Larvae of several swallowtail caterpillars (e.g., Papilio glaucus and P. troilus) have morphological features that are similar in appearance to the heads of snakes. Although several authors have speculated that these morphological patterns increase their survival by reducing predation by birds, to my knowledge, this assumption has never been tested. To test this assumption, we built caterpillars out of modeling clay, which are often used to assess relative rates of bird predation. Specifically, we designed 6 caterpillar models, with or without the following three features: swollen thoracic region, yellow spots on the dorsal side (“eyespots”), and smaller black spots inside the yellow spots (“concentric spots”). We affixed all six models to branches in each of 57 black cherry (Prunus serotina) trees, which are one of the major hosts of P. glaucus larvae. After four days of natural exposure to predators, we measured the height of each model in the trees, and the impressions of bird beaks left in the models. 

Results/Conclusions

Results indicate that although birds were less likely to attack models with swollen front regions or eyespots, these effects were not significant. However, birds attacked models with concentric spots significantly less than other models. The effect of height in the tree was not significant, but the effect of tree was significant, perhaps indicating that if birds encountered one larva on a tree, they were likely to find others on the same tree. Forty-nine models with eyespots (or concentric spots) were attacked, ten of which were attacked on the spots themselves. Given the fractional surface area of the spots relative to the portion of the caterpillar models exposed to predators (the upper sides), the number of caterpillar models with eyespots that were attacked in the eyespots themselves was greater than expected by chance. This indicates that although many avian predators may avoid attacking larvae with concentric spots, the birds that do attack are attracted by the spots. Two alternative hypotheses remain to be tested: 1) birds that attack the spots are curious about the spots because they are novel or unexpected features, or 2) birds are intending to disable potential predators or prey items by attacking their eyes.