I’ve always thought that praying mantises are way cool. If I were to write a science fiction story about a race of giant mantids which invades Earth, how big could they be before their exoskeltons couldn’t support their weight, they’d have to eat constantly and thus wouldn’t be biologically feasible, etc.?
I’m thinking they could be at least twice the height of an average human being, easy. True?
Ever heard of Starship troopers?
I’d wager that you’re more likely correct if you don’t make them much larger than the largest insects we have on hand… a foot or two tall, at the most.
But for the purposes of fiction, bear in mind how many Godzilla movies there have been. Make 'em as big as you want.
I am not a biologist, but from hearing my biologist friends rant over Sciffy movies, your real problem is going to be their lungs. Insects are itty-bitty because there’s not enough Oxygen in our atmosphere (anymore).
I seem to recall that somewhat-largish (but-still-not-giant) spiders are more feasible, but that usually comes up at my ‘dude, get a life’ point, so my biologist friends don’t get to finish their rant.
Not even close. Structural strength scales up with the square, but weight scales up by the cube of the linear dimensions, so your giant insects can’t get much larger than they are without extensive modifications to support structures. Their legs would have to be thicker and/or positioned more directly under the body. That’s one limitation. Another is the way insects breathe, lacking lungs means they must stay small to oxygenate their tissues sufficiently. The largest insects which ever lived weren’t much bigger than couple of feet or so in size, excluding wings and other appendages.
If insectoid critters on an Earthlike world evolved into large (intelligent) beings, they’d need to evolve features like true lungs, a good circulatory system, and of course a vastly more complex central nervous system, but I don’t see why they couldn’t retain some kind of exoskeleton. They’d need additional structural support, though – maybe some internal cross-braces to make the exoskeletal segments stronger.
As for food, though, I believe that problem goes in the opposite direction – that is, small creatures need to eat more often than large animals. If I recall, a shrew eats close to its own body weight every day.
I recently read an article about the absolute maximum size of insects given the current oxygen content of the atmosphere (and I’m talking real insects, not some hypothetical insect-like alien) and the constraints imposed by breathing through spiracles and leg geometry, but I’m damned if I can find it now.
The upper limit seems to be about six inches.
You certainly couldn’t scale up a mantis as it is and expect it to survive – as QED correctly points out, square-cube law problems would be lethal. But if we’re talking about creatures that evolved, rather than just getting suddenly blown up in size, then I can imagine that large bug-like creatures could conceivably exist with massive body architecture changes. (They might have ultra-light chitin with air channels. Or have carapaces of something other than chitin.)Especially if they lived in a lower-gravity situation.
Certainly this hasn’t stopped writers from including such creatures. Alan Dean Foster has a whole series with creatures that are man-sized mantises. There was a novel about a planet with oversized insects called “The Lost Planet” by some classic golden age author (whose name I can’t remember, or Google right now). And the anthology Future Boston features an alien who looks like a mantis with the legs of a pillbug. Presumably these have different architecture than your small-size mantids.
Don’t forget moulting. How is a 2m tall mantid going to survive shedding its exoskeleton and then waiting for the new layer to harden?
Simple answer is: as big as you want them to be. They’re aliens, not mantids. They might look like praying mantises but they’re not, really. All you have to do is provide reasonable mechanisms for the systems to work. You don’t even have to provide many details—in fact you shouldn’t provide many details or it’ll be boring. Offhand remarks about physiology when one of them is injured, or occasional contrasts with terrestrial insects is about all you need.
Alan Dean Foster’s details on the Thranx, mentioned by someone above, are pretty sketchy and have been thrown in a little at a time over several books. They have spiracles, but a partially closed circulatory system. They mass less than a similar sized human. Their chitin isn’t really chitin, but it is similar. They smell good to humans. This is probably because they still use some form of scent communication. They have 4 pairs of limbs, two dedicated “hands,” four dedicated “feet,” and one set of intermediates that can be used for either task. Their language includes gestures as well as sound, and presumably scent, though I don’t think that has been explicitly discussed.
The characters are well used to the appearance of Thranx, so these things are mentioned only in passing. I think it took me two of his books before I was sure of exactly how many legs they had! Don’t get bogged down in the details. You can get away with only light description of anything beyond surface appearances unless it directly pertains to the plot. You should know your background so that you don’t have to make it up as you go and introduce inconsistencies, but you don’t need heaps of detail.
Even if you’ve got that detail, you most likely don’t want to introduce it into the story. Heinlein spent most of a week working out orbital mechanics for a situation. That week of effort is condensed into about two sentences in the story. “What,” “who,” and “why” are far more important than “how” in most storytelling.
Orson Scott Card’s buggers are, as a character eventually points out once in, I think, Xenocide are warm-blooded with an internal skeleton and so on. They look like a bunch of insects and act much like a bunch of insects but they’re reasonably close to mammalian or avian in terms of internals.
Under water?
You’ve obviously never met my Uncle. 
I prefer the Men In Black approach. Why do they have to be big to become a power in the universe? Given enough of them, and sentient thought, why they could spread out like…like…like…locusts.
Cartooniverse
Let the exoskeleton split along its long axis. The underlying soft stuff bulges up and forces the edges of the split apart. The fresh exoskeleton hardens rapidly. Have the old exoskelteton gradually flake away from the edges of the split, continually (if slowly) replaced by rapidly-hardening material from below. There would be two bands of leathery-hard, not-yet-dry exoskeleton that gradually move around the body and meet again on the opposite side. The old exoskeleton is not shed in one fell swoop.
Or, have the exoskeleton continually grown like a fingernail. On a person, the germinal matrix forms the nail, which migrates slowly over the nail bed (which adds material and thickens the nail) until it detaches from the nail bed at the hyponychium. On a big bug, the ‘nail’ would be stationary on the ‘nail bed’, and the germinal matrix would move slowly around the body as a band. The cells of the germinal matrix need not move; the ‘nail bed’ cells could be chemically induced to change into germinal matrix cells briefly before returning to their role as stationary ‘nail bed’ cells. As the ‘germinal matrix wave’ passes below mature nail/exoskeleton, the ‘nail’ lifts off and either wears away or is trimmed off. This would be a continuous process rather than a series of discrete molts.
One of my favorite past threads (it made Threadspotting on my recommendation): let’s engineer a giant cockroach
As pointed out, exoskeletons don’t scale up very well; they have a poor weight-to-strength ratio compared to internal skeletons. There’s no reason why an alien with an internal skeleton can’t have external armor as well, however, and be as large or larger than humans. Remember knights; they functioned just fine despite all that heavy steel armor. For that matter, look how big some armored dinosaurs got.
This big enough
Hmmm. although the address reads right there is seemingly a fault in it.
http://www.dn.se/DNet/jsp/polopoly.jsp?d=597&a=580677&previousRenderType=3
Maybe that will work better.
Some in this thread have suggested the BEMs evolve in a low-gravity environment. Remember the OP posits they’re going to invade Earth. Either they can function in our gravity or they have technology to compensate.