Insects are not designed to be big. All this talk about how an ant can lift 10 times its weight is fine as long as the ant remains tiny. But enlarge it to the size in the movie “Them,” and they wouldn’t be able to walk.
Many of the reasons why insects are small were discussed in my February 13 post: Life on large planets (Are Earth-sized planets not the best size for life? II) I mainly discussed the surface-to-volume ratio problem that insects have: as surface area doubles, volume triples. Smaller creatures have a greater surface area to volume ratio, which means that small creatures have greater water and heat loss compared to large creatures (this is why children can dehydrate and become hypothermic much easier than adults). This is why insects have exoskeletons—the outer shell helps insulate and “waterproof” them. But this shell also slows down their growth—all growth has to occur within the shell before the shell is shed and, as quickly as possible, replaced.
However, there is another reason for their small size that I didn’t cover in the post mentioned above: their respiratory mechanism. Without going into great detail, because of their exoskeleton and their need for all their organs to help restrict water loss, insects have a separate respiratory system from their circulatory system. All air that enters their respiratory system must be able to diffuse to every cell in the body; thus, insects have a network of air tubes. They also tend to have air sacs, which can contain an extra supply of air, so that in dry environments they can close their spiracles (openings in their exoskeleton through which air passes) to help reduce water loss. Such a respiratory system works well for small creatures, but not for large ones; it would be difficult for air to diffuse to every cell in a large creature unless it was riddled with air tubes, and even then, it would be difficult to ventilate the tubes so that air would flow through them.
Ah, but insects were large—no, huge!—in prehistoric eras. One large monster bug, which lived around 286 to 360 million years ago, was the giant dragonfly (Meganeura monyi), which had a 27-inch wingspan! As reported in this month’s Science Illustrated magazine, “Scientists think an oxygen-rich atmosphere helped boost their size” (p. 29). What probably helped too is that the Earth was particularly hot and humid at that time, so heat loss and water loss were less of a problem.
Thus, for a sentient alien race to be insectile, like the insectoid Xindi from Star Trek: Enterprise or the large insects threatening the Earth in Starship Troopers, it looks like they need to evolve on a small (low gravity) planet that has a hot, humid, and very oxygen-rich atmosphere.
And it might not hurt if the planet was mostly water, because insects evolved from crustaceans, and being in water helps with buoyancy (allowing them to grow larger) and there is no need to worry about water loss (though there may be a need to worry about salt content because creatures adapted to fresh water do not survive in salt water and vice versa).In Starship Troopers, the large, dry, rocky bug home world is simply not hospitable to large sentient insectile creatures.
Of course, insectile creatures on other planets are not destined to be exactly like Earth insects or crustaceans. Universal biology will allow for a great range of diversity. Insectile creatures on other planets may have some similarities to Earth insects, but they will also have differences, especially if they are able to evolve high-level (i.e., technologically-based) sentience.
Yet more can be said on this subject, but it’s way past my bedtime.
