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Let us imagine a human-sized mantis (1.7m tall, 70kg / 5'8", 154 lbs.), but with the forelegs modified to have a shape closer to human arms. So each arm would have two main segments, and end up in a multi-segmented, multi-fingered, grasping-capable hand.

In order to make this creature feasible without violating the square-cube law, let us imagine that it evolved a closed circulatory system and a pair of huge, highly vascularized and alveolated lungs, capable of expanding and contracting just like ours. Add a diaphragm, just because. The mantisman would have tracheas only in the abdomen, and those tracheas would lead into those lungs. Its metabolism and phisiology would be on par with ours.

The point where such a creature differs from us is the skeleton. This creature's skeleton is its external chitin layer. The muscles are inside and attach to their skeleton in a different way, compared to us.

I wonder is besides grasshoppers there are also lawnhoppers and woodshoppers

I know, not a mantis, but it's the best image showing insect muscles that I could find.

What I would like to know is how this creature's musculature would compare to ours regarding strength, more specifically lifting and carrying weight. Other aspects of strength, such as arm-wrestling or doing pull ups would be appreciated, but are not needed for this question.

Edit: I know that some insects and arachnids are capable of feats of strength, such as ants and spiders being able to lift many times their own weight. What I don't know is if that would scale up with size, nor if the reason they are able to do so is directly related to their muscle arrangement.

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    $\begingroup$ No, this would not scale up with size. The reasons such insects are capable of incredible feats of strength is their small size. Because of the square-cube law, their strength to mass ratio increases with decreasing size, which is why they can carry objects many times their own weight. A scaled-up version would not retain this advantage. $\endgroup$ – Gryphon May 11 '18 at 16:39
  • $\begingroup$ @Gryphon I thought that could maybe be the case. However, if we abstract scaling away for a moment, how would an exoskeleton based, human-sized creature compare to a human? $\endgroup$ – Renan May 11 '18 at 16:47
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    $\begingroup$ @Gene Could the skeleton have openings for the muscles? Could there be some sort of elastic skin like organ that would give the muscles some space? Or would that actually make it worse? $\endgroup$ – Nuloen The Seeker May 12 '18 at 10:12
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    $\begingroup$ I don't have the chops to answer this question, but perhaps it will be useful to note Eurypterid , the largest arthropod ever (2.5m, aquatic or maybe amphibius); and Arthropleura, the largest land arthropod ever (2.3m). Eurypterid was an apex predator, so it was possibly quite strong. Arthropleura was armor plated, so it was probably pretty muscley as well. Could they strangle a man in a dark alleyway? For the sake of your worldbuilding, I certainly hope so. $\endgroup$ – Pink Sweetener May 14 '18 at 13:58
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    $\begingroup$ @IlmariKaronen That could work to OP's advantage, though. A longer period of helplessness means a greater incentive to evolve more rigorous social structures. If OP wants these insects to be intelligent, that could be a good jumping off point. $\endgroup$ – Pink Sweetener May 14 '18 at 18:48
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Very general answer, because you do not seem to care about scaling a mantis, but rather seem interested in something human-sized that looks like a mantis and has an exoskeleton (man-tis):

  • The peak force of muscles scales with their cross-sectional (c-s) area. Things like holding force, lockability, fatigue, they are all dependent on the concrete physiological implementation. So a man-tis with human-size, human-style muscles is human in that department.
  • The breaking force of muscles and tendons also scale with c-s area, which is good news, because with an exoskeleton, there is more area to attach the tendons to - no more pulled tendons for man-tis!
  • Joints will be a major headache - humans often use the ball-in-pan system or one of the variations, which has the nice property of keeping the area involved in taking the strain somewhat constant with angle and, more importantly, keeping the area involved as big or bigger than the c-s of the bones forming the joint. Man-tis will not have that. Joints will be more like flexible rings of the exoskeleton, stabilized by fibre-direction (in those leathery membranes) and hydrostatic pressure. They will hold up fine, but quick movement might go out the window because those pressures need to be coordinated. Furthermore, limb-angles may exist that penalize force applied over long time.
  • Tubes are much more stiff, compared to rods of the same weight, so that bodes well for the exoskeleton, but making a tube with the outer diameter of a muscular arm from just the material contained in an arm-bone will result in a terribly buckle-prone (because thin walls) tube. You can offset that with internal pressure (which will need some structures containing and producing it) or additional material - both solution take away c-s area that could otherwise be used for muscle (and is, in a human)
  • this point is very important, from the point of view of your question: Armwrestling. As you know, if you have armwrestled, angles, lengths, ratios, geometry of the contesting arms and attached wrestlers - all need to be considered. With everything else being equal, the slightest advantageous twist of the hand will produce a superior angle of attack, leading to the inexorable defeat of the other. In our scenario not all is equal - general mass, yes, muscle physiology, yes, but everything else is different, and not better/worse different in a clear-cut way. One could just as easily construct a man-tis winner or looser - it all depends on the specific angles of attack and muscle c-s coming to bear, in a prolonged contest maybe even the circulatory performance to muscles under strain.
  • In a (wo)man-tis/(wo)man olympiad, with its different contests ranging from fatigue over speed, acceleration, strength to repeatability of movement, i'd see the humans on top, primarily because an olympiad was conceived by humans, and is therefore tailored to their abilities (a man-tis has two (three?) sets of arms, which one is allowed in weight-lifting? or may man-tis use them all?)
  • To resumé: Contests will be tailored so the contestants are comparable, which is why most sports have categories for children, elderly, men, women... a human/man-tis olympiad will be boring, because there will be no contest. Some categories will be won by man-tisses all the time, some by humans. Boring. 'Lifting force': I struggled with lifting my printer the other day, although i routinely lift cement sacks five times its weight, simply because the box it came in was designed in such a way as to make every way of holding it incredibly disadvantageous. Man-tis may be able to lift ten times more weight than humans, but only five inches up, and a tenth of the capability of humans over its head - does that make it stronger or weaker than humans? Man-tis can exist, with the alterations you already proposed to its respiratory system, but the exact strengths and weaknesses are completely up to you.
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An explanation of the body stresses that a scaled up insect would have to deal with is here under Session 5 Giant Ants Attack! - http://fathom.lib.uchicago.edu/2/21701757/

It talks about the mechanical stresses on the exoskeleton to support the weight of the larger insect, and the joint structure of insects. A key paragraph about the joints from the above article -

One estimate suggests that mammalian joints are called on to withstand forces as much as 100 times the animal's body weight during normal locomotion. (In humans, the peak forces on the knee during running can be 15 times body weight.) Arthropods, even though they have more legs than mammals, have it even worse; their joints may see forces as great as 3,000 times body weight, 30 times higher than mammals. Because joint contact areas are much lower in arthropods than vertebrates, the difference in stresses must be much greater. Now we come to the heart of the matter. As you scale up an ant, body mass must increase faster than joint surface area--indeed, the stress on the joint should increase in direct proportion to size.

The preceding paragraphs explain why the arthropod joint is required to withstand the higher forces.

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    $\begingroup$ I have been reading about this. Some arthropods have developed structures that make their exoskeleton much more resistant to buckling, and for some insects, the forces applied on their legs while walking has been measured to no more than 50 times body weight, which is more favourable than the stress faced by mammalian joints. $\endgroup$ – Renan May 14 '18 at 14:31
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    $\begingroup$ Huh, interesting! Perhaps this is why history's heaviest arthropods are aquatic or many-legged? $\endgroup$ – Pink Sweetener May 14 '18 at 15:20
  • $\begingroup$ My article talks about direct scaling of an existing creature to a larger size. If a creature were to evolve to human size, the exoskeleton would have to be altered to resist buckling and reduce stresses. Your attached articles show some of the adaptations that already exist to allow better mobility for arthropods. The referenced 50 times body weight for walking is still 3x what your own legs experience while running. $\endgroup$ – Futoque May 14 '18 at 16:18
  • $\begingroup$ If the exoskeleton reinforced itself from within, via some sort of latticed framework, I wonder if that would make enough of a difference. $\endgroup$ – Callum Bradbury May 15 '18 at 13:24
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I know that some insects and arachnids are capable of feats of strength, such as ants and spiders being able to lift many times their own weight. What I don't know is if that would scale up with size, nor if the reason they are able to do so is directly related to their muscle arrangement.

What reliably scales with size is power output of the muscles. Power is produced chemically and is subject to several hard constraints. ATP and Pcr stores in the muscle limit peak power; the cardiovascular system's oxygen throughput limits sustained power.

Based on the power constraint alone, a creature of a given size can carry 1000 times its weight at 1 cm/second, or 1 times its weight at 10 m/s (force*distance/time=const).

How this power is distributed between force and speed will depend on the creature's exact physiology. I've seen feats of nature impressive enough to lose belief in evolutionary impossibility. The outward effects of adaptation for high strength/low speed would be thick short limbs and vice versa.

The mammalian endoskeleton is generally more favorable for high speed and high instantaneous force than an exoskeleton. The joints are compact, the muscle is well-cooled and has a lot of room to change shape. In comparison, an exoskeleton is like an aircraft fuselage, it's good at distributed static loads, but not at strong shocks and high acceleration. Force on the skeleton is determined largely by acceleration, so running fast would be riskier for the mantis than a mammal. Carrying weight, however, would be a lot more efficient, with most loads going through the exoskeleton.

So the human-sized mantis would likely have good strength and excellent ability to carry loads, but not be as good as a mammal at jumping and running. Its sustained strength would be determined by its cardiovascular system. To make good use of its muscle, the mantis will need to circulate at least twice more blood, as both a coolant and an energy source.

In the gym, it's going to be very good at most weight exercises, doing dozens of reps (given good circulation). But not as good at complex full-body exercises, because of the system's limited instantaneous capacity. They won't be good at throwing punches, as that takes explosive strength and speed, but will be good at holding force.

Carrying as much as 5x own weight over long distances, but not very fast, should be quite doable. Perhaps 10x if someone else puts it on their back. It takes a lot less muscle effort to keep their exoskeleton straight than it does for a flexible bipedal human.

Arm-wrestling between one another will be viable, but more like how non-wrestlers imagine it - arm vs arm, slowly forcing the opponent down. A human athlete puts their body's force on the opponent's arm in one strong move; it's forceful and would be dangerous. Due to exoskeleton limitations, the mantis population will likely be a lot more uniform between athletes and non-athletes than humans. So they could be able to arm-wrestle amateurs, but at risk of permanent injury if they come against an athlete.

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  • $\begingroup$ For specific burst movements it is possible to store the energy in some elastic component with low internal friction, essentially a spring, that then can be triggered, for instance for high jumps or devastating punches - your point for the power output still holds true, but we have to realize that this is only about the output of the muscles, not of the system, so the time-averaged power output of the system will be lower than that of the muscles, but that says nothing about the size of peaks. $\endgroup$ – bukwyrm May 15 '18 at 7:09
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    $\begingroup$ It's theoretically possible, but I was going off what known living creatures use. A limitation of exoskeletons is that the punch can be devastating for the one throwing it as well. Compare ramming something with a stick vs a tube (or pickup vs aircraft). A tube is stronger against static loads, but breaks easier on impact, where the stick would flex. $\endgroup$ – Therac May 15 '18 at 7:19
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What I would like to know is how this creature's musculature would compare to ours regarding strength, more specifically lifting and carrying weight.

Yes, it would be comparable.

So, let's look at the square-cube law another way: the larger something is, the slower it is in all areas relative to its size. It's metabolism and reaction times slow down, its top speed slows down, etc. It also lifts less mass relative to its own body weight.

So scaling a mantis up to human size would result in a mantis with human strength, human speed, human reflexes, and so on. Keep in mind that the average human weighs 45 - 50 kg, and most cannot lift more than half their bodyweight without specifically training in a gym to do so. Even lifting 20 kg (50 lbs for us Americans) repeatedly would be a struggle for some.

You might be interested in these videos. They'll point out some of the problems with increasing the size of an insect without allowing for how scale determines the way an organism interacts with its environment and changes its metabolism, respectively.

https://www.youtube.com/watch?v=f7KSfjv4Oq0

https://www.youtube.com/watch?v=MUWUHf-rzks


Edit: I know that some insects and arachnids are capable of feats of strength, such as ants and spiders being able to lift many times their own weight. What I don't know is if that would scale up with size, nor if the reason they are able to do so is directly related to their muscle arrangement.

No, this does not scale up, particularly if the mantisman has the same metabolism as a human. In a certain sense, metabolism is a way of talking about how much energy an organism is capable of producing. Because smaller organisms have faster metabolisms, they produce the energy necessary to lift many times their own mass. They also have a higher surface area to mass ratio, providing more surface area for interior muscles to connect to and generate force from.

In contrast, a mantisman with human metabolism is going to have much less energy to lift with, and a lower surface area to mass ratio that reduces the amount of area the muscles can use.

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    $\begingroup$ +1. However, I am not so sure about equal size meaning equal strength. Gorillas are about 1.6 meters tall and weight round 150 Kg... I am quite sure that a 1.6m tall human weighing 150Kg would not have the same squeezing strength as a gorilla. $\endgroup$ – Renan May 14 '18 at 15:24
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    $\begingroup$ @Renan You are correct in that a gorilla of a comparable weight to a human would be far stronger than said human. But this is because the Gorilla would have a far greater amount of muscle relative to it's overall weight than the human. Whereas the human would have a higher body fat% (and a heavier brain that consumes a largely disproportionate amount of calories for its size). Among the great apes, we humans are weak bitches. We just make up for it by having huge brains. $\endgroup$ – Jason Desjardins May 14 '18 at 16:46
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    $\begingroup$ Also, a human that is 1.6m tall and weighs 150kg would look like a walking water balloon they'd be so fat. $\endgroup$ – Jason Desjardins May 14 '18 at 16:48
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The issue with human-sized insects, or the Attack of the 50ft Whatever, is as mentioned in the comments, the Square/Cube law.

Specifically, if you double the dimensions of an object, it's mass increases by eight times, however muscle strength only increases by the cross-dimension surface area, which will increase by 4 times. Thus, a higher proportion of the creature's strength must be dedicated to moving itself, leaving less available for other tasks, like lifting food to your mouth.

The reason why muscles only increase by a factor of two, is because we care about muscle strands. It doesn't matter how long the strand is, what matters is how many there are. Thus making your arm twice as long just makes each strand double in length. It's only when you also increase it in the other two dimensions does you muscle gain power, as there is now four times as many muscle strands.

The other issue with giant insects is that they don't have lungs, but instead use Spiracles, but that's not the focus of your question.

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    $\begingroup$ I feel like you're mostly just repeating the author's premises rather than answering the question. $\endgroup$ – Pink Sweetener May 14 '18 at 13:43

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