Should “a/an” articles be used before general concepts?

Question

Let me give you some examples from my textbook, that were confusing for me.

1. How many strings does ___ bass guitar have?
2. How many players can be on court in ___ basketball team?
3. Which European country hasn't fought in ___ war since 1815?
4. Is __ spider ___ insect?

I "feel" that they can be spoken in either way, i.e. with article or without, but I'm not sure.

And by general concept I mean some class of common objects, like biology taxon... Now, when I've mentioned it, I'm starting to think that taxons cannot be singular words.

In these examples I can consider guitar, team, war, spider and insect as objects in general or as some "one" object. It's hard to explain, but I think it's an influence of my native language and Japanese. I suppose, any singular countable noun in English should be preceded by an article (or other specifying word). And general entities should be used in a plural form. Please, correct my conclusions if I'm wrong.

Answer 1

All of them take the article. The exception is “fought in ___ war”, which can be fought in a war or fought in war.

The reason for this is that the word war means both the event (as in “another war has started”) as well as the state/practice (as in “war has claimed many lives”). However, in this specific construct, fought in a war is more common.