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What possible evolutionary reasons would there be for insects to increase their physical size over a long period? What benefits would be gained by an insect being larger? For example, say ants increases in size from today's size to the size of a football.

There is another question that asks can insects evolve to increase in size. I am asking for reasons why they would.

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Before asking why insects would grow bigger, a better question to ask would be why would anything grow bigger or smaller. A great answer for this can be found here.

Now from that link one will find that insects benefit from being small because it's just so beneficial for them - literally every answer on that list applies (except for hibernation and athleticism, I guess).

So then we can look at the food-web and ask ourselves what the role of insects are. So, insects like ants and such eat other insects, plants, fight other ants, and eat dead things (so they're scavengers and part of the decomposition process). If we zoom in on ants alone, ants are in many ways the #1 predators on the planet. They're not hunting down elephants, but as a species (or group of species) they kill more organisms than almost any other species in terms of variety and raw number (except maybe humans). As prey, insects are eaten by creatures larger than themselves, like birds, lizards, snakes, anteaters (who only seem to be interested in ants), and others.

So why would ants or any insects grow bigger? This could be due to a combination of a few of different reasons:

  1. Lack/Removal of predators: One reason why insects could grow bigger is because nothing wants to kill them. Now, increasing in size is typically considered a good thing because you can ward of bigger prey, but being too small to see/aim at is also pretty beneficial. That's what most insects are now, but if nothing was eating insects, it is plausible that over millions of years insects would grow pretty big (check out ancient dragonflies). That isn't to say ALL insects would - most likely there would be a number of smaller insects that would be preyed upon by bigger insects. This already happens now, but now in your world the biggest insects are bigger than ever.

  2. Reduced reproductive rates: I don't know what could cause this, but it's easier to have a million babies when all your babies are small. Insects typically have many eggs, and although not all eggs survive and hatch, enough do. If this reproductive rate of some insect species decreased suddenly and significantly enough, this could quickly make this species extinct as they'd be 'outbirthed' by rival species. Assuming that this decrease in births takes place gradually, a good way to get around the problems that come with this would be for said species to get bigger. This would hopefully also increase the successful birth of a given baby insect. If you have a million babies and only 10% survive, that's still 100k babies, so your species will probably be ok. If you have only 5 babies 10% survive, that's a concern. What could cause fewer births? If some insect species was the Bruce Lee of insect species and had a formidably high kill-rate per individual, there would probably be no reason for that species to outbirth other species. This could be a reason for why insects get bigger as well.

  3. Abundance of food: Average human sizes in every country has gone up as food became more accessible. How does this relate to insects? Many insects are herbivorous. If for some reason we had a sudden increase in vegetation, allowing there to be more than enough food for insects, it is plausible that some species would grow in size. This could also mean that the predators of these insects could also grow bigger to effectively take on these large, ripped, vegetarians, or grow smaller to be able to effectively swarm larger prey.

I think that you'll have to combine multiple reasons to cause long-term size growth evolutionarily speaking. You can throw in a catastrophe here and there too. For research purposes, look up the Carboniferous period (swampy part of our history - all our coal comes from here) and the early Devonian period (this is when insects first came about). Let's not forget that both these periods had very different atmospheres - only 21% of our air is oxygen, as opposed to 30-35% back then. Also, depending on how much you hate them, consider eliminating birds. They were also a big reason why insects got smaller.

I hope this helps!

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    $\begingroup$ My understanding is that insects became super large back in the dayfor 2 reasons. 1) oxygen toxicity, small larvae have difficulty regulating the amount of oxygen in their body. As size increases surface area to mass decreases. Making larger larvae have a much higher survival chance. 2) insects are not very good at breathing and rely on somewhat passive methods to absorb oxygen, this limits their size in oxygen poor environments. $\endgroup$ – Garret Gang Dec 4 '19 at 23:19
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    $\begingroup$ The reason I didn't mention oxygen level increase because I was assuming that the question-asker would want this to take place gradually but not too gradually. Increasing oxygen levels instantly (like if we increased the % of O2 in our atmosphere by 10% TOMORROW) seems unrealistic. The answer I gave is plausible even without oxygen-level increase. But you do have an interesting point in that interestingly, we could end up having more oxygen in our atmosphere if the conditions I listed happen over time, and the increased oxygen-levels would contribute to insect growth the way you describe. $\endgroup$ – cyber101 Dec 6 '19 at 2:40
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    $\begingroup$ I think it would be a concern if only half a baby survived $\endgroup$ – Greenie E. - Reinstate Monica Dec 10 '19 at 8:18
  • $\begingroup$ For a 1000 parent insects with 5 kids each, a 10% survival rate comes out to be 500 babies. Some of these parents may die childless, others with a full success rate, but there is no way to be sure - this makes it more of a statistics question than an evolution question. $\endgroup$ – cyber101 Dec 11 '19 at 4:01
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First of all, keep in mind that without drastic changes in the atmospheric composition, insects won't grow much bigger than they are today, because oxygen availability practically limits their size.

That apart, an obvious reason for an insect to grow bigger is because it has to face bigger enemies. Say a wasp preys on flies. If some flies randomly happen to be bigger, they will be more difficult to be captured from the wasp. Therefore their genes would be more easily passed upon the next generation, making it bigger.

This would likely start an arm race, in which both sides try to outcompete the other.

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If being larger afforded an insect greater availability of food it would create an evolutionary pressure that favors larger insects. This pressure would be most present in times of food shortage, perhaps droughts or flooding.

Being larger could also make it favorable in terms of the insect being able to defend itself or its territory.

Evolution is fairly simply, if there is something that increases the odds of a specimen with that trait surviving and passing on its genetics it creates an evolutionary pressure that favors that trait. Heck, their size increase could be simply because the insect is highly selective during mating and prefers larger mates. We see this with peacocks, there is no advantage to having a large colorful tail aside from it being a preferred trait for mating.

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There are a lot of good details in other answers already, but I feel like no one has actually answered the real question here: For what reason would insects grow in size over a long evolutionary period?

As others have pointed out, if the atmosphere becomes more oxygenated, they could become football sized. The thing about insects is that they have passive respiratory systems. Rather than a heart and lungs, they absorb O2 through their exoskeletons and it diffuses to the rest of their body. The bigger the bug, the harder it is to get oxygen where they need it. This is the main reason why they cannot compete with other animals in terms of size and why they have evolved to such extremes in terms of reproduction and rapid life cycles so that their specieses can survive despite high attrition to predation by larger animals.

To increase the O2 saturation in the atmosphere you need a mechanism for sequestering low oxygen carbon compounds. In our own world's history, this happened in the Carboniferous period with the evolution of cellulose. With a chemical formula of (C6H10O5)n, cellulose has a lower oxygen to carbon/hydrogen ratio than the CO2 and H2O that plants consumed to make it. When plants first evolved cellulose, it was a great evolutionary advantage, but there were not decomposers yet that were able to metabolize it turning it back into CO2 and H2O so all the cellulose that plants made just got buried depriving the ecosystem of these molecules and dumping tons of unremovable waste O2 into the atmosphere over the course of millions of years until other organisms finally figured out how to digest the stuff.

Once oxygen is more prevalent, all the other evolutionary pressures already exist for insects to do better at larger sizes where they could better compete with avians, reptiles, and mammals.

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Being small offers many advantages to insects compared to bigger creatures like:

1) Less energy expenditure and more strength/speed: Insects can handle weight many times more than theirs, because they are small. If you just scale them up, that advantage is easily lost. Same goes for flying insects.

2) less food requirement: They expend less energy since they are so small and hence require less food

3) Immunity against external factors: If a building collapsed on you, or you encounter a rock-slide, an ant your size will most probably die, normal tiny ants wont since they would fit in the gaps. Remember smaller things can enter large places, not the other way around.

4) The forces of nature: At the size of insects, forces of nature behave very differently. Surface tension triumphs gravity, air resistence as well. Try throwing an elephant from burj khalifa and an ant, and you will know. Same goes for the lack of momentum, which makes conservation of momentum lot less threatening. If you were to undergo the same kind of aerial maneuvers as a fly (scaled accordingly), th G's will kill you.

So for evolution to make the bugs grow big, you need an environment that nullifies all these advantages of being small. Plenty of food and no predators are obvious answers. Low gravity and higher density fluid environment can also accomplish the same.

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There are many benefits to being large*, that is why large size has evolved so many times. The real reason insects today are not large is because they can't get that big without drastically changing their anatomy, in particular their breathing mechanism which is very size limited. At larger sizes spiracle/tracheal breathing just can't exchange much oxygen because is is a largely passive system relying on diffusion. So insects of the size you want can't occur unless the oxygen content of the air is much higher. In the past when it was higher you got some huge insects.

The other thing you can do is has a them evolve some form a lung, like the branchiostegal lung of coconut crabs (which evolved from gills), it will not be that believable becasue it is a big change but it will at least make them possible.

*including, better defence, better offense, better metabolic efficiency, better thermal efficiency, better ability to store resources, ect.

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As L. Dutch answered, the critical componant is more atmospheric oxygen. Insect's breath through a series of holes in their exoskeletons called "spiracles". Because of the square cubed law, "spiracles" cannot gather enough oxygen for the insect the size of a football to survive. It would get oxygen for sure, but it would not be efficient for biological functions to continue... and CO2 couldn't be vacated efficiently either... and anything that is expelled from a body usually causes health issues if it is allowed to remain.

That said, there was a point where insects the size of footballs not only existed, but were considered puny to average insect sizes of the time. The largest known insect ever was the Meganeuropsis permiana which lived 280-290 million years ago and was 28 inches long and 17 inches wide. This is also a period of time where the earth's atmosphere was at "peak oxygen" with the air containing 30% oxygen concentration (Today, Oxygen in Atmosphere is 20%).

This isn't just the reason for large bugs, but also why other animals such as dinosaurs were larger then animals today. In theory, any creature evolving in these conditions will also get significantly larger over time. That said, it will make for some trouble for humans specifically as we're reliant on fire for so many of our tools and cooking... the eras of peak oxygen were plagued by frequent and devastating wild fires that would make the ones we have today look tame by comparisons.

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