Insects.

The history of insects dates to the Devonian period, about 400 million years ago. Numbering more than one million species in the twenty-first century, insects are the most diverse class of animals on Earth. Insects populate almost every habitat except the deep oceans and permanently frozen land masses. Insects are specialized to live underground, in live or rotting trees, in fast-flowing rivers, or in stagnant puddles. Parasitic insects live attached to the outside (hair, skin) or inside (stomach lining, respiratory tract) of other animals. Crafty insects modify their habitat by building their own houses. Spittlebugs mix air into slimy anal secretions to form shelters. Bagworms spin silk to hold leaves together around them. Some African termites glue soil particles together with saliva into colossal nests measuring six meters high and nearly four meters across.