Why is holometabolism so successful?

Here is a question about nature, and specifically about insects, that I didn’t think to ask until I came on inaturalist and started posting so many pictures of insects!

There are two types of insects. Hemimetabolist, and holometabolist. The hemimetabolist insects hatch (or sometimes are born alive), in a form that pretty much resembles the adult, but smaller, and often without functional wings. A holometabolist animal is born as a larva, usually a worm-shaped one, and lives for a long time as a larva before they pupate and become an adult.
Common hemimetabolist insects are dragonflies, grasshoppers, bugs and aphids, cockroaches and preying mantises.
Common holometabolist animals include four big families: bees and ants, moths and butterflies, beetles and flies.

Hemimetabolist insects are certainly successful, I can walk through a field and see dozens of grasshoppers flying around. But in terms of biomass, species diversity, and shaping the ecosystem, the holometabolist insects are much more successful. (Although maybe that is a matter of judgement…)

So the thing is, holometabolism seems like a much more risky business. Holometabolist insects hatch in one form, live in that form, and then form a vulnerable pupa before becoming an adult. A hemimetabolist insect, on the other hand, hatches and is ready to go, with no intermediate steps. In the case of some aphids, they are born alive and ready to eat. It seems that from an evolutionary standpoint, holometabolism is more risky—there are so many more steps to take. And yet, holometabolist insects have outcompleted hemimetabolist ones. Is there a reason for that?

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Oh man, this could turn into a really informative, awesome discussion. Great questions! Hopefully some folks who are a bit more qualified to speak on this than me will weigh in but the first thing that occurs to me is that (at least anecdotally) this can serve as a way to decrease intraspecific competition.

Most hemimetabolous organisms utilize the same food source across different stages, but when holometabolous ones go through these drastic changes, it seems that a large number of them utilize different resources in the immature stages than the adult ones.

Interested to see what others say!

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Not necessarily an expert myself, as I’m just starting my journey into entomology. I’ve always been told that one of the major factors for the success of the holometabolous insects was due to fact that the adult stages and larval stages often utilize different niches and foods. This means the adults and larvae aren’t competing for the same resources. Some adults don’t even eat and focus only on reproduction, reducing intraspecies competition even further. Hemimetabolous insects would be competing with their nymphs as they use the same resources and niche.

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Welcome to the Forum, and thanks for your response :)

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It’s definitely less risky than if both larvae and aduts need the same resources and can all die out, it’s explained right like why aphids have males in autumn, because you need diversity if situation changes, no matter if it’s genetic or ecological, aphids live in stability on one plant for season generation, there’s no need for them to have different larvae as they can feed on the same plant in great numbers as they actually do.
Also all insects need to moult and you can find many soft grasshoppers and catch them and eat if you were a predator.
In fact this question has many online sources as it’s something you could be aske about on an exam, but the #1 answer is in 1st comment. But also you should really think out of insects, there’re different animals undergoing on or another type of process and you can see why they succeed because of that.

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Like the amazing transformation from algae-grazing tadpole to insect-hunting frog. And what makes it even more amazing is that there is no pupal stage marking a sharp break – in some ways they seem “hemimetabolous” in that they transform gradually, yet the lifestyle differences between larva and adult could scarcely be wider.

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Coming from an ecology standpoint my take on it is that holometabolism allows the organism to take advantage of different food sources at different life stages as well as providing a mechanism for delaying the “growing up” process if conditions aren’t right.

It also means that the adults and the juveniles are occupying different niches and not competing with each other.

A quick search seems to confirm this, with emphasis being placed on the removal of competition between adults and juveniles:

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In addition to utilizing different resources, as others have said, larvae and adult insects can be anatomically specialized for their “role” in the life cycle. So, a caterpillar or a maggot is a no-frills eating machine, a worm with a mouth and a digestive tract. They can spend their time on a single plant, just gorging themselves, and they are just perfect for that! They are not very mobile, though, which would make it tough to find mates and new habitats for their offspring. But, thankfully, after metamorphosis a butterfly or fly is a strong flyer designed to mate and disperse. In some species, such as saturniid moths and bot flies, they don’t even have a functional digestive system as adults, and rely on energy reserves collected as a larva!

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A different perspective - I get kind of annoyed by these types of questions/debates. A trait persists because it is no disadvantage, not necessarily because it is any advantage. While there may have been some ‘advantages’ (different food sources etc.) I tend towards a more random approach. Somehow a mutation - which is actually part of insect makeup (picture the change from a dragonfly larva to an adult) - demonstrated itself. It was no disadvantage to the new line, so it proliferated. The new organisms exploited the environment in ways that the hemimetabolous insects did, so they proliferated. Hemimetabolous insects did not vanish. Holometabolous insects did not replace them, so one life cycle is not ‘better’ than the other, just different.
This is my perspective on evolutionary matters - as long as a mutation/trait is not a disadvantage, it may persist, given that all other environmental changes can be dealt with.

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This is true! But I think it is striking that four of our five most diverse insect orders (flies, beetles, moths/butterflies, and wasp/bees/ants) are holometabolous (the fifth is true bugs, which are hemimetabolous). The innovation of complete metamorphosis was followed by an explosion of diversity. Seems to me that they’re doing something right!

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It could also be the case that whatever evolutionary pressures, or circumstances produced the first holometabolous insects is not what led them to later radiate.
It is also hard to know because the early history of insects, which started 400 million years ago, is not very well represented in the fossil record. Insects already existed, and were successful, for maybe 100 million years before the first holometabolous insects (that we know about, at least). Holometabolism might have evolved to deal with a specific extreme condition, such as the conditions at the Permian mass extinction event.

But it also seems like it took a while for it to catch on: most of the holometabolous insects had to wait another 200 million years, until the Cretaceous to become successful, and a lot of that seems to do with the success of flowering plants.

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Yes, I was thinking that as I wrote. I blame it on God, who seemed to be inordinately fond of beetles! But @mnharris makes a very good point. Insects have been around for a very long time, and for most of that time (as far as we know), they were hemimetabulous. The development of flowering plants seemed to at least promote the radiation of holometabolous insects (which came first, the flower or the fly?). We really don’t know how long they had been around before that, sort of like the way mammalian evolution remained subdued until the dinosaurs declined.
I think what bothers me most about these kinds of conversations is that we (humans) conflate current ‘success’ with ‘better’.

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Diptera, or true flies, represent a huge fraction of the insects. This paper illustrates how the gall midges, or Cecidomyiidae, perhaps outnumber the entire Coleoptera (beetles), and this family fits squarely within the paradigm I describe below. https://royalsocietypublishing.org/doi/full/10.1098/rstb.2015.0333

The archetypal Diptera life history is 1) larva and pupa entirely submerged in wet, mushy, or underground substrate 2) adult winged, disperses widely, often a flower visitor. There are very few exceptions. Having wings and being good at dispersal makes it harder to take advantage of these other types of habitat. There are a huge number of ecological niches that are thus only available to holometabolous insects, and more specifically, Diptera :)

To summarize, holometabolous development enables larvae to be more extensively specialized for feeding and growing, and adults to be more efficient at dispersal and mating.

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Well, that is an explanation I have seen for the subimago of mayflies: that in most insects, it was advantageous to synchronize the molt into sexual maturity with the molt into the winged stage, but mayflies are so short-lived, there isn’t a strong enough selection on them to bring them into that pattern.

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To summarize, holometabolous development enables larvae to be more extensively specialized for feeding and growing, and adults to be more efficient at dispersal and mating.

The same statement could be applied to Odonata.

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Can’t speak for insects, but in amphibians most frogs (and toads) have a free-swimming tadpole stage which is analogous to insect larvae. However, where free-standing water might not be available for reproduction, the tadpole stage is kept within the egg and the young are “born” as fully formed terrestrial froglets. The strategy across anuran species is essentially: use the free-swimming larval stage when conditions are good for that, but it’s not necessary, and evolutionarily it seems to be a flexible method of reproduction. (The tadpole stage can last for a couple weeks or years, depending on species.)

The different strategies would seem to boil down to: putting your resources into lots of cheap, rather uncomplicated young which can forage on their own, and a minority will make it to the adult stage. Or produce fewer, more expensive young that are essentially smaller versions of the adult but which will likely have a higher survival rate.

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Yup, I was thinking of Odonata and Ephemeroptera while saying that! Odonate larvae still have legs and the same general body structure as adults. The total reduction of appendages and body structures in holometabolous insects goes much further, as we find taken to an extreme in the larvae of parasitoid flies and wasps. These groups all feed within the host tissue, which perhaps wouldn’t be possible for larvae of hemimetabolous insects, and these lineages are correspondingly the most diverse among insects.

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Totally! The strategy of resource/anatomical specialization between life stages seems to be a popular one in insects, as dragonflies, mayflies, stoneflies, and others have adopted it separately from the Holometabola. For this reason, people occasionally distinguish “paurometabolism,” gradual metamorphosis like in grasshoppers, cockroaches, and true bugs, and “hemimetabolism,” which is a more dramatic metamorphosis without a pupal stage like in dragonflies, stoneflies, and cicadas.

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I’m not sure I buy the argument that it’s obviously “less risky” for the larvae and adults to occupy separate niches. For the species to survive, both of those niches need to remain available. One could argue that this should make it harder for holometabolic organisms to evolve in the face of a changing environment.

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I was thinking something similar. The argument that one of the advantages of holometabolism is that the adults and juveniles are not competing against each other for the same food source makes sense…but it doesn’t account for the fact that they have to compete against other species. Aphids might have to compete against their own young for food, but Beetles of Species A have to compete against other species as larva, and then compete against a whole other set of species as adults, with the added burden that they have to store up energy to make the change.

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