Sometimes a caterpillar eats eggs, another caterpillar, a pre-pupa, or a chrysalis, even when there is enough food. Why?
When we stop to think about it, we realize that an caterpillar is simply a mature egg plus host plant. A pre-pupa is addition of time and more host plant. A chrysalis is the addition of … you get the idea.
Because a caterpillar, pre-pupa, and chrysalis are made entirely of the host plant, so to speak, the taste of the insect would be tasty to caterpillars that eat the same host plant, whether they are the same species or not. We know that some caterpillars that eat distasteful plants are distasteful to many predators. The taste of the host plant controls the taste of the insect.
The instinct of some species of caterpillars, when they hatch, is to eat its egg shell. Once that shell is eaten, if there is another egg nearby, that instinct may cause it to continue eating eggs.
Although it happens more often when caterpillars are contained and run out of food, cannibalism happens even when there is plenty of food available.
Cloudless sulphur caterpillars are well known for cannibalism, out in nature. Even on a large host plant, they often attack and eat other caterpillars. They are one species that are often raised separately, one per plant stem.
Pipevine swallowtails and gold rim (polydamas) swallowtails are extremely cannibalistic in captivity. Once they are near pupation, many farmers raise them separately. As young caterpillars, they are gregarious, staying together on a leaf. Once they grow to three or four instars, they naturally separate on the plant.
To be safe, simply make sure they have plenty of food and learn which species tend to cannibalize in captivity. Separate them at the appropriate age.
Butterfly Fun Facts 
I have just witnessed painted lady caterpillar cannibalism yesterday & today. I had run out of commercial diet just as most were beginning to pupate, but 5 of the caterpillars were a day younger than their siblings and not quite ready. I gave them yarrow from my garden, and most went up and formed chrysalids soon after. Unfortunately, 3 of them fell in the process of making that last molt. I was going to let them harden before trying to hang them back up, but before I knew it, their young siblings had started eating them! They went straight for them, as though the fresh chrysalis gave off a scent that attracted them. I caught another one trying to nibble on one that was still hanging! I gave them more fresh yarrow, but if I see another incident, these last 3 are going outside.
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Cannibalism is fairly common when they run out of food and, for some species, even when they have plenty of food. It is quite shocking at first but once we stop to think about it, a caterpillar is basically made out of its host plant since that is all that it ingested since hatching.
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One of my monarch cats ate a hole in his sister’s chrysalis, does anyone know if/how I can help the chrysalis?
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Some species are cannibalistic, even wild in nature with enough food. Others only sometimes eat one another when they are in captivity. It hasn’t been recorded as happening in the wild (although perhaps it does). In captivity, they are closer together than in the wild. As they are eating, they sometimes simply eat a chrysalis or pre-pupa. It is made of milkweed, in a sense, and probably has close to the same taste – milkweed. Once a hole is eaten in it, it is usually lost. If you’d like to send me a photo of the chrysalis, please send it to edith@buyabutterfly.com.
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Yesterday I saw a two week old cat which had been immobile for around 8 hours expell a smaller black cat complete with legs and antennae, then turn around and consumed the whole thing . It has since continued to munch and grow . I have never seen this before . Is it common? Was it a parasite? Explanation please, if you know . I did get a video .
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The caterpillar was molting. It crawled out of its old cuticle/skin, It usually then turns around and eats the old cuticle (depending on the species of butterfly or moth). It does this four times before it pupates into a chrysalis. It’s an amazing thing to watch!
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