Insects are members of the phylum Arthropoda, class Insecta, the largest and most diverse class in the animal kingdom. The number of described insect species is estimated to be 750,000 and the actual number of living species is perhaps 3 million. Even at 750,000 species, however, insects outnumber all other plant and animal groups.
The small size of insects aids in their dispersal to more favorable habitats. Thrips and aphids are carried by frontal systems for long distances. Small insects have been collected by airplane at 3,000 m (10,000 ft). Migratory butterflies and locusts may fly hundreds of kilometers in a few weeks.