Oregon Swallowtail Caterpillars Ecdyse into Fourth Instar

Our oldest third instar Oregon Swallowtail caterpillars (Papilio machaon oregonius) have ecdysed into fourth instar.

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Admittedly, there hasn't been too many interesting observations to make about our Oregon Swallowtail caterpillars so far since they are almost identical to Anise Swallowtail caterpillars which we have raised thousands of. However, during the third instar, the caterpillars seemed slightly larger and lighter in color than typical anise swallowtail caterpillars raised under the same conditions and we suspected from the start that a main phenotypic difference between the two species' larvae was in their color based on internet searches. Anyway, today, two of our Oregon Swallowtail caterpillars molted into fourth instar, and we were amazed to see that both were green morph fourths, one being very green and the other somewhat whitish green. This is a clear difference between the two closely related species, as Anise Swallowtail caterpillars are typically black in the fourth instar when raised indoors and are only green when subject to very high temperatures and low humidity. In fact, we have only ever had a single indoor raised green morph fourth instar Anise out of thousandss we have reared. Thus, the fact that two of two of the fourth instar Oregon Swallowtail caterpillars reared under the same conditions as Anise are green is clearly not a coincidence and must be typical of the species.

Since the Oregon swallowtail caterpillars are already green in the fourth instar, we assume they will be very green fifth instars with extremely thin black bands since this is typical of fifth instars produced from green morph fourth instar Anises. When speculating from an evolutionary standpoint, it would make sense for the oregonius caterpillars to be more green in the final instar with very thin black bands since most non-umbel feeding Swallowtail larvae are green with no black bands. The machaon group Swallowtails diverged from Rutaceae feeding Swallowtails by jumping onto Apiaceae, or umbels (possible due to phytochemical stimulant similarities) and all have very characteristic black bands and yellow or orange spots in the final instar unique to other Swallowtail larva which make them quite cryptic on umbels. However, since switching over to umbels, some of the machaon group swallowtails such as oregonius now almost exclusively utilize Artemisia in the wild rather than umbels, again due to phytochemical stimulant similarities. Thus, since oregonius no longer feed on umbels any more, their once cryptic black bands are probably not very cryptic anymore and thus greener caterpillars with thinner black bands are more cryptic and have higher fitness. Don't quote us on any of this though, since these are pure speculations that came to mind when comparing oregonius to zelicaon. 

- Alan