What flights were significantly affected by insects hitting the fuselage?
It is conceivable that an accumulation of bugs on the leading edge of an airfoil would affect its performance.  In fact, the in flight accumulation of bugs occurs at such a low rate, and only at low altitude, that it's not a problem.

Above a few thousand feet AGL, bug density is negligible.  A car, driving at night, or a small plane flying for hours at low altitude may accumulate a mess of bugs.  Even so, such change in airfoil shape and affect on drag is miniscule on powered aircraft that normally spend moments, not hours, at such low altitude, above which there is no bug density.

The common hazard of bugs hitting the fuselage is restriction to visibility through the windscreen.  Bugs on the windscreen can make it difficult to see other aircraft and creates a myopic focus point for the eyes.

Another conceivable problem is blockage of inlets and ports, through most certificated aircraft are designed with alternate systems, e.g. bypass and redundancies to deal with such emergency.  Flight into icing conditions, heavy rain and ash are of concerns here.