Thursday, December 17, 2009

Darwin Contemplates the Caterpillar - Arsenura Armida


The photo above is from a family vacation to Mexico. It has taken me four years, but I have finally identified the creatures on the tree. The tour guide just called them "giant caterpillars", but they were much bigger than any caterpillar I had ever seen.

Thanks to the wonderful website WhatsThatBug.com, I found out that four other people had seen the same caterpillars, one in Honduras and three in the same area we visited near Cancun (the tour guides must bring everyone past the same tree!). They are Arsenura Armida, a type of silk moth from the family Saturniidae. Other than being unusually large and producing low quality wild silk, these caterpillars stand out due to their bright coloring.

My photo shows these caterpillars in their final "instar", or final molting phase, before they transform into pupae. Here is a website that shows some of the other instars, along with a lot of other technical details. You can see that the caterpillars are even more brightly colored in their earlier instars. If you compare the photos from the first instar to the last you would hardly say they were the same species. (You can tell my photo is the final instar because their "horns" have fallen off.)

An interesting historical note is that the great Charles Darwin was stumped by the colorful displays of caterpillars like these. Darwin's explanation for bright colors was that they aided in sexual selection (think of bright red male cardinals competing for the attention of brown females). But brightly colored caterpillars did not fit Darwin's theory, since caterpillars do not mate.

Darwin turned to his colleague Alfred Russel Wallace for help in answering this question. Wallace had published a paper on natural selection before Darwin did. Wallace made many of the same observations on his visits to the Malay Archipelago that Darwin made in the Galapagos Islands. Some science historians have suggested that Darwin "borrowed" ideas from Wallace for his On the Origin of Species, and that Wallace should have received some of the fame that went to Darwin.

It was Wallace, with the help of experiments completed by John Jenner Weir, who proposed that the caterpillars that had evolved bright colors were the same ones that had evolved a bad taste to birds. The bright color was a warning to their predators, helping them remember which caterpillars tasted bad. Fitting in perfectly with this theory was the fact that caterpillar species that evolved camouflage coloring were the ones that tasted good to the birds (when they could find them!).

This concept of warning coloration developed by Darwin's associates is known as aposematism.

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