Ant Wars

Agreed, the idea of a rival species is an idea I've thought about and about which there is a good deal of sci-fi written.

And it probably makes the most sense of the rival species inhabited a different section/habitat like the ocean (which covers far more area than the land). Or inside the earth. How to fight, deal with a completely different species has a lot of varied aspects given that people can barely understand different cultures so a completely different species would be worse.
Also, if you look at how vulnerable humans are in the water (submarines, boats, scuba equipment), a species like an evolved intelligent octopus would be a huge threat. I mean intelligent enough to build machines, and to organize themselves.

Sig, you want a sig, here's a SIG-sauer!

The actual biological reason is actually oxygen. There is not as much oxygen present in today's atmosphere as there was billions of years ago. Scientists have proven that with higher than normal oxygen intakes, they could grow spiders larger than normal sizes. Using that data, you can draw the conclusion that it's the same with other insects and arachnids. Quite simply not enough oxygen in the atmosphere to grow the critters to a good size.

However, small size doesn't stop some of these creatures from being deadly in their own right.

There are reports every year of a species of ant in the Amazon or Africa (I forget which) takes down a human or baby and literally devours it within minutes. Most of these claims are unsubstatiated and there's no proof that these things happen... But it's probably a safer idea to believe the locals on such matters. They do live with the things year in and year out. They would know.

Also, what usually stops ants from growing far more intelligent than they are, is the basic lack of intelligence. They do build cities, but they build them underground for safety. However, they have no need for cars or elevators. They use each other for these purposes, and they never stop working. Very seldom do they ever remain still. They also possess a single intelligence in the colony. The queen is it. And everything is done for the survival of the queen and her eggs. There's no need to provide "comfort" or "distractions" to a queen in the species. She's always eating and laying eggs, that's what she does. And with only one individual, you can't really turn a "hive mind" into an "intelligent mind". Not unless you put some VERY SERIOUS environmental pressures on the species as a whole, and over a long period of time. You can see some signs of this already. What with the "animal husbandry" and "farming" some of the ants do. Even some of the "bridge building" and "boat making" some ant species do. But these behaviors happend out of necessity for the species to survive. And the world is a VERY hazardous place for a creature as small as an ant.