Fritz_Brown
Super Moderator
Well, these folks won't be from a low-g world, but the feetses is an interesting idea. I will use that elsewhere, certainly!
http://en.wikipedia.org/wiki/CarboniferousTheir large size can be attributed to the moistness of the environment (mostly swampy fern forests) and the fact that the oxygen concentration in the Earth's atmosphere in the Carboniferous was much higher than today .[21] This required less effort for respiration and allowed arthropods to grow larger with the up to 2.6 metres long millipede-like Arthropleura being the largest known land invertebrate of all time. Among the insect groups are the huge predatory Protodonata (griffinflies), among which was Meganeura, a giant dragonfly-like insect and with a wingspan of ca. 75 cm (30 in)
Unless your exoskeleton race is native to a low g world they are going to be very small.
this need not be the case. humans are fully able to don metal armor suits that approach their own body weight so there's no absolutely necessary reason why an exoskeletal form must be small...
One of the biggest limiting factors on insects, at least on earth, is not only their muscular/skeletal system, it's there internal systems. Terran insects and related species are limited by their respiratory and circulatory system. with no lungs, heart or enclose circulatory system,
http://en.wikipedia.org/wiki/Respiratory_system_of_insects
http://www.cals.ncsu.edu/course/ent425/tutorial/circulatory.html
At one point Arthropods could reach lengths of 2 meters ad were one of the dominant land dwelling life forms on the planet. Due mainly to radical differences in atmospheric oxygen.
.
http://en.wikipedia.org/wiki/Carboniferous
http://en.wikipedia.org/wiki/Meganeura
http://en.wikipedia.org/wiki/Arthropleura
Also the lack of a complex circulatory system limits the size, and complexity of the brain. The brain is an oxygen hungry beast, so to support higher brain function you need a LOT of oxygen.
To evolve past "garden pest/predator" stage an alien species would need an oxygen rich environment, or an enclosed complex circulatory system. The more advanced system can supply muscles and brain with lots of O2. ( or what ever gas they need to live.)
An alternate method would e to have the creatures breath a gas with smaller molecules, that can be more easily transported through the simple respiratory system of an arthropod more effectively, Such as hydrogen.
So if the species evolved an enclosed circulatory system, lungs, and a fairly advanced hybrid endo/exo skeleton. It would be well within the biological limits, as we know, them to evolve intelligence and tool use..
Bug threads are like bugs - it's hard to tell if they're really dead.
Humans can also take off that armor. Consider that a wound to an exoskeleton serves both as a break in the "skin" and a fractured "bone". A fleshie that isn't killed outright may have a better chance of surviving a wound and living to learn from the experience, not to mention reproduce, than a beastie with an exoskeleton. Consider also that humans in metal armor are not incredibly fast. For all their prowess from horseback, armored knights produced equivocal results when fighting equally determined lightly armored foes on foot. While it is possible to conceive of a form evolving the metabolism and structures to overcome the disadvantage of carrying extra weight on land, it is also possible that the disadvantage is too great to overcome competitive selective pressures in nature. It might be that it's easier to make a man-sized exoskeletal land form artificially than it is to see one evolve naturally - unless you introduce some factor that makes the exoskeleton an advantage. Maybe a higher level of background radiation.
A problem with extrapolating from primitive insect species is that they're primitive. Hard to tell what might have occurred had someone else not expanded into neighboring ecological niches that left them as prey to large fast predators. The competitors to the giant insects of that era were the carboniferous amphibians - likely effective predators for the era, but not in a class with the reptiles that would follow. And, the era was noted not just for oxygen concentration but for warm swamp-like conditions: note the significant size difference between modern tropical rainforest insects and their cousins of the more temperate regions. As it is, they were trapped in those niches that emphasized characteristics that enabled them to survive as species despite being eaten by anything and everything up to and including carnivorous plants. They produce large numbers of offspring, grow fast, and they try their darnedest not to be seen by predators - in which contest it pays to not grow so large that you can't hide in the local foliage.
What is needed is a world where the soft-body forms don't get a chance to get out of the water and evolve. Perhaps a world with little or no metallic core to form a protective magnetic field. Without that, interactions with charged particles from a sunlike or larger star would make it very difficult for an atmosphere to maintain the kind of layers that protect Earth from ultraviolet. The insects would have an advantage, and the soft-forms that managed to make the transition to land would likely also have protective armor or other protective strategies. On the other hand, it would affect plants too; that could make for a really odd ecology as plants evolved ways to take in and use solar energy without having their cells killed by high frequency UV-B.
Hydrogen is not an option for a breathing gas. Leaving aside other issues, it is simply too light - on any but an impressively massive world, it floats off and escapes the planet. You could maybe argue hydrogen for the airborne life floating through a Jovian atmosphere, but that kind is not going to want an exoskeleton to make it heavy.
The exoskeleton could have evolved to be not so much as and external support system, but mainly for protection. It is thick keratin with a leathery structure that has thick chitinous scutes to protect the main organ cavities and provide sharp and heavy edges to limbs and such. Not everything has to be (nor would it even in insects here)the same thickness everywhere so the joints and thoracic areas would have thinner layers of keratin (and more flexible) so the beast can move and breathe. Internally, the creature could have plenty of internal support structures of honeycombed bone or some analog of such, just like large crustaceans have today. However, since the outer layers would be less rigid and heavy (being that only the most important areas, and along the spine, etc..) the internal structures should add that much weight overall. Scale will be important, but there's nothing to say that a being that is around 4-5' tall couldn't have a carapace that combines some of the properties of an exoskeleton, enhanced by internal support structures to provide bracing for the outer shell and muscles.
Bug threads are like bugs - it's hard to tell if they're really dead.
Humans can also take off that armor. Consider that a wound to an exoskeleton serves both as a break in the "skin" and a fractured "bone". A fleshie that isn't killed outright may have a better chance of surviving a wound and living to learn from the experience, not to mention reproduce, than a beastie with an exoskeleton. Consider also that humans in metal armor are not incredibly fast. For all their prowess from horseback, armored knights produced equivocal results when fighting equally determined lightly armored foes on foot. While it is possible to conceive of a form evolving the metabolism and structures to overcome the disadvantage of carrying extra weight on land, it is also possible that the disadvantage is too great to overcome competitive selective pressures in nature. It might be that it's easier to make a man-sized exoskeletal land form artificially than it is to see one evolve naturally - unless you introduce some factor that makes the exoskeleton an advantage. Maybe a higher level of background radiation.
A problem with extrapolating from primitive insect species is that they're primitive. Hard to tell what might have occurred had someone else not expanded into neighboring ecological niches that left them as prey to large fast predators. The competitors to the giant insects of that era were the carboniferous amphibians - likely effective predators for the era, but not in a class with the reptiles that would follow. And, the era was noted not just for oxygen concentration but for warm swamp-like conditions: note the significant size difference between modern tropical rainforest insects and their cousins of the more temperate regions. As it is, they were trapped in those niches that emphasized characteristics that enabled them to survive as species despite being eaten by anything and everything up to and including carnivorous plants. They produce large numbers of offspring, grow fast, and they try their darnedest not to be seen by predators - in which contest it pays to not grow so large that you can't hide in the local foliage.
What is needed is a world where the soft-body forms don't get a chance to get out of the water and evolve. Perhaps a world with little or no metallic core to form a protective magnetic field. Without that, interactions with charged particles from a sunlike or larger star would make it very difficult for an atmosphere to maintain the kind of layers that protect Earth from ultraviolet. The insects would have an advantage, and the soft-forms that managed to make the transition to land would likely also have protective armor or other protective strategies. On the other hand, it would affect plants too; that could make for a really odd ecology as plants evolved ways to take in and use solar energy without having their cells killed by high frequency UV-B.
Hydrogen is not an option for a breathing gas. Leaving aside other issues, it is simply too light - on any but an impressively massive world, it floats off and escapes the planet. You could maybe argue hydrogen for the airborne life floating through a Jovian atmosphere, but that kind is not going to want an exoskeleton to make it heavy.
One of the biggest limiting factors on insects, at least on earth, is not only their muscular/skeletal system, it's there internal systems. Terran insects and related species are limited by their respiratory and circulatory system. with no lungs, heart or enclose circulatory system,
...At one point Arthropods could reach lengths of 2 meters ad were one of the dominant land dwelling life forms on the planet. Due mainly to radical differences in atmospheric oxygen.
...Also the lack of a complex circulatory system limits the size, and complexity of the brain. The brain is an oxygen hungry beast, so to support higher brain function you need a LOT of oxygen.
...So if the species evolved an enclosed circulatory system, lungs, and a fairly advanced hybrid endo/exo skeleton. It would be well within the biological limits, as we know, them to evolve intelligence and tool use..
Well, what about if on the world they evolved on, this species of big critters did develop something akin to a circulatory system, managing to push enough oxygen-rich goo about a circulatory system with its several pumps to each of its brain units. Its outer covering being a combination of rigid chitin and tough leathery material was an adaptation that enabled it to survived breaks that killed other species that only had a hard outer covering.
Less implausible now?