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We were probably all told to quit picking our noses at one point, and by most standards, this is good advice. But if you were a toad, it might come in handy to scratch away blowfly eggs.

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If you think what’s in your nose is gross, just be glad that there isn’t a human equivalent of the toadfly. This species of blowfly lays its eggs exclusively in the nostrils of live toads, and even by blowfly standards, it’s… kind of horrifying.

Blowflies in general have some pretty macabre reproduction habits. Most species lay their eggs either on the carcasses of dead animals or on poop.  Their larvae, or maggots, then consume whichever delightful food source they find themselves on until they’re big enough to pupate and emerge as adults. Some blowflies lay eggs in the open wounds of live animals, including - I’m sorry - humans.

If you’ve heard of screwworms, for instance, those are blowfly maggots.  But the toadfly takes this idea to a whole new level of awful. Found in Asia, Europe, and North America, this insect lays its eggs at the entrance of the nostrils of a live toad. When the larvae hatch, they crawl into the toad’s nasal cavity, which is where they hang out while they grow and develop.

The baby toadflies actually munch on the inside of the toad’s head, destroying its airways and eventually killing it. What makes this even wilder is that toadflies can do this even though the toads they attack secrete toxins from their skin. Scientists aren’t really sure how!

What they do have a good sense of is how and why this weird lifestyle evolved.  You see, based on studies of the blowfly family tree, it seems like this behavior of attacking live frogs probably evolved about 5 million years ago. The toadfly has one sister species that also probably infests live frogs, but most of its other closest relatives go after dead stuff. So the toadfly’s ancestors were likely carrion-eaters.  And dead things are, by nature, an ephemeral resource: they go through boom and bust cycles.  That leads to lots of competition among the animals that depend on them.

So, to beat out other carrion feeders, scientists think proto-toadflies may have evolved to arrive at frog carcasses earlier and earlier, until they ended up going after frogs that weren’t even dead yet! Now, obviously a toadfly infestation is terrible news for an individual toad, but scientists are still figuring out what parasites like toadflies might mean for amphibian conservation. We don’t yet know whether they can affect their hosts at the population level, like causing their overall numbers to decline.

And we don’t know if or how many effects could be complicated by human-caused problems like pollution, habitat loss, and climate change. But intriguingly, some scientists think that with more research, species like toadflies could be used as bioindicators: species that can help us gauge how healthy the environment is and how it’s changing over time. And if so… we’d owe the toadflies some thanks, I guess?

But they still make me glad I’m not a toad. Thanks for watching this episode of SciShow! If you want to learn more about weird parasites, I recommend our episode about six that live inside cells.

I also want to give a quick shout out to all our awesome patrons! If it weren’t for you, we wouldn’t be able to talk about wonderfully weird creatures like this.  And if you’re not a patron but want to support more episodes like this: you can learn more about our patron community at  [♪ OUTRO].