What layer of the tropical rainforest do most animals live in?

Answer 1

There are almost as many interesting websites about the rainforest and its populations of flora and fauna as there are ways of being interested in the rainforest, itself. Here are but two of them.

The first is blueplanetbiomes:
http://www.blueplanetbiomes.org/rainforest.htm

The second is from Caltech (the California Institute of Technology), and is called "What is a Rainforest?":
http://www.srl.caltech.edu/personnel/krubal/rainforest/Edit560s6/www/what.html

If you look, you will find many more--some even with animal soundtracks, much like that with which you will be regaled on Caltech's "What is?" website.

To simply answer your qusetion, though: The canopy layer and its understory house most of the rainforest's animal species. When one realizes most animal species there and everywhere are insects, this will make even more sense.