When I was a little boy

the opening in the front of my short trousers was called a 'spare',
as if the buttons on it were spare buttons to replace those attached to braces (which Americans call suspenders*).

*What the British call suspenders are, I think, a garter-belt in the USA.  What, then, do Americans call the leg-bands to hold up socks or stockings which the British call garters ?

As I got older, the opening was called a 'fly' or 'flies' (as in a theatre ?),
and was no longer closed by fumbly buttons but by nifty zip.

When I was a little boy and was driven in a car
the windscreen was soon speckled by dead flies.
Some cars even had fly-deflectors at the front.

Now, almost no flies are smashed on windscreens.
Flies (like most insects) are becoming rare, and definitely not spare.