Terrestrial insects by definition are insects that are born or living on land rather than in the sea or the air. The only time that terrestrial insects get into the water is by accidentally falling or accidentally landing or by being washed into the water. Ants are by far the most common terrestrial that can be found as you have discovered if you have ever planned a picnic.
Little surprise that ants are a good source of food for trout especially after a heavy rain when the ants have been washed into the streams. A good place to start is in the current seam where a small feeder stream meets a larger stream. There will be ants carried along in the current especially after a heavy rain.
All ants have three body sections and our “Perfect Flies” do indeed have the three sections just like the real thing. Obviously the trout do not count the sections and reject anything does not fit that pattern, but the more realistic a fly is, the more productive it will be especially in the calmer waters.
The best ant patterns are made to sink just as the real ants do. And as such they can be fished as a dropper one or two feet below an attractor fly.