I grew up in New England, and moved to Colorado, the most stunning difference to me was the complete lack of insects. No misquitos, no flies, nothing. I could leave the windows open at night.
I was told this was due to the lack of moisture. That makes sense to me. Misquitos need water to breed, flies probably need water to keep rotting shit moist to lay babies in, etc.

However I was watching a documentary on the North African campaign in WW2, and it talked about how the biggest issue the soldiers faced was the flies.
Where were these flies coming from? Isnt it an arid desert? What are these flies normally eating when human armies are not around to host them?