Many caterpillars are cryptically colored and resemble the plants on which they feed. They may even have parts that mimic plant parts such as thorns. Their size varies from as little as 1 mm to about 75 millimetres (3.0 in). Some look like objects in the environment such as bird droppings. Many feed enclosed inside silk galleries, rolled leaves or by mining between the leaf surfaces. Caterpillars of Nemoria arizonaria that grow in spring feed on oak catkins and appear green. The summer brood appear like oak twigs. The differential development is linked to the tannin content in the diet.