Ah. I see. Yes, you're getting into some interesting stuff now, Jack.

As far as I'm concerned, plants are living beings too...although they're certainly a different type of living being than an animal. Do they feel pain? Yes, according to fairly recent scientific research on the matter. That research has been done with an instrument somewhat similar to a lie detector, and it can detect a plant's energetic (but not physical) response to a threat. It appears that plants can even "feel" hostile or loving thoughts being directed toward them...as well as feel physical pain when damaged...but most plants have no way of visibly or audibly showing us their reactions. (An exception to that would be certain small tropical plants I've seen which will recoil, for instance, when you touch them, but those are a very rare instance among plants.)

I'm not trouble by your assertion that human beings are a species of animal. Fine with me. We get into various mental habits that animals don't seem to burden themselves with (as far as we can determine), but that may be because we've developed a more complex brain and consciousness while still being a species of animal.

Note that I said "may be". I'm not saying that I know. I'm theorizing.

Words or phrases are most definitely mental constructs, as you say. Humans made up all those words at some point, and we periodically make up new words as we go along, all of which serve as labels and mental concepts.

I know what an apple is by direct experience (or at least I think I do...). My choice to call it "an apple", though, is an arbitrary choice that was made a long time ago by other people, and I'm repeating that arbitrary choice whenever I use the word "apple". The apple itself is real. It's observable. It can be experienced. The word "apple" is arbitrary...but we (in the English speaking population) have agreed to use it as a label for the real thing.

"Is it immoral to experiment on fruit flies?"

I don't know. I could arbitrarily decide that I did know...and then say "yes" or "no", but the real fact is that I don't know if it's immoral to experiment on fruit flies, and I don't particularly mind admitting that I don't know. There are a great many things that all of us don't know....they considerably outnumber the things we do know...and we shouldn't be afraid to admit that, in my opinion.

In the end...not knowing...we are faced with making the best decisions or compromises that we can in the face of our partial knowledge...we have to go on our gut feelings and our sense of "right and wrong" (which is greatly affected by the culture we grew up in). We have to make compromises between various extremes and various outcomes, based on our feelings and our beliefs of "right and wrong".

My gut feeling is that we shouldn't do hurtful experiments on chimps or other animals (incuding humans) if we can possibly avoid doing so. I'm not too worried about doing experiments on fruit flies. But that's just my own personal feeling.

Everyone has to decide for themselves about these things. No one has the final answer for everyone else, but only for themselves.