We estimate the terminal velocity for a man falling in air. For air
we have
We approximate the man to have cross sectional area 0.2 m
and mass
m=70 kg. The corresponding terminal velocity is
(ps: I miscalculated this on the blackboard)
This will typically be a lethal fall. We can estimate the distance
required to reach the terminal velocity to be
This estimate neglected drag just asking after what distance
a free fall reaches the terminal velocity.
Lets consider how this result scales with the typical linear dimensions
of the falling object, L. We have 
and
so finally
Thus if we consider a bug with typical dimensions L=0.005 m ie. 100 times
smaller tha a human, its terminal velocity will be
times smaller than for a human ie. 14 mph. Hitting
the earth at that velocity probably is harmless. Thus we now understand
why a bug can fall from arbitrary heights without serious
bodily injury. Its mass to area ratio is so small that the terminal
velocity is not "terminal".