Anise Swallowtail Chrysalis Horns!

Today while we were tallying our stock of Anise Swallowtail (Papilio zelicaon) chrysalises, we took a few pictures of a few chrysalises that apparently formed while the caterpillar's osmetarium was still everted.

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During the transitional phase between the larval and pupal stage in holometamorphic (complete metamorphic) insects, some parts of the larval morphology that are irrelevant to the formation of the adult are often dissolved and released as waste upon adult eclosion (sometimes also immediately before pupation). Contrary to popular belief, however, the pupal stage doesn't completely break down all of the larval parts into some sort of nutrient soup. While in some respects, this is certainly true, much of the adult body parts and organs (particularily the reproductive structures) are already at least partially formed in the caterpillar stage; the change between stages is not nearly as dramatic as it is made out to be. (The larval head will eventually become the adult head, the true larvae legs will become the adult legs, and so forth.) After all, there is strong evidence that holometabolism evolved from hemimetabolism (incomplete metamorphisis). 

An example of a caterpillar organ that is probably not utilized in adult formation is, apparently, the osmetarium in swallowtails (Papilionidae). This pair of stinky horns sequesters or synthesizes certain toxins (usually some form of butryic acid) designed to ward off both motion-sensitive (via retraction speed and waving of the horns in a batting motion) and chemical-sensitive predators. No such organ counterpart exists in the adult. Even when the caterpillar becomes a prepupa, the horns still remain functional to some extent: they are able to evert the horns when sufficiently agitated but are unable to retract them back into the thorax (at this stage the thorax is probably no longer quite the same as it was before!). Evertion of the osmetarium appears to be a reflex and thus even the caterpillar seems to have limited control over this. Once the horns are out, they hang out even after pupation! 

- Brian