Instagram has lately been known to add a lot of features to its photo sharing service, and today is no different. Unlike other occasions, however, this time around it's only a small thing that's been changed. Small, but important.
Starting with version 24 of Instagram for Android and iOS (which should be available now in the Google Play Store and iTunes App Store, respectively), comment threads are a thing. This makes Instagram more like Facebook, its parent company, as that social network has had comment threading from time immemorial.
So, from this point on, when you hit Reply underneath any comment on Instagram, your response will be grouped in a thread - and the comment you reply to will be the thread starter. It's all neatly portrayed in the center screenshot above. This functionality makes it easier to keep track of various conversation topics that exist in the comments for every post.
Although Instagram hasn't explicitly said this, we assume threading will work exactly as it does on Facebook, meaning that you can't go more than one level down - so a reply to a reply that's already in a thread will not create a new, even more indented thread.