No, it’s not a giant mosquito, nor does it eat them! This is a crane fly, probably Tipula paludosa, which is an imported species from Europe. Since it arrived in North America it has spread widely, becoming one of the most common crane flies. However, there is a decent chance this is some other species of crane fly. There are more than 4,200 species described so far, and many of them are devilishly hard to ID.
Crane flies are some of the oldest flying insects. They were the first flies to evolve, some 225 million years ago when the very first dinosaurs were stomping around and the earliest mammals were trying to avoid getting stomped on. It is amazing that an insect as delicate as a cranefly could not only survive for 200 million years, but diversify into a family of more than 4,000 species worldwide.
Crane fly larvae are called leatherjackets because of their thick cuticle. Many kinds live in moist soil or leaf litter. If this one is T. paludosa, it might have grown up in the moist litter underneath the large bushes in the east corner of the garden.
Some kinds of leatherjackets are pests of lawns, gardens, and crops. Most are not. As adults, crane flies rarely eat. They are not favorites of insect collectors because they tend to drop their legs at the slightest hint of trouble. You can’t catch them in nets, you have to use a camera!
Discover Life has a nice page on Tipula paludosa. The Bug Guide has lots of pix of crane flies. There’s a crane fly article at Wikipedia. Nature Spot (England) has some nice pix of T. paludosa. And lastly, a blast from the past: when I was writing Cool Fact Of The Day for The Learning Kingdom way back in 1999, I wrote about crane flies.