Late Season Dark Form Anise Caterpillars

This late in the breeding season, the usually highly variable Anise Swallowtail caterpillars (Papilio zelicaon) under different conditions are invariably darker forms. 

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There is quite a bit of observational evidence that many caterpillars take on specific color forms based on temperature or photoperiod either as a way to enhance camouflage during a specific time of year or to help with thermoregulation since darker colors retain greater amounts of heat than lighter ones. We have noticed this in a number of species we have reared, particularily the Anise Swallowtail (Papilio zelicaon). We have also observed 100% of our Achemon Sphinx (Eumorpha achemon) and Imperial Moth (Eacles imperialis) caterpillars taking on brown forms when reared completely indoors (where there is no sunlight exposure). While we have not compared this result to outdoor rearing (we have not had the chance yet), this is somewhat unlikely unless some other factor besides random phenotypic distribution is at work. We are somewhat doubtful that host plant has a significant impact on this as some sources claim.

The Anise Swallowtail is one species that we have had the chance to observe quite closely though, and in a earlier post this year we described how we noticed their tendency to lean toward lighter morphs when exposed to greater amount of sunlight or darker morphs when reared indoors. With this particular species, both thermoregulation and camouflage seem to be reasons for this phenomenon. Sunlight exposure increases body temperatures and therefore it would be beneficial to have less black coloration on the body so as to retain less heat. And towards the end of the season when there is less sunlight, their umbel (Apiaceae) host plants (particularly fennel [Foeniculum vulgare], which also happens to be their preferred host plant in our locality) are usually mature and therefore have much more "thinned out" leaves (less dense needles) so it would make sense to have thinner green bands on the caterpillar.

Most of these observations, as stated before, were comparing outdoor reared summertime individuals with summertime indoor reared individuals and less frequently with individuals actually reared in the fall time where temperatures are naturally lower and there is less sunlight. But now we do; now that it is November, our three dozen fifth instar Anise caterpillars that we reared from eggs layed by a wild caught female in October are invariably dark forms. They have been reared outdoors stating from the third instar which usually results in extraordinary light morphs in the summer. This is not a coincidence. 

- Brian