“AN INORDINATE fondness for beetles.” That was the reply of J.B.S. Haldane, a British scientific polymath of the early 20th century, when he was asked if there were anything that could be concluded about God from the study of natural history. There are 380,000 catalogued species of beetle, making them the most species-rich group of insects—and insects are the most species-rich group of animals. But why there are so many has been a mystery.

As they report in the Proceedings of the Royal Society, Dena Smith of the University of Colorado and Jonathan Marcot of the University of Illinois think biologists have been barking up the wrong tree on the matter of beetle diversity. Previous attempts to explain it considered reasons why new beetle species evolve: their catholic tastes in food, for example, would open lots of ecological niches. Dr Smith and Dr Marcot have looked from the other end of the microscope and asked if the explanation might rather be that, once a beetle species has appeared, it is less likely to become extinct than other animal species are.

To assess this idea they examined beetles’ fossil record. In general, insects are not well preserved down the ages, so their fossil record is patchy. But beetles, which have strong exoskeletons, preserve better than most, and the two researchers were able, by searching the world’s palaeontological archives, to assemble a list of 5,503 fossil species collected from 221 sites.

They divided the past 300m years, the period during which beetles have existed, into a dozen 25m-year blocks and looked at the number of consecutive blocks in which each known beetle family was represented. (A family is the taxonomic classification level above a genus, and it is used by palaeontologists interested in extinction rates because the randomness of preservation makes it hard to know when, exactly, a species or a genus really has vanished from the face of the Earth.) About 90% of modern beetle species belong to a group (technically, a suborder, which is one level up the classification ladder from a family) called the Polyphaga. And that dominance, and thus, in essence, the dominance of the beetles, Dr Smith and Dr Marcot found, is indeed because polyphagans seem hard to exterminate.

Since the Polyphaga appeared about 220m years ago, no family belonging to the suborder has become extinct—even at the end of the Cretaceous, 66m years ago, when an asteroid strike did for the dinosaurs and many other types of animal. As new families have appeared, therefore, the diversity of the Polyphaga has inevitably increased. Indeed, as they have prospered, other groups of beetles have withered. When they first appeared, they were one of ten coleopteran suborders. Now, only three other suborders remain.

Why this group of beetles has such resilience to extinction remains unknown. But at least biologists musing on why there are so many beetles around can now look in the right direction for the answer. Were Haldane alive today, he might want to refine his answer to “an inordinate fondness for Polyphaga”.