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I'd be a bit cautious of the fly on the windscreen evidence unless someone where to drive the same car today they drove back then as ever more attention is paid to aerodynamics and it may be that insects are being carried over the roof of the car and away from the windscreen.
Used to have aerofoils on the windcreen wipers at one point to maintain contact between the wiper blade and the screen but fewer cars have those now.... because of changed wiper design or aerodynamics?
We don't know unless the observation is made under identical conditions as previously.... too many factors to consider.
That's a good point J, that the aerodynamic design of cars may have improved a bit over the decades, and that might be contributing to the perception that insects are in decline.
There was an attempt to quantify the number of insects being splattered by cars by the RSPB charity (Royal Society for Protection of Birds) back in 2004, which involved a device distributed to a substantial number of UK motorists called a 'splatometer', described in this BBC news article:
The splatometer was basically just a piece of cardboard with gridded squares that was attached to the car's front numberplate, and 40 thousand UK motorists participated in the exercise. Use of the front numberplate to assess splattering would be expected to reduce the importance of the issue of the aerodynamic design of the car, but it might also concentrate on the proportion of the insect population that tends to fly about a foot above the ground.
The results from the exercise in 2004 were that one insect got squashed for every five miles of road travel. Presumably in the years after 2004, RSPB staff have then continued to use the splatometer themselves to monitor the UK insect population, and the absence of news stories about the splatometer since then suggests that the insect population measured by the device has not declined further.
One insect getting squashed every five miles means that it would take only 50 miles of travel by car to get to Chairman Mao's 10 per day fly swatting quota, and that would be achieved just with a portion of the front numberplate of a car.
By the way, I seem to recall an article many years back about the French fondness for Frogs legs.
It appeared that in India the harvesting of frogs proved a modest but appreciable money earner with the consequence that so many frogs were dismembered (and many of them while still alive) that the insect population exploded where this took place.