They say that cockroaches will still walk the earth long after nuclear war has wiped out humanity, so it's not surprising that Insect aliens are a popular choice of non-humanoid alien. Starship Troopers is probably the most literal interpretation of this type of alien, as the Bugs that declare war on humanity are based on a broad spectrum of insect forms and operate in a hive-like structure controlled by an autocratic intelligence. The Zerg from StarCraft are another good example, especially when they start rushing.