Milt,
And do they need oxygen masks at high altitudes.
Yes, the typical Australian sea level acclimatised fly has an operational ceiling at about 6000 to 7000 feet.
As a young F/O I often flew with a Captain who would amuse himself by observing stow-away flys exceeding their TUC (Time of Useful Consciousness) and falling into a comatose state at about 6000 to 7000 feet. Phase 2 of the amusement was to liberally douse the fly in a pool of 100% Oxygen from the nearest oxygen mask. Recovery would typically take place within about 5 seconds, whereupon the recovered fly would buzz of to do fly things. TUC outside the O2 rich environment was about 10 seconds, whereupon a further crash took place.
Merv did get bored on long flights.
Regards,
Old Smokey