These pesky things live in your lawn and sit and eat the roots of your grass. They are called Leather Jackets and turn into Cranefly, commonly known as Daddy Longlegs. They can decimate lawns extremely quickly and sadly there is no chemical treatment to kill them. Nematodes are a good option – something you can buy online. Or you can heavily water the area, cover with black plastic, and check in the morning. Then collect them and destroy them anyway you choose!
Either way, left to their own devices they can kill your lawn. You’ve been warned!!!