The Beast and the Beauty

I found a surpassingly ugly caterpillar the other day.

It took me a while to identify it, and even longer to understand the significance of what I was looking at. This unappealing grub transforms into more than one possible butterfly, in a twist on one of nature’s greatest mysteries that has become a famous evolutionary case study.

Round where I live in Maine, the caterpillar is likely to metamorphose into this (photographed last year):

This is a White Admiral, Limenitis arthemis arthemis. Notice the red spots and the blue scalloped border. But go a little further south, and it becomes the Red-spotted Purple, Limenitis arthemis astanyax, totally lacking the broad white wing bands.(photo from Wikipedia)

The southern form mimics the blue hindwing coloring of the very toxic Pipevine Swallowtail, Battus philenor, and their two ranges coincide. This “Batesian mimicry” deters predators. DNA evidence shows that the two colorations hybridize across a zone starting in southern New England, shown below in dark grey (from Ries and Mullen 2008)

Interestingly, the presence of even small numbers of the toxic model in the hybrid zone is enough to maintain some instances of the mimicking coloration. Only when there are absolutely no toxic models present, as in Maine, do the butterflies revert entirely to the non-mimicking coloration. The data are a wonderful example of the importance of citizen science, since they were all taken from the annual North American Butterfly Association’s July 4th Butterfly Surveys going back to 1975. If you’d like to take part in a count, click here for details: https://www.naba.org/butter_counts.html

The really challenging question is how lepidopterists decide whether these two wildly different wing patterns should now count as two species. Although they look entirely different, they can nonetheless interbreed and produce fertile offspring (hence the hybrids), so they are not genetically isolated from each other. *

The controversy continues.

*It reminds me somewhat of dogs. These two animals, a mastiff and a Yorkie, are both of the species Canis familiaris. Cross-breeding to produce a hybrid might be mechanically challenging, but the resulting Yorktiffs would no doubt become the height of fashion.