Insects are not the most numerous animals on Earth, despite being the most diverse. Mites are more numerous, with an estimated 10:1 ratio to insects. However, nematodes, or roundworms, are the most ubiquitous and numerous animal, with an estimated 100:1 ratio to mites and up to 1022 on Earth.
When asked which is the largest animal in the world, most people assume insects. The Entomological Society of America states that insects are 1.6 billion times more numerous than humans, in which case there would be about 1019 insects. However, insects, although they are the most diverse animals (with over a million species described by science), are not the most numerous animals: they are too large, usually measuring in millimeters or centimeters. To find the largest animal, we need to zoom in on a smaller scale.
Mites, a form of arachnid, not an insect, are ubiquitous in virtually every environment and are often considered one of the most numerous animals on the planet. Like insects, mites are thought to contain over a million different species, but this is even more impressive than in the case of insects. This is because mites (Acarina) are only a subclass of the organism, while insects (Insecta) are a Class, a higher taxonomic division. The mites have a smaller average size, about one millimeter. These small sizes, coupled with their rapid reproduction rates and ubiquitous range, make it reasonable to estimate that mites outnumber insects by at least 10:1. If so, there are more than 1020 mites on Earth.
There are even more numerous organisms than both mites and insects. These include various species of microfauna, especially roundworms, also known as roundworms. Nematodes are very small (many have around 1000 cells and measure 1-2mm in length) and very numerous. They cover virtually every square inch of land and sea. Nematodes account for 90% of all life on the ocean floor. In some fertile areas of topsoil, there are over a trillion nematodes per square mile. Most biologists agree that nematodes are both the most ubiquitous and the most numerous animal on Earth. It is reasonable to estimate that nematodes outnumber mites 10:1 and probably more than 100:1. If so, then there are 1021-1022 roundworms on Earth, making them the most numerous animal.
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