Surprise, Surprise: Sixth Instar Pipevine Swallowtails

For a while we thought we may have done something wrong with our captive raised Pipevine Swallowtail caterpillars (Battus philenor hirsuta) that caused our fifth instars to become severely undersized, but today we were both surprised and relieved to find that one of them was in apolysis for sixth instar!

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Most butterflies and moths exhibit five larval instars. It's just typical; most people who find a caterpillar in the wild can assume that it has five instars even without identifying the species. Of course there are plenty exceptions, especially when we're talking moths and also extremely large species of moths or butterflies. For example, the famous Atlas Moth (Attucus atlas) and Queen Alexandra's Birdwing (Ornithoptera alexandrae), the world's largest moth and butterfly respectively, both can have six instars in order to obtain their large size. 

However, instar number doesn't have to be that strict. Under the right conditions, it has been shown many times that a sixth instar can be induced in species in which five instars normally occur such as in Tobacco Hornworms (Manduca sexta) in order to make up for small size. We have even shown that the same is true in Anise Swallowtails (Papilio zelicaon) by rearing a severely undersized, nutritionally deficient caterpillar on citrus leaves which eventually molted into sixth instar. In some other species, such as some birdwings (Ornithoptera), there can be anything between four and six instars that seem to occur more or less randomly.

Well, today we have finally realized that the Northern California Pipevine Swallowtail (Battus philenor hirsuta), a relative of the tropical birdwings, can also apparently have six instars unlike other swallowtails. Despite the fact that it is such a popular species to rear, we have never seen it documented anywhere that they can naturally have six instars, so it took us all the way until we witnessed it with our own eyes to figure it out! For the past week we have been struggling to find an answer as to why our fifth instar caterpillars appeared to be so severely undersized when compared to some wild caught caterpillars that we thought -- at the time -- must have also been fifth instars. Our captive raised caterpillars looked like an entire instar behind because they were so small and it was beginning to get really depressing. But now, it appears that they really are an instar behind and we have misidentified the wild caught ones as fifths when they are really most likely sixth instar because we have just discovered that one of our captive raised fifth instars is in apolysis for sixth. What a relief that was -- we didn't mess them up too much after all and they will turn to "normal" size in the sixth instar!

We hypothesize that perhaps it is because the California Pipevine (Aristolochia californica) must be a less than ideal host compared the the pipevines they utilize in the other parts of the continent (the Pipevine Swallowtail's appearance in California is much more recent) and so, it is to some extent similar to the nutritional deficiency experiment with feeding Anise Swallowtail on citrus. There's some evidence for this: the leaves are hairier and tougher then other varieties, making it harder to eat, and the hirsuta subspecies likes to lay eggs in larger clusters than the species, perhaps because a larger group of caterpillars has an easier time feeding on the leaves. It is likely that the species is still in the process of adapting to the new region and transitioning with weird numbers of instars and will very soon completely diverge and become a new species of its own.

In any case, we should have seen it coming from a mile away. For a while now, ever since the first day that we found the Pipevine Swallowtail caterpillars this year on 6/18, we have always been super confused over why it seemed like there were such big size variations in the species. The variation was so dramatic that we had a hard time identifying the instars of all of our caterpillars -- it just seemed so ambiguously confusing! Eventually, we reasoned that all of the extremely small caterpillars must been small because they had been parasitized. But that can't possibly be correct because a few of our huge chrysalises have also turned out to be parasitized just the same. Now we strongly believe that it must be linked to the number of instars. A few of the fifth instar caterpillars must somehow take the normal route and pupate into very small chrysalises while most others will move on to sixth and become monsters in size. What we sincerely hope then, of course, is that most of our fifth instars will take the second route!  

- Brian