Some Experiments On the Camouflage of Stick Caterpillars1)

Experiments were made to find out whether caterpillars of Ennomos alniaria, Biston strataria and B. hirtaria are protected from Jays and Chaffinches by their resemblance to sticks. Being used to the inedibility of sticks, the birds do not peck at the larvae. However, once it has found one by accident, a bird will usually peck at all similar objects until it is discouraged by finding only sticks. The original situation is then restored. The caterpillars are confused only with sticks of their own food plants, and some birds are able to distinguish them even from these. Even in its present highly refined form, therefore, the adaptation does not yet give the maximum possible protection, and all steps in its evolution will have had a considerable selective advantage.