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.
More aggressive self-defense measures are taken by some caterpillars. These measures include having spiny bristles or long fine hair-like setae with detachable tips that will irritate by lodging in the skin or mucous membranes. However some birds (such as cuckoos) will swallow even the hairiest of caterpillars. The most aggressive caterpillar defenses are bristles associated with venom glands. These bristles are called urticating hairs. Most urticating hairs range in effect from mild irritation to dermatitis.