Arthropods are the most abundant and diverse phylum of animals. Arthropods live in all of the major habitats. Small insects and spiders have been found in the upper atmosphere and crustaceans live at the bottom of the ocean. They are predators, herbivores, and scavengers. The smallest arthropods can only be seen with an electron microscope and the largest were prehistoric dragonflies with wingspans over a foot in length.
Arthropods were likely the first animals to become terrestrial. For a long time in Earth’s history, the only organisms on land were plants, fungi, and arthropods. Because of this, there has been a lot of plant/insect coevolution. Many species of plants cannot live and reproduce without some interaction with insects. There are many species of insects whose whole lifecycle is dependent upon a single species of plant. Humans are dependent upon insects because they are the major pollinators of the plants whose fruits and grains we eat.
Arthropods were the first organisms to develop flight. Millions of years before birds and bats, insects dominated the air. Today, flight is so common in arthropods that the major classification characteristic for insects is the structure of their wings.