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

Here we compare the 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 our hybrid ♀ Papilio ​polyxenes asterius × ♂ P. zelicaon larvae have molted to fourth instar, we can compare them with the pure breeds. As before, the hybrid seems like a pretty good intermediate. The fourth instars are light forms like P. polyxenes asterius and look almost identical except for a few minor differences. The black stripes on the saddle are not quite complete, unlike P. polyxenes asterius, but are also not as broken as light form P. zelicaon; the black stripes are also slightly thicker than P. polyxenes asterius in general. The tubercles of the hybrid seem also to be intermediate between the two species. Like P. polyxenes asterius, there are no spots on the thick thoracic stripe close to the head. This all seems to be pretty compelling evidence that the color form is a genetic trait and not just environmental.

On 7/23, a few more hybrid larvae molted to fourth instar and are of a darker form than the first one. Compared to P. zelicaon dark forms, they have more distinct white stripes between the segments and some have a more solid looking saddle like third instar P. polyxenes asterius​ third instars. 

Comparing the fourth instars of the pure breeds again now that they have reached the end of the instar, we see that the P. polyxenes asterius is still much smaller and less thick. The tubercles are much reduced in proportion to the body in the P. poyxenes asterius, making them look much like fifth instar P. zelicaon. Surprisingly, they are not actually very green despite being light forms. The spots of the P. polyxenes asterius are very small, which goes well with the very fine and complete  black bands, and are invariably yellow. In P. zelicaon, orange seems to be more common in the fourth instar and, according to some sources is a genetically determined dominant trait. For some reason, the P. polyxenes asterius also seems to have a slightly more thickened thorax, giving it a more tiger-like larval shape. 

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Authors
Brian Liang

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