Anise Swallowtail Update: Still Hanging On!

The last two fifth instar anise swallowtail (Papilio zelicaon) caterpillars from an October brood still have yet to pupate after two and a half months!

---

On 10/8 we caught a wild female anise swallowtail (Papilio zelicaon) adult who laid three dozen or so eggs that we collected and raised. On 11/24 we harvested the chrysalises formed from this brood of caterpillars. At the time of this chrysalis harvest, there were still two young fifth instar caterpillars that had been lagging behind. Today, they are still somehow with us. They have made gradual progress over the weeks, having clearly eaten and grown over time, and they appear to be completely healthy. Needless to say, we are quite baffled.

As herb feeders, anise swallowtails normally have an extremely fast growth rate. At room temperature, say 60=70 degreed Farenheit, these caterpillars have absolutely no problem completing development from the time they hatch to pupation in three, possibly four weeks (assuming other rearing conditions are not decent enough). When our October brood of caterpillars were fourth instar, they were only about at day or two apart in development but by the time they reached fifth instar, only about have of them seemed to develop at a normal rate and completed larval development in the expected three weeks. The other half pupated over the course of November in what seemed to be a complete random order. By the time we collected the chrysalises on 11/24, a few had only just finished pupating. The ones that lagged behind were cleary growing at a raye that is completely out of the scope of what should have been normal even given that the temperatures have gradually dipped a few degrees over the course of the month. But nothing can compare to the two individuals that are still eating and growing as this post is being written, almost two months since the first few individuals of this group pupated. These are caterpillars that hatched from eggs laid on the exact same day!

Why did this happen? These are not sickly individuals. They are not like some sick caterpillars that fight a prolonged battle with a virus or a parasite that they inevitably succumb to. We have seen plenty of those in our time rearing lepidopterans. No, these two fifth instars appear and act completely healthy and are not undersized (see the left and middle images). And as seen in the picture at the right, they have produced quite a bit of frass in one day, no less than what we would expect of them. The temperatures are currently 50-60 degrees Farenheit, no where near intolerable (bear in mind that the temperature wasn't always so low back in October and November, when the lived most of the lives thus far). So how are we supposed to explain this one? 

The only logical assumption that we can make is that this is caused by having experienced too many stressors, including the temperature (as of late). And, of course, the lagging behind only results in a downward spiral where caterpillars that are already slightly behind are more susceptible to getting even more behind; the two individuals right now have experienced a nightmare of rain and cool temperatures that the faster grower never experienced and this is making them lag behind even more than they would have. Its plausible that when stress reaches a tipping point, things start not working right as they do in so many other animals including ourselves, including stunted growth. 

- Brian