Pipevine Swallowtail Caterpillars Pupating After Both Fifth and Sixth Instar

Today, another one of our fifth instar Pipevine Swallowtail (Battus philenor) caterpillar has had runny stool rather than entering apolysis for the sixth instar.

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A while ago, we made a post about how surprised we were to observe six instars in the Hairy Pipevine Swallowtail (Battus philenor hirsuta) rather than the typical five instars observed in the majority butterfly species. All of our oldest fifth instar caterpillars at the time successfully molted into a sixth instar. Since then, they have matured and the first of them have begun to pupate.

Our younger caterpillars followed suit. Almost all of our caterpillars are now sixth instars save for a few fourths and fifths that were wild-caught in an earlier instar. But today, for the second time this week we have witnessed one of our fifth instar caterpillars have runny stool, begin to wander, and form a legitimate prepupa. The prepupa today is only 0.3 grams; the average for our first healthy twelve wild caught chrysalises was 1.2g! This caterpillar was from the same clutch as the other fifth instar caterpillar that had runny stool earlier this week, so perhaps there is some genetic factor involved affecting the plasticity of the instar number. 

While it is true that in most species, the female is significantly larger than the male as early as the caterpillar stage and sometimes the female will need an extra molt to achieve this size, this is highly unlikely the case here. With 45 sixth instars currently and only about five fifth instars left, the ratio cannot possibly be so skewed. We still suspect instead that this species originally evolved on five instars and that the ambiguity between five or sixth instars is not a natural or, at least, intentional occurrence. Whether the mechanism behind the hormonal decision to molt into the sixth instar is the same as that of the supernumery instars in other lepidoptera caused by unfavorable growth conditions in the laboratory we still are unsure. 

All of top and bottom left: the smaller one is the fifth instar; the larger are sixth instar prepupae. The bottom right shows fifth instar chrysalises compared to sixth instar chrysalises from our caterpillars back in June.

- Brian