There are many dragonflies in a recent place. Why?

There are many dragonflies, indicating that the air pressure in this area is low and there will be showers. This also means that parks and street trees spray less chemicals. In order to hold some activities, some cities sprayed a lot of chemicals, and many insects disappeared, especially dragonflies. Although there are fewer pests, soil, air and some waterways are polluted, and insect ecology is unbalanced. It is also possible that some insects are genetically modified. Let's take mosquitoes for example. Hum is rare. . . . . . .