that has been dining on nice green leaves and your hindgut is packed with chewed up and partly digested leaf fragments that you are finished with and are about to consign to the wider world, but before you do, you have to retrieve some of the water in the leaves to keep yourself from desiccating, and you have to adjust your ion balance from plantlike to animal-like (with respect to sodium and potassium).  You’ve got a handy-dandy organ (called the cryptonephridium) in which your excretory tubules collaborate with your hindgut to form a sort of insect kidney.  This organ can resorb water and exchange ions in both directions, so all seems well in caterpillar land.

But you are a tiny caterpillar, and caterpillars are in the business of growing, and as you grow without changing shape (hang on, here comes the math!), your surface area (and that of your gut) increases with the square of your dimensions, and your volume with the cube.  For every two-fold increase in your dimensions, you will have half as much surface per unit volume, and therefore half as much surface relative to volume across which to resorb water and balance ions. The “need” for exchange will increase twice as fast as the capacity for exchange. What to do?

The solution lies in changing shape as size increases, and the gut does this by evolving six deep longitudinal folds in the gut wall, like the segments of an orange, increasing the gut surface area more than 2.5-fold compared to a simple, cylindrical gut.  It is these folds that impart the artful shape to the droppings.  The folds, along with a parallel increase of the cryptonephric organ keep the exchange capacity up to speed.  Water and ions move according to need, and form and the function are still related.  The unintended consequence of this adaptation is fancy excrement, mixed media art.

End and side view of droppings of a large caterpillar. Note the division into six deep, longitudinal grooves.

One of the predictions from this story is that across species the poop of large caterpillars should be more “artfully sculptured” than that of small ones.  I have surveyed my stock of caterpillar poop (three species, to be exact) for supporting evidence.  For the medium sized caterpillar of the sycamore snipper, Misograda unicolor, there is still a lot of artfulness (below) in the 3 mm droppings, but for the smaller tent caterpillar (up to 2 mm), the artfulness seems more half-hearted and variable with some dents but no deep grooves, especially in smaller droppings.  Would still smaller caterpillars have simple, grooveless droppings? 

(left) Droppings of a mid-sized caterpillar, the sycamore snipper. (right) Droppings of tent caterpillars of a range of sizes.

A testable hypothesis has emerged from my observations, i.e., the larger the caterpillar the more artful the poop.  What a great Ph.D. dissertation that would make! It could even be interdisciplinary, incorporating art and philosophy!  No expensive tools would be needed, material would be easy to come by, and the opportunity for an attention-grabbing title would be irresistible!.  This would assure a large audience at the dissertation defense, and a stand-out paper to follow!