Papilio polyxenes, zelicaon, & ♀ ​polyxenes asterius × ♂ zelicaon Comparisons

Here we compare the third and fourth instar larvae of the black swallowtail (Papilio polyxenes asterius), anise swallowtail (Papilio zelicaon) and a ♀ Papilio ​polyxenes asterius × ♂ Papilio zelicaon hybrid cross. 

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Now that we have mature third instar larvae of all three types of larvae, we decided to compare them side by side. The major difference between Papilio polyxenes asterius and P. zelicaon third instars is the amount of white. The latter always has a checkered white saddle and has minimal white dots along the body, especially near the rear. The P. polyxenes asterius larvae have a more or less solid chunk of white on for the saddle where even the tubercles are white, allowing the faint yellow spots there to be more easily seen. Another obvious difference is the size; P. zelicaon is indisputably larger, which would make a lot of sense because they have much larger eggs that hatch into larger first instars. As for the hybrid larvae, it is intermediate in size between the two species (perhaps even as large as P. zelicaon since the individual in the picture isn't quite full grown) but has a pattern that matches much more closely to P. polyxenes asterius. Some of the spots at the rear are blacked out though. 

We don't quite have any fourth instar hybrid larvae yet, but we can compare the fourth instars of the pure breeds. The fourth instar P. polyxenes asterius larvae is a markedly different thing than the P. zelicaon. Like the third instar, the P. polyxenes asterius is much lighter (all of the fourth instars, reared indoors, we have so far are light whereas only a very small percentage of indoor reared fourth instar P. zelicaon are light), even lighter than the light form fourth instar P. zelicaon in the photographs. In fact, the P. polyxenes asterius could almost pass as a newly molted fifth instar P. zelicaon because the white saddle is completely gone. All of the black stripes there are complete and consistent, unlike P. zelicaon light form larvae, and the tubercles are extremely reduced. Another indisputable difference that could not possibly be from natural variation within the species are the dots on the thickest thoracic stripe right before the head. The P. polyxenes asterius larvae are completely lacking in them, a characteristic that is completely unique to the fifth instar in P. zelicaon. 

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Authors
Brian Liang

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