Fifth Instar Pipevine Swallowtail Pupation Failure

One of our fifth instar Pipevine Swallowtails (Battus philenor) that had become a prepupa actually managed to pupate without dying and ended up as a caterpillar-chrysalis intermediate.

---

Over the past week we have had a total of three of our fifth instar Pipevine Swallowtails (Battus philenor) unexpectedly messy our containers with their loose stool. We have actually documented a sixth instar in every single other of our other captive raised caterpillars thus far; as of today there are 27 sixth instar caterpillars, 11 sixth instar formed prepupae, and 14 sixth instar formed chrysalises. But for some reason these three sickly looking individuals in a particular colony decided to do this. 

Clearly, the endocrine functions in these Pipevine Swallowtails is out of control. Their size fluctuates naturally fluctuates more than any other caterpillar we have ever reared, and not just because the females tend to be larger than the males (this is actually the case in many species), we suspect, due to juvenille hormone (JH) levels in the caterpillars being so out of whack. Five larval instars is not a normal occurrence, at least in our particular regional race and is probably simply caused by a lack of JH. Suggested, of course, by the fact that the vast majority of them went on to the sixth instar and the ones that didn't just died.

Today, we finally got one of the fifth instar prepupae to pupate though, without just dying. The first two in the colony had just sat there, developing into pupa but never actually completing final ecdysis (so basically, they died as a chrysalis wrapped in caterpillar skin). We had a similar experience with the fifth instar pupaters we had earlier in June. But this time, though the fifth instar really did pupate, it ended up turning into a caterpillar-chrysalis intermediate, supporting our conclusion of hormonal involvement. It didn't have enough JH to stimulate the sixth larvae-to-larvae molt, but had just too much such that the chrysalis still had caterpillar parts. At this point, it will probably die within a day.  We can't image what would happen if it didn't die and actually eclosed as some strangely deformed butterfly, especially because it is a puny 0.3 grams; it would clearly be incapble of breeding!

On another note, we noticed today that what we believe to be a normal fifth instar is surprisingly large. For week we have been complaining about how our captive raised caterpillars are, on average, much smaller than their wild caught counterparts due to us rearing them at extremely high densities, poor food quality, and disruptive disease outbreaks from bad sanitation. The largest sixth instar caterpillar we have is currently only 2.4 grams (they shrink to roughly half the size when they pupate!) and our largest chrysalis only 1.2 grams while our largest wild caught formed chrysalises from June were 1.5 grams. The large fifth instar in question is now in apolysis for sixth and already a whopping 1.4 grams, with a head capsule unrivaled by any of the other seven fifth instars we have left or any of the 20-30 fifth instar head capsules that we saved. Though big, its head capsule isn't quite as big as the average sixth instars' but at first glance we still could have mistaken it for a small sixth instar trying to make up by going into a seventh instar! 

The left and middle show the fifth instar formed chrysalis, complete with a distinguishable caterpillar head! The left shows it next to a normal (and normal sized) chrysalis. 

- Brian