Probably depends on how thick their exoskeletons are.
Ducks are not flightless. In some places, ducks are migratory birds, flying long distances in winter.
Antarctica... the north pole... but even in those places, there is no guarantee that there will be no insects whatsoever.
returdation
Because insects have evolved that way. This wider adaptability of insects have maximized their population also.
Crabs
Probably because the majority of insects actually prefer dark, damp and cool places than direct sunlight. They cannot regulate their temperature like mammals can - so they move from warm spots to cooler places to maintain their temperature.
Mainly in the forests but may find a couple other places
c: Falling
Insects live can in steamy tropical jungles, cold places, deserts, sea, caves, or flying up in the air - just about anywhere.
more and different places bcoming crowded
They work in the places that bugs are the most often!
Yes, if fact, it's essential to keep them in damp places as they can get hurt while moulting.