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Ask any casual science enthusiast what animal group has more species than any other, and they'll probably say beetles. And there are a ton of beetle species— more than 350 thousand, in fact.

That's about 22% of all species that we've identified and named so far, including stuff like bacteria. But some scientists think beetles' time at number one should come to an end. Other animal groups, they say, may be more deserving of the title of “most species”.

Though it's tough to say which group that actually is, and it's not just about knowing for curiosity's sake. Having good estimates of how many species of beetle, plant, or bird there are is vital to tracking how those numbers are changing, and what we have to do to preserve them. It's hard to actually pin down how many species there are for a couple of different reasons.

One is that species estimates are based on the number of species we currently know about, and we're not familiar with an equal proportion of every group of animals. It depends on how interested we are in them and how easy they are to find. Beetles in particular were a popular collector's item with old-timey naturalists, who gathered them for their exciting array of colors.

And that interest meant there was ample funding and opportunity to go look for more beetles. Meanwhile, less charismatic animals, or ones that were smaller and harder to count, missed out—. Animals like Hymenoptera, the group of insects that includes sawflies, wasps, bees, and ants.

A 2018 paper published in the journal BMC Ecology argued that there are more species of hymenopterans than anything else, thanks in large part to parasitoid wasps. And if you're thinking of the yellowjackets in your garden, think again. These wasps can be barely bigger than a grain of sand and spend most of their life cycle inside other insects, making them very easy to miss.

Parasitoid wasps target pretty much all insects, including ones that live in the ocean, in the Arctic, and deep down in the soil. And scientists think that these wasps are pretty specialized, meaning each species only targets a specific type of host. So according to the 2018 study, every insect, including all those beetles, might have not just one but potentially multiple species of parasitic wasp hunting them down.

Also, insects have multiple life stages, and different wasps can target both the larval and adult forms. So two wasps for every beetle? I mean, you do the math.

That's not even counting parasitoids that target other parasitoids or ones that go for non-insect arthropods, like spiders. And it's not just one study making this claim. Studies of sand dunes in California and insects in Canada estimate that hymenopteran species outnumber beetles by about two and a half times in that particular habitat.

Now, if hymenopterans are not the most speciose animal group—that's the technical term for having a lot of species—it's probably still some sort of parasite. And that's because parasitism is really popular in the natural world, evolving more than 200 separate times in 15 different groups of animals. And it makes sense!

Hosts are both food and shelter rolled into one. That makes nematodes, or roundworms, another contender for the most species-rich group. Not all nematodes are parasites, but many are and they target not just insects, but also other animals and plants.

One 1993 study estimated there may be well over a million nematode species, though other estimates have varied wildly because they're so understudied. Another paper from 2008 suggested that there may be between 75,000 and 300,000 species of nematodes parasitizing vertebrates alone. And nematodes have been found everywhere, including sediments deep down on the ocean floor.

There's probably nematodes in places humans have never even been yet. Which is an important point: species estimates rely on what we can find, which means they rely on where humans can go. Now, you may have noticed that we're comparing nematodes, which are an entire phylum of worms, to wasps and beetles, which according to the tree of life are technically orders.

You know, kingdom, phylum, class, order. But even if you compare whole phyla, nematodes could still win out over arthropods, the phylum that contains wasps and beetles. We think there's roughly a million of each, but there's so much variation in those estimates that it's tough to call.

And we don't assign those groups—phyla, classes or orders—based on how big they are, but that's a whole other story. And then there's the question of charisma, which could explain why Diptera, the order that includes true flies, have been overlooked. They might grab your attention when they're buzzing around your living room, but there aren't many people who'd say they… like flies.

Not like people like beetles. But when scientists do get to studying them, they find a surprising number of species. Like a study from 2018 that found more than 4000 species of true flies in a four-hectare patch of Costa Rican rainforest.

Current estimates put Diptera at more than a million species, easily surpassing beetle numbers. There's that million number again … I mean, we told you it was tough to call. Flies can live almost anywhere on land and, as larvae, can feed on everything from plant nectar to vertebrate blood.

And that's important too: for a group of animals to stand a chance at being the most speciose, it has to show variety in where it lives, what it eats, and how it survives in general. And that could mean making yourself comfortable inside a host like a parasite does, or being able to find homes in some of the most remote places on Earth. But, if there's anything we've learned from trying to count all of the animals, it's that what's right in front of us isn't always a good indicator.

If we want to know what's out there, we've got to go look for it. Thank you for watching this episode of SciShow. If you want to help support the channel, you can do so over on Patreon.

We've got the best community of supporters around, and you can meet and chat with them—and some of our team—over on our patron-only Discord. If you want to get started, head over to patreon.com/scishow. [OUTRO ♪].