I want to ask a question related to ants. On which Stack Exchange site should I ask? I found Biology.SE.com but its a broad subject. I want a QnA site just for animals and insects.
Specific questions about animals are on topic on Biology. In fact, the zoology tag is currently used on 340 questions. entomology is used on 199 questions.
Ants turn up quite a bit there; they are mentioned 148 times.
You won't find a more specific site than Biology, I'm afraid.
I assume this is related.
This is strongly dependent on what type of question you have to ask.
As @HDE226868 pointed out, for questions about Zoology (animal behavior) and entomology (insects) Biology.SE is your best choice. I would not worry about your question being 'too broad' - it's already specifically about ants, and I assume you aren't looking to ask something like "how do I write a report on ants", which would not be accepted on any SE site.
If you're interested in their effect on your garden, or how they interact with other insects in your garden, gardening.SE is more ideal, and will cater to specific questions in that regard.
If you're interested in getting an ant farm, pets.SE is the site for you, and they'll answer any questions you might have about personally-owned pet ants.
If you want to know about dealing with ants on a camping/picnic trip, outdoors.SE can help you there.
And if you have questions about ants in fiction, there are several sites that you can visit to answer your questions: SciFi.SE for giant or otherwise mutant/alien ants from a specific work of fiction, Worldbuilding.SE for writing fictional ants into your own stories, Movies&TV.SE for TV shows and Movies depicting ants, just to name a few.
Most importantly, if you have several questions about ants that you need to ask, and they don't all fit into one SE, try to divide them into individual questions to ask on the appropriate SE sites - otherwise, you will be flagged for a too-broad question, no matter what SE site you pick.