Dragonflies and mosquitoes...
and the incomplete nature of evolutionary explanations.
If you take walks in the country in the summer, you have almost certainly observed that you will get bitten by mosquitoes much more often in the shade than in the sunlight. Why? Well, the evolutionary explanation that I've heard (and it seems perfectly sound as far as it goes), is that a major predator of the mosquito is the dragonfly, and dragonflies only fly in the sun, so mosquitoes have evolved to fly in the shade in order to stay away from them.
Excellent... But then why haven't dragonflies subsequently evolved to be able to fly in the shade, and go get the mosquitoes where they are?
Again, I'm not saying that last question shows that the explanation on offer is wrong: I think it is most likely correct, as far as it goes. I am just noting that it leaves much unexplained.