Cinema Insomnia), those with giant insects tend to be science fantasy at best. 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 which 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, their respiratory system is separate 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 - heat loss and water loss being 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 maybe it wouldn't hurt if the planet was largely a water planet as insects are evolved from crustaceans and being in water helps with buoyancy (thus can grow larger) and no need to worry about water loss (though may need to worry about salt content - creatures adapted to fresh water tend not to survive in salt water and vice versa). The large, dry, rocky bug home world in Starship Troopers just isn't a hospitable planet for large sentient insectile creatures.
Yet more can be said on this subject, but 'tis way past my bedtime.
"How did insects evolve?" Ask Us. Science Illustrated. Sept/Oct. 2008: 29. Print.
Myer, Dr. John R. "Insect Physiology: Respiratory System." General Entomology - ENT 425. North Carolina State University. 1 Nov. 2006. Web. 16 Aug. 2008. <http://www.cals.ncsu.edu/course/ent425/tutorial/respire.html>.