Anyone who has read the famous children’s book The Very Hungry Caterpillar knows that caterpillars have a reputation for eating a lot, and caterpillars have a reputation for eating a lot because they do eat a lot. But then this raises the following question asked by a reader the other day: “how do caterpillars eat so much food when they have tiny bellys?” Phrased slightly more technically, how can caterpillars, given their relatively small size and stomach, eat the enormous amount of food they do? Doesn’t the caterpillar’s body size limit how much food a caterpillar can eat? In order to address these questions, we will first consider how much caterpillars actually eat, and we will also explain how their body size relates to the amount they consume.
Although what constitutes eating “a lot” is inherently relative, caterpillars eat an enormous amount by virtually any reckoning. Indeed, essentially all a caterpillar does is eat, as the energy supplied by the food they consume is what fuels their maturation process, and caterpillars, like other larvae, are constantly maturing. In particular, they are growing larger as they prepare for their eventual transformation into butterflies or moths. (How caterpillars turn into butterflies is one of the great marvels of nature, as a relevant side note.) During only a few weeks, a caterpillar can eat thousands or even tens of thousands of times their body weight. Applied to a larger animal, this is an almost unfathomable amount of food to consume. A 200 lbs. (90 kilogram) person, for instance, would have to eat hundreds of thousands of Big Macs (to pick a somewhat universal reference point) a day to keep up with the way many caterpillars eat. For this reason, the reader’s question becomes all the more interesting: how on earth can a caterpillar eat so much?
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Like other animals, caterpillars pass food from their bodies in the form of fecal matter. Of course, they do this far more frequently than most other creatures because of their constant eating, which is why caterpillars more or less leave a trail of frass (often in pellet form) wherever they go. Caterpillars consume an incredible amount of matter, but much of that matter passes right through them. If it didn’t, a caterpillar would practically be the size of a whale by the end of its life cycle.
While caterpillars clearly don’t grow to be the size of whales, they do grow, and this is another reason they can eat so much. It’s not as if the nutrients that caterpillars eat don’t contribute to physical growth – that is by and large the point of their furious eating regime – and in fact some caterpillars grow to be 1,000 times larger than their original size. Caterpillars have malleable skin that can handle some of their rapid growth, but they still molt several times during their lives to accommodate their increasing size. These developmental stages are called instars, and the average caterpillar goes through about five or six in a lifetime.
Thus, caterpillars are able to eat so much because they grow rapidly and immediately pass much of the matter they consume in the form of waste. It still seems incredible that caterpillars can eat so much, and it is, but of course many natural phenomena are incredible. Indeed, many natural phenomena involving caterpillars specifically are amazing, like their transformation into butterflies that we cited earlier, or the ability of some species of caterpillars to live on both dry land and underwater. It is also worth keeping in mind that caterpillars start their lives extremely small; their size keeps their eating feats in perspective, but only a little bit.
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