P. zelicaon Caterpillar Thermoregulation?

For years since we first began collecting and comparing wild-caught Anise Swallowtail caterpillars (Papilio zelicaon) as well as rearing caterpillars in captivity from eggs, we have noticed some pretty dramatic color variations in the older caterpillars that we believe has something to do with thermoregulation.

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Pretty much all organisms have morphological variations or polymorphisms. Humans certainly do -- I mean just take a look at how physically diverse we are as a species! A lot of scientific literature exists describing the mechanisms behind these differences but little, if any, has been written on caterpillars. Not surprisingly. Studying these variations in caterpillars can be tricky since there are endless little details to investigate and scrutinize. For starters, one must first determine whether variations are genetic (variations within a species, population, race, etc. gets tricky!) or environmental. 

In Anise Swallowtail caterpillars (Papilio zelicaon) there are two basic fronts for color variation. The first can be found in the little spots that line the body that typically appear in the third instar. These spots can be any shade of bright yellow to red-orange and can be constantly changing between molts. This, we believe is due in part to genetics, though environmental factors do seem to also play a role (described later). Caterpillars reared under the exact same conditions, even siblings, seem to produce both yellow oriented and orange oriented spots entirely by random. In any given group, there may be a higher amount of yellows or vice versa for no apparent reason other than they were born to be so. Interestingly though, we find that an overwhelming proportion of fourth instars will have orange spots.

However, sometimes it does seem possible to induce a particular spot color form through lighting. This actually brings us to the second front for variation: starting in the third or fourth instar, caterpillars can either take on a white striped form or maintain their black "bird dropping" mimicry; by the fourth instar, some caterpillars that were white forms in the third instar may become increasingly white dominant to varying degrees, or even green like a mature fifth instar (green morphs); by the fifth instar, these light morph fourths are likely to be extremely light green with very reduced black stripes and a white rather than black dominated underside. Just take a look at the two fourth instars we found today! To the opposite extreme, some later instar caterpillars may be extremely black dominant dark morphs with some fifth instars severely lacking any green coloration. It is the light morphs that more often than not will have yellow spots while the dark morphs will almost always have orange spots. What induces these differences in "lightness" seems to be a combination of light intensity and humidity. 

Anecdotally, we have found that an alarming number of all wild-caught third or higher instar caterpillars are light morphs while almost all indoor, captive raised individuals are neither extremely light or extremely dark. By this, we mean that they follow a more typical or "average" sequence of appearance development in which the caterpillar maintains the dark form all the way until the fifth instar which has roughly equal parts of dark green and black and only sometimes leaner towards a certain pole of the spectrum (dark green morph). We have never, after rearing thousands of these caterpillars for many years, witnessed an indoor reared specimen molt into green morph fourth instar. It would be ridiculous to call this a coincidence and leave it uninvestigated; In the outdoor, P. zelicaon caterpillars routinely receive dramatically higher amounts of light and less humidity than the indoors (away from open windows, of course). To test this, we have experimented with rearing these caterpillars completely outdoors in similar conditions in which we find wild-caught caterpillars. This has always succeeded in producing a higher average amount of light or green morphs caterpillars than rearing indoors. 

We theorize that molting into the light form during times of intense sunlight and dryness (peak summer conditions in our region) is a means thermoregulation. As most people know or have self-observed, darker pigments (black being the darkest) are the most absorptive of all wave lengths and therefore trap to most heat from the sun while the opposite is true of lighter pigments (white being the lightest). This means that a lighter body color -- cued by the light intensity and humidity of hot summertime weather -- would be better suited for defending against extremely high temperatures for a cold-blooded animal such as a caterpillar (again, the opposite is true of a darker body color). Caterpillars that we have found or reared during the late fall or early winter months have always been very dark morphs.

This darkness factor also affects a third front for color variation not mentioned before is in the intensity of a set of blue bands along a dark green morph fifth instar caterpillar's body. These bands may be extremely exaggerated in some individuals that the caterpillar may take on an overall turquoise color. We have never observed this in lighter green morphs (light green with white); in fact, the blue bands are commonly nonexistent in these. 

- Brian