That is a Dobsonfly, Corydalus cornutus (Megaloptera formerly Nueroptera): Corydalidae).
The dobsonfly and its larval stages known as hellgrammites are among the largest and most primitive looking North American insects. Both may be over three inches in length and certainly qualify as insect dinosaurs. They belong to the insect order Megaloptera, which was formerly included in the Nueroptera. Other Megaloptera include insects known as fishflies and alderflies dobsonflies. Hellgrammites are aquatic insects that live under big rocks in larger, well-aerated streams and rivers. Being large and "showy" insects they tend to be secretive, hiding tightly wedged under large rocks to avoid predation (being eaten by something else).
Hellgrammites make excellent fish bait. As they lie under rocks, they feed on any insect or invertebrate that crawls by. They don't chase creatures that swim by, because they lack the ability to swim strongly in the fast-flowing water environment that they live in. Actually, they don't swim very well in any water. They have very large pincers. They use these powerful pincers to catch and tear up their prey. When handling these creatures one must be very careful because they move very fast and can inflict a painful bite. They're not poisonous, but they are powerful enough to draw blood.
Hellgrammites spend two to four years of their lives in the water living under rocks, like we mentioned above. After spending a few years in a stream they crawl onto land--sometimes several hundred yards from the water--where they pupate. After a week or two they emerge as adults, in much the same way that caterpillars turn into butterflies. In Michigan, adult hellgrammites (dobsonflies) begin to appear in mid to late-June. Dobsonflies fly along streams and rivers exclusively at night. They're sometimes attracted to lights.