First find the total bug population after spraying: 80% * 400 bugs = <<80*.01*400=320>>320 bugs
Then find the total number of bugs each spider eats: 12 spiders * 7 bugs/spider = <<12*7=84>>84 bugs
Then subtract that number from the number of bugs left after spraying to find final number of bugs: 320 bugs - 84 bugs = <<320-84=236>>236 bugs
#### 236