They're not...the word 'fly' in that context refers to the flap of cloth which covers the zip. In the same way, the 'fly' of a flag is the part furthest from the pole which 'flaps' about in the wind.
Pedantically, On Line Etymolgy says this about the origin of the word "fly" in this useage: "fly (v.1)
"to soar through air...The O.E. plural in -n (cf. oxen) gradually normalized 13c.-15c. to -s. Notion of "flapping as a wing does" led to sense of "tent flap" (1810), which yielded (1844) "covering for buttons that close up a garment." (Hope your summer has improved since last we communicated Q...)
Yes, C, both weatherwise - to some extent - and France-wise. Last repeat of D-Day...ie 'invasion' of France...due on September 25th.
Sorry, John, for hijacking your thread.