Each year, researchers discover dozens of animal species to add to the millions on our planet. One group, in particular, contains an exceedingly large chunk of Earth’s biodiversity. So, of all the animal groups on our planet, which has the most species?

According to estimates from Camilo Mora, an associate professor in the Department of Geography and Environment at the University of Hawaii at Mānoa, insects are the most diverse animal group. His investigations predict that there are a whopping 3 million insect species, meaning roughly half of all animal species on Earth are insects.

One type is particularly prolific. “Of the insects, the group that has the most species are beetles: there are almost 400,000 species of beetles,” said Jessica Ware, an evolutionary biologist and associate curator of invertebrate zoology at the American Museum of Natural History in New York City. In fact, that means 1 in 3 animal species is a beetle, making them the most species-rich creatures known on Earth. For comparison, whereas there are almost 3,000 snake species, “there’s twice that many of [just] ladybird beetles!” Ware told Live Science.

Why have beetles and the wider insect class become so successful? There are many possible reasons. One possibility is that insects have been around for almost 500 million years, making them one of the oldest animal groups. That’s allowed “time for speciation to occur, for species to accumulate,” Ware said. Most insects are also short-lived, which typically coincides with quicker rates of reproduction and more offspring, thus creating more opportunities for mutations that lead to new species, Mora explained.

High levels of speciation have also given insects more opportunities to adapt and thrive in thousands of ecological niches worldwide. For example, beetles’ outer wings not only enable them to fly but also serve as a protective casing that lets them burrow underground, Ware said. “The other possibility is that maybe there are other groups that are just as diverse [as insects], but we haven’t studied them,” Mora said. In fact, in a 2018 study, researchers proposed that a group of insects known as parasitoid wasps may contain even more species than beetles do, but we’ve just devoted less time to studying them.