Giant Insects Aliens

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jewish_scientist
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Giant Insects Aliens

Postby jewish_scientist » Tue Jun 04, 2019 10:11 pm UTC

I was thinking about what insect aliens that could survive on Earth would be like and came across a 3 problems, 2 of which I though of solutions for.

1) Square-Cube Law: Whenever you try to scale something up, the first objection people raise is the square-cube law. I never really saw this as such a problem since you can solve it by scaling load-bearing structures (e.g. exoskeleton) faster than everything else.

2) Breathing: One of the common reasons given for why there are no giant insects is that they have an very inefficient cardio-respiratory system, meaning that they need high O2 concentrations to survive. This problem can be solved by just giving the aliens a mammalian cardio-respiratory. An insect with lungs and veins is still an insect after all.

3) Molting: This is the problem I could not solve. The larger an insect is, the more dramatic the molting process becomes. Given how dense an insect's exoskeleton is, aliens that are even close to human size would have to frequently throw away almost 1kg of body-mass! The only way I could think of to get around this was to have each limb molt independent of one another, but I do not think that would work given how exoskeletons are fundamentally a single, continuous piece.
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Eebster the Great
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Re: Giant Insects Aliens

Postby Eebster the Great » Tue Jun 04, 2019 10:51 pm UTC

It's actually more than 1 kg, per 1). It's probably the biggest reason the arthropod bauplan doesn't scale up. Lobsters that get too large actually stop molting and eventually die as a result. It's kind of amazing that the giant spider crabs can manage it.

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Re: Giant Insects Aliens

Postby gmalivuk » Wed Jun 05, 2019 2:39 am UTC

Molting and joints would both be problems if you wanted to stick with exoskeletons, according to this discussion.
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Eebster the Great
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Re: Giant Insects Aliens

Postby Eebster the Great » Wed Jun 05, 2019 3:32 am UTC

Some insects do actively respire, contrary to the implication in that page. But I don't know if it would be good enough for a human-sized air-breathing insect. The lack of a true circulatory system is a real bummer.

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Heimhenge
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Re: Giant Insects Aliens

Postby Heimhenge » Tue Jun 18, 2019 10:14 pm UTC

First, let me say I've been on the forum for 5 years and this is the first time I've seen this subsection ... hidden away at the top of the Science section menu I just never glanced in that direction. Or did this subsection just get moved from somewhere else?

Anyway, wanted to chime in on the insect respiration thing. Once, on a dare, I put a live cockroach into a bell jar and pumped it down to a pressure of around 500 mb. This was a high school lab vacuum pump and that was the best it could do. Puffed up marshmallows really nice. But the cockroach didn't seem fazed by it. Or puffed up. Just kept walking around trying to get out. Blew our minds. We got tired watching after 2-3 minutes and shut off the pump and let the air back in. Wondering if it would have eventually succumbed had we waited longer.

EDIT: To clarify (before someone responds with "Yeah, it would've succumbed in a few days from lack of food/water."), what I'm wondering is if 500 mb is enough for a cockroach to respire? I have very little knowledge of insect biology.

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Re: Giant Insects Aliens

Postby commodorejohn » Wed Jun 19, 2019 3:16 am UTC

Since we've already given a nod to square-cube considerations, we definitely ought to link to the definitive scholarly work on the subject.
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Eebster the Great
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Re: Giant Insects Aliens

Postby Eebster the Great » Wed Jun 19, 2019 4:33 am UTC

The subforum has been here for quite a while, but it's easy to overlook.

I have no idea what air pressure is required for cockroach survival. I do know that flies look funny as air is pumped out of their enclosure.

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Heimhenge
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Re: Giant Insects Aliens

Postby Heimhenge » Thu Jun 20, 2019 6:47 am UTC

Interesting. Flies puff up but roaches don't. Probably an exoskeleton thing. Abdomens inflate easily compared to shells. Didn't really see any size change in the roach though.

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Eebster the Great
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Re: Giant Insects Aliens

Postby Eebster the Great » Fri Jun 21, 2019 4:54 am UTC

Yeah their body plans are pretty different. But also, flies gradually lose the ability to fly as the air gets thinner, which is the reason I usually see it demonstrated.

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Heimhenge
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Re: Giant Insects Aliens

Postby Heimhenge » Fri Jun 21, 2019 7:21 pm UTC

Eebster the Great wrote:Yeah their body plans are pretty different. But also, flies gradually lose the ability to fly as the air gets thinner, which is the reason I usually see it demonstrated.


I would not have expected that, but yeah, with a good enough vacuum they'd obviously lose flight. Never tried it on flies with that HS vacuum pump. Since lift is proportional to fluid density a fly would only get around 1/20 the lift at 500 mb, but not being an insect person I wonder how much "excess lift" a fly has to start with. Would be an interesting experiment. Do you recall at about what level of pressure they stopped flying?

I ask because I'm writing an article about that drone NASA wants to put on Mars, and the way they solved the lift problem was pretty obvious ... two counter-rotating 1.2 m props, which they say improves the airflow to the bottom prop by 10%, as well as negating gyro effects. The props spin at 2800 rpm. That's slower than commercial drones, but like 10x faster than a helicopter. It's already been test flown in a low pressure chamber. Would be cool to weave in a story about flies under a bell jar. :)

Then again, at the dimensions of a fly the Reynolds number must be pretty low so I'm not sure usual aerodynamic rules apply.

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Eebster the Great
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Re: Giant Insects Aliens

Postby Eebster the Great » Fri Jun 21, 2019 7:40 pm UTC

In this video, flies seem able to remain in the air with difficulty down to around 0.6 bar, but not much lower. Bees have apparently been found at somewhat lower pressures at around 5,600 m on Mt. Everest.

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Heimhenge
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Re: Giant Insects Aliens

Postby Heimhenge » Fri Jun 21, 2019 8:57 pm UTC

Awesome video, thanks for the link. Not surprised someone actually tried this with all the amateur scientists out there trying to be myth busters. I'll include that link in my drone article. I'm not sure the average person realizes how pressure affects aeronautics, and this is a great demo!


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