虻蜂取らず(あぶはちとらず)= 虻も取れず、蜂も取れない
虻あぶ a horsefly 蜂はち a bee
First of all there are a horsefly and a bee on a spider's web.
Then a spider went toward a horsefly, but it changed it's mind on the way, so it turn to a bee.
After all a spider did not catch both of them.
Do you understand my explanation in this English? Please correct if possible.