Why do dragonflies have these special little spots on their wings?
Pterostigma is what those spots are. Wikipedia article is at https://en.wikipedia.org/wiki/Pterostigma.
I read the summary of the technical article below.
The pterostigma are heavier than the rest if the wing, and so the insect can glide faster without a destabilising flutter beginning.
The author also believes that the pterostigma contribute to more efficient active flight.
Technical details at:
Norberg, R. Åke. &quot;The pterostigma of insect wings an inertial regulator of wing pitch&quot;. Journal of Comparative Physiology A. 81 (1): 9–22.
doi:10.1007/BF00693547.